名称和语源

中文的「月」为象形文字，在甲骨文中月像一弯眉月的样子。东汉许慎在《说文解字》一书中分析月的字型时说：月，阙也。人们经过观察，发现月圆的时间少，阙（弦月或眉月等）的时间多，于是就照眉月的样子创造出这个象形字。古称太阴、玄兔、婵娟、望舒等。

在英语中月的专有名称是「」。该名词源于原始日耳曼语的「mǣnōn」，在725年之前的古英语被称为""，1135年为「」，大约在1380年变为「」，之后再变成现在的写法。月球在现代英语的主要形容词是「」，源自拉丁文的「」。另一个比较不常用的形容词是「」，则源自古希腊文的「」（），是衍生自字首「」（像是「」）。古希腊塞勒涅（）和古罗马的狄安娜（）或称辛西娅（）的女神都是月球的名字。辛西娅和塞勒涅是反映月球处于不同轨道期如远月点、近月点的专门术名，狄安娜一名连接死亡，意指白天。

以天体位置来看月球也能称呼为地卫一（，），但以天体位置来称呼在天文学的术语使用上较为罕见。

形成

有数种机制都认为月球形成于年之前，即大约是太阳系诞生之后的3000万至5000万年。这些机制包括分裂说、捕获说和地月同源说（孪生说）等。分裂说认为月球是由于离心力从地壳分裂出去，但要产生如此大的离心力，需要地球在诞生初始时有超高速的自转。捕获说则认为月球是在成型时被地球引力场捕获的天体，但这种假说需要地球拥有一个有非常大的大气层来消耗月球通过时的能量，减缓月球运动速度。同源说认为地球和月球形成于同一原生吸积盘，但这种假说无法解释月球上金属铁的匮乏，也不能解释地月系统的高角动量。

现今主流的地月系统形成理论是大碰撞说：一颗火星大小的天体（被称为特亚，神话故事中月球女神塞勒涅的母亲）与原生地球碰撞，爆裂出的物质进入环绕地球的轨道，经由吸积形成月球。

该假说虽然不是很完美，但也许是最好的解释。在1984年10月有关月球起源会议召开前的18个月，比尔·哈特曼（Bill Hartmann）、罗杰·菲利普斯（Roger Phillips）和杰夫·泰勒（Jeff Taylor）挑战月球科学家同事们：「你们有十八个月的时间，下定决心，回到阿波罗数据，回到电脑中，做所有你们必须做的事。不要来参加我们的会议，除非你们有了有关月球诞生的话要说。」1984年夏威夷科纳的会议上，大碰撞假说成为最受欢迎的理论。在会议之前，有三种「传统」理论学派，加上少数开始认真思考大撞击理论的人，以及为数众多，认为辩论永远不能解决问题的中间派。会后，学术界实质上只分为两派：大碰撞阵营和不可知论者。

大碰撞说认为：在太阳系诞生的早期，巨大的撞击是很常见的。电脑模拟的大碰撞模型表明，这样的撞击后产生的双星系统具有充分的角动量匹配目前地月系统的轨道参数，而且也可以解释月球具有相对较小核心的原因。此外，大碰撞说还可以合理解释地月成分的不同：月球的大部分组成成分都来自撞击前的天体，而并不是原生的地球。但是这个假说仍然不是很完善，例如对陨石的研究却显示内太阳系的其他天体，如火星、灶神星等，其氧和钨的同位素成分和地球不同，而地球和月球有非常相似的同位素成分。一个合理的解释是导致地月系形成的撞击混合了地球和月球形成时挥发的物质，有可能导致两个天体之间同位素的组成变得均衡，但这种解释仍有争议。

大碰撞中所释放的大量能量和之后在地球轨道上再作用的物质会熔化地球的外壳，形成岩浆海。新形成的月球也会产生自己的月球岩浆海，估计它的深度范围为500公里（310英里）至1737公里(1079英里)，最深相当于月球自身的半径。

尽管它准确地解释许多证据，但大撞击假说很难完全解释一切，其中大部分涉及月球的组成成分。

另外一种假说则认为大碰撞产生了两颗在同一轨道上的卫星，一个就是月球，而另外一个较小，直径只有约1000公里。在数千万年后，两个卫星缓慢相撞，最后合二为一。这种假说解释了月球一面地势平坦，另一面则地势起伏不平的原因。

2001年，华盛顿卡耐基研究所的一个研究团队报告了对月球岩石同位素最精确的测量值，研究小组惊讶地发现，阿波罗计划所带回岩石的同位素特徵，与地球岩石相同，而不同于太阳系几乎所有的天体。这完全出乎于以前认为的进入轨道形成月球的大部分物质都来自于忒伊亚的想法。2007年加州理工大学研究人员宣布，忒伊亚同位素特徵与地球相同的概率低于1%。2012年发表的阿波罗月球样品的钛同位素分析同样表明，月球和地球的组成成分相同，这完全有悖于大碰撞假说预期的月亮形成于远离地球的轨道或来自忒伊亚。

物理特性

月球是地球的同步自转卫星，它绕轴自转的周期与绕地球的公转周期是相同的，这使得它几乎永远以同一面朝向地球。它之前以较快的速度旋转，在后来由于地球产生潮汐摩擦，让其自转速度减慢，直到最后以同一面持续面对地球，即潮汐锁定。我们将月球朝向地球的一面被称为正面，而相对的另一面则称为背面，背面通常也称为"暗面"，但是事实上它如同正面一样会被照亮。当月相为新月时，我们看到月球的正面是黑暗的，而月球的背面则被太阳照亮。

月球是一个南北极稍扁、赤道稍许隆起的扁球。它的平均极半径比赤道半径短500米。南北极区也不对称，北极区隆起，南极区洼陷约400米。但在一般计算中仍可把月球当作三轴椭圆体看待。物理天平动的研究有助于解决月球形状问题。通过天平动研究还表明，月球重心和几何中心并不重合，重心偏向地球2公里。这一结论已为阿波罗登月获得的资料所证实。

表面地形

科学家曾经使用雷射测高仪和立体影像分析对月球表面的地形进行测量。月球表面最明显的地形特徵是位于背面的巨大撞击坑南极-艾托肯盆地，其直径有2,240公里，是月球上最大的陨石坑，也是太阳系中已知最大的。它的底部是月球上海拔最低的地方，深度达到13公里。而月球海拔最高的地点则正好就在它的东南方，有人认为这个区域是造成南极-艾托肯盆地的撞击所形成的隆起。月球上的其它大撞击盆地，如雨海、澄海、危海、史密斯海和东方海等，也都拥有低海拔的区域和高耸的边缘。月球背面的平均高度比正面高1.9公里。

火山地形

在月球表面上用肉眼可以清楚看见有黑暗的，相对平坦的平原，我们称之为月海，这是因为古代的天文学家认为这些地方充满了水。现在，我们知道这些黑暗部分是古代火山爆发后熔岩浆在洼地凝结成的广大玄武岩。和地球的玄武岩类似，月海中的玄武岩含有丰富的铁，而完全缺乏因水流过而出现的矿物。大多数喷发的熔岩浆流入与撞击盆地相连接的洼地，形成月海。现在科学家已经在月球正面的月海中发现几个拥有盾状火山和火山穹顶的地质分区，这些是熔岩浆凝结形成月海的证据。

几乎所有的月海都位于月球正面，占正面面积的31%，相较之下，在月球背面只有少数的月海，只涵盖了背面2%的面积。这被认为和通过月球探勘者的伽玛射线光谱仪所描绘的月球化学图上所看见在月球正面地壳下的生热元素的浓缩有关。生热元素的浓缩会造成地函下的温度上升，部分熔解，并上升到表面造成喷发。大部分玄武岩的喷发都出现在30至35亿年前的雨海纪，但也有少部分样本的辐射定年显示其形成于更古老的42亿年，也有一些相对年轻的样品，最年轻的喷发物经由撞击坑计数测定年限发现其发生在12亿年前。

月球上较亮的部分被称为「高地」，因为它们高于大多数的月海。经由辐射定年测定它们是于44亿年前形成的，这意味著这些高地可能是在月球岩浆海形成时的斜长岩堆积所产生的。月球上没有任何一个主要的山脉被认为由地质构造事件产生的，这和地球的情况刚好相反。

撞击坑

平均每天月球都会承受超过3吨宇宙物质的撞击，当中的小行星或彗星撞击月球表面时会形成撞击坑，是另一个主要会影响月球表面地形的主要地质事件。现在估计单在月球正面直径大于1公里的陨石坑就大约有300,000个，其中有些陨石坑以知名的学者、科学家、艺术家和探险家的名字命名。月球地质年代是根据月面上的重大陨石撞击事件进行分界，包括在酒海、雨海和东方海等的撞击事件。这些撞击事件的结构特徵是产生多层物质隆起的环，通常是由数百至数千公里直径的围裙状喷发物沉积形成一个区域性的地层视界。由于月球没有大气层、天气变化，在最近几十亿年也没有地质活动，大部分环形山都保存得很完好。虽然有几个多环盆地明显的已经很久远，它们还是能用于分派相对的年龄。由于撞击坑是以恒定的速率累积，计算单位面积内的撞击坑数目可以用来估计表面的年龄。阿波罗任务收集撞击熔化的岩石以辐射测定年龄，群集在38亿和41亿年的年龄：这已被用来解释撞击的后期重轰炸期。

覆盖在月球地壳上的是高度粉碎的（碎裂成更小的颗粒）和撞击园艺下的表面层称为风化层，是由撞击过程形成的。最细微的风化层，是二氧化矽的月球土壤玻璃状物体，有著像雪一样的纹理和闻起来像用过的火药。较老的风化层表面一般比年轻的表面厚；在高地的厚度在10-20米之间，在海的厚度则是3-5米。

在细致的粉碎风化层下面是「粗风化层（megaregolith）」，厚达数公里高度碎裂的基岩。

表面地质

月球表面的化学元素分布极不均。按照其丰度依次为：氧、硅、铁、镁、钙、铝、锰、钛。氧的含量估计为42%（按重量）。碳和氮只有痕迹，似乎只存在于太阳风带来的微量沉积中，氢主要集中在月球的两极。

月岩

月球表面主要有四类岩石：月海玄武岩、高地岩石（主要包括斜长岩与富镁的结晶岩套）、克里普岩和角砾岩，且分布不均。

月壤

月壤是月球表面的细微表岩屑，主要是玄武岩和斜长岩物理崩解的结果，由多年来持续的流星撞击和太阳及星际带电粒子轰击所造成。

内部构造

月球是一个已经分异的天体，即它拥有地壳、地函、和核心。月球的内核富含固态铁，半径大约为240公里，此外还有一个流体的外核，主要成分是液态铁，半径大约为300公里。核心周围是部分熔融的边界层，约有500公里的宽度边界层结构是在45亿年前月球形成不久之后，由月球岩浆海通过分离结晶形成的。岩浆海的结晶可以经由沉淀形成由镁铁质和沉积的橄榄石、斜辉石和斜方辉石等矿物组成的地函。四分之三的岩浆海结晶之后，可能形成密度较低的斜长石并浮在地壳的顶部。最后才由液体结晶的部分会被夹在地壳和地函之间，并且含有大量不相容和发热的元素和之相符的是从月球轨道上遥感绘制的月球地质化学图也显示其地壳几乎都是由斜长岩组成。通过对部分熔融的地函喷发出的熔岩流冷凝下来的月岩样本的研究，科学家确认地函含有比地球更丰富的铁，其主要成分是镁铁质。通过地球物理技术发现月球地壳的平均厚度约为50公里左右。

月球是太阳系内密度第二高的卫星，仅次于木卫一埃欧。但是月球的内核并不大，半径大约是350公里甚至更小，只占月球大小的约20%，相较之下，其它地球型天体的比例约为50%。它的组成尚不是完全清楚，可能是由金属铁组成，同时含有少量硫和镍。对月球随著时间变化转动的分析显示月球核心至少仍有部分是熔融的。

重力和磁场

月球表面的引力约为地球的六分之一。月球的重力场已经通过围绕月球旋转的探测器发射无线电信号的zh-hans:多普勒效应;zh-hk:多普勒效应;zh-tw:都卜勒效应;所测量的。月球重力场主要的特徵是拥有质量瘤，即在一些巨大的撞击盆地却反而出现较重的重力分布，这可能与组成这些盆地的玄武岩熔岩流密度较大有关系，这些异常对环绕月球轨道的太空船有极大的影响，如果经月球这些地域时，假如太空船与月面距离足够低，而且轨道不加修正的话，那么太空船会在数个月或数年间在月球表面坠毁。但令人困惑的是，熔岩流密度本身不足以完全解释重力异常，有一些质量瘤的存在明显和月海中的火山作用形成的熔岩流无关。

月球拥有一个外在磁场，其强度不到地球磁场百分之一，范围在1至数百特士拉之间。已发现月球上有类似质量瘤的异常的磁场区。这些磁场区有明显不同于其他地方的磁场强度。天体液体金属核心可以生成的全球性双极性磁场，但现在月球的磁场并不是由液体金属核心产生的，而可能是在月球演化的历史早期被磁化而一直保留至今的地壳磁场，月球磁场另一种可能来源是在大碰撞事件期间生成的瞬态磁场残馀的磁化，通过撞击产生的电浆云包围，扩大了磁场的范围，这种说法受到最大的地壳磁场撞击盆地对面出现对跖点的支持。

大气层

月球有一个非常稀薄、接近真空的大气层，总质量低于10公吨。如此小的大气质量在月球表面产生的压力大约是3atm（0.3nPa），数值随著月球一天的时间不同而改变。月球大气的来源包括出气和溅射，如太阳风的离子轰极月球表面释放出的原子。过往曾经检测到由溅射产生的原子包括钠和钾，相同的情况也曾在水星和木卫一埃欧的大气中发现过。月球大气的氦-4来自太阳风，氩-40、氡-222和钋-210则来自月球地函相关元素放射性衰变后的溅射。但月球大气中缺乏存在于月球表岩屑的氧、氮、碳、氢和镁等自然元素的原子或分子，目前原因尚不清楚。月船1号已经在月球大气中发现水蒸气的存在，其含量随著月球纬度的不同而改变，大约在纬度为60-70度时水蒸气的含量最高。这些水蒸气可能是由月球表面表岩屑的水冰升华而生成的。月球大气层的气体有些被月球的重力吸引回到表岩屑，有些由于太阳的辐射压，或者被太阳风的电离后逃逸到太空中。

季节

月球的转轴倾角只有1.54°，远小于地球的23.44°。由于这个缘故，太阳照射对月球季节变化的影响很小，反而是月球表面地形对季节变化有重要作用。在2004年，约翰·霍普金斯大学的Ben Bussey博士率领的小组研究克莱芒蒂娜探测器在1994年获得的影像，发现位于月球北极的皮尔斯环形山边缘有4个区域在整个月球日中都被阳光所照亮，形成永昼峰，而在月球南极地区没有类似的区域。而在极区的许多环形山底部是永久黑暗的，没有受到阳光照射。这些黑暗的环形山底部是极低温的：月球勘测轨道飞行器在夏天的南极环形山底部测得的最低温度是35K（−238 °C），而在接近冬至时在北极测得埃尔米特环形山的温度只有26K（−247 °C）。这个温度比冥王星的表面温度还要低，是太空船在太阳系中所测得的最低温度。

水的存在

月球的表面不存在液态水，因为太阳辐射会使水被光解并快速逸入太空。但从1960年代以来，科学家假设由彗星撞击所带来的水、或者来自太阳风的氢和含氧丰富的月岩反应所产生的水，都可能以冰的型态沉积下来，并在月球两极撞击坑低温的永久阴影区留下可以追踪得到的痕迹。电脑模拟月面的永久阴影区约有14,000平方公里。在月球上可用水的数量是一个重要的因素，可以决定建设一个月球适居区计画的成本效益，因为从地球运水到月球的费用极为昂贵。

近年来，已经在月球表面发现水的特徵。在1994年，安装在克莱门汀号太空船的双向雷达实验，显示有少量、冰冻的水存在接近表面的凹穴内。但是，后续使用阿雷西博天文台的雷达观测，又认为此一发现可能是由新撞击坑中的岩石近被撞击的岩石喷出的。在1998年，月球勘探者携带的中子能谱计显示，在极地附近深度1米的风化层存在著高浓度的氢。在2008年，对一颗由阿波罗15号带回的熔岩珠的分析，显示有微量的水存在于球状硅酸盐玻璃内。

在2008年，印度的月船1号太空船使用在载月球矿物绘图仪确认表面有水冰的存在。分光计观测在反射的阳光中侦测到羟基的通用吸收谱线，提供了有大量水冰在月球表面的证据。太空船显示浓度可能高达1000PPM。在2009年，月球坑观测和传感卫星送了一个2,300公斤的撞击器到极区永久阴暗的环形山，并且从喷出的羽状物质中至少检测到100公斤的水。LCROSS另一个实验的数据显示侦测到的水量，更靠近155公斤（± 12公斤）。

与地球的关系

轨道

月球相对于固定的恒星以27.32天的周期完整地绕行轨道一周。更正确的说，月球的平恒星周期是27.321661天，和平回归周期（从分点至分点）是27.321582天（「天文历书的补充解释」, 1961, at p.107）（它的恒星周期）。然而，因为地球间同时间也绕著太阳转，它对地球呈现相同相位的时间就会较长，大约是29.53天（它的会合周期）。与其他行星大多数的卫星不同，月球的轨道比较接近黄道平面，而不是地球的赤道平面。月球的轨道受到太阳和地球而有许多小、复杂并且相互影响而难解的摄动，例如月球轨道平面的渐进转动，这影响到月球其它的运动状态。卡西尼定律以数学叙述出后续的影响。

其中主要的轨道变化有：偏心率变化、轨道倾角变化、拱线运动、交点西退、中心差。

偏心率变化

月球轨道偏心率变化在1/15到1/23的范围内，偏心率的平均值为0.0549，接近1/18。

严格来说，地球与月球围绕共同质心运转，共同质心距地心4,671公里（即地球半径的2/3处）。由于共同质心在地球表面以下，地球围绕共同质心的运动好像是在「晃动」一般。从地球北极上空观看，地球和月球均以逆时针方向自转；而且月球也是以逆时针绕地运行；甚至地球也是以逆时针绕日公转的。

很多人不明白为什么月球轨道倾角和月球自转轴倾角的数值会有这么大的变化。其实，轨道倾角是相对于中心天体（即地球）而言的，而自转轴倾角则相对于卫星（即月球）本身的轨道面。这个定义习惯很适合一般情况（例如人造卫星的轨道）而且数值是相当固定的，但月球却非如此。

拱线运动

月球围绕地球的椭圆轨道，在它自己的平面上也不是固定的，其椭圆的拱线（近地点和远地点的连线）沿月球公转方向向前移动，每8.85年移动一周。中国早在东汉，贾逵就提出月球视运动的最疾点每九年运动一周，这实际上正是拱线运动的结果。

轨道倾角变化

月球轨道（白道）对地球轨道（黄道）的交角（黄白交角）变化在4°57～5°19之间，平均值为5°09。

月球的轨道平面（白道面）与黄道面（地球的公转轨道平面）保持著5.145 396°的夹角，而月球自转轴则与黄道面的法线成1.5424°的夹角。因为地球并非完美球形，而是在赤道较为隆起，因此白道面在不断进动（即与黄道的交点在顺时针转动），每6793.5天（18.5966年）完成一周。期间，白道面相对于地球赤道面（地球赤道面以23.45°倾斜于黄道面）的夹角会由28.60°（即23.45°+ 5.15°）至18.30°（即23.45°- 5.15°）之间变化。同样地，月球自转轴与白道面的夹角亦会介乎6.69°（即5.15° + 1.54°）及3.60°（即5.15° - 1.54°）。月球轨道这些变化又会反过来影响地球自转轴的倾角，使它出现±0.002 56°的摆动，称为章动。

交点西退

白道与黄道的交线，其空间位置并不固定，而是不断地向西运动，每18.6年运行一周。这一现象早在东汉末年就为刘洪发现，并用于月食预报计算中。

中心差

由于月球轨道是椭圆而不是圆形，月球公转速度并不均匀。月球运动同均匀的圆周运动比较，时而超前，时而落后，其半振幅为6°.29，周期为27.55455日。

几何天秤动

由于月球轨道为椭圆形，当月球处于近地点时，它的自转速度便追不上公转速度，因此我们可见月面东部达东经98度的地区，相反，当月处于远地点时，自转速度比公转速度快，因此我们可见月面西部达西经98度的地区。这种现象称为经天秤动。又由于月球的自转轴倾斜于公转轨道平面（白道面），而白道与黄道又有约5度的交角，因此月球绕地球公转一周时，极区会作约7度的晃动，这种现象称为纬天秤动。再者，由于月球距离地球只有60地球半径之遥，若观测者从月出观测至月落，观测点便有了一个地球直径的位移，可多见月面经度1度的地区。这种现象称为周日天秤动。

如同绝大多数天体运行，月球绕地球的长期轨道痕迹是一个甜甜圈，月球轨道远离的现象会到目前轨道的大约1.4倍为止，然后再慢慢绕回来。

相对大小

月球相对于地球的大小是最大的：直径略大于地球的四分之一，质量约为1/81。就卫星与行星的相对大小比例来说，它是太阳系最大的卫星（虽然冥卫一凯伦与矮行星冥王星相对来说更大）。

然而，地球和月球仍然被认为是一种行星-卫星系统，而不是双行星系统，因为它们的质心，一般所谓的质量中心，位于地球表面之下约1,700公里处。

潮汐效应

地球上的潮汐主要是来自月球牵引地球两侧引力强度的渐进变化的潮汐力造成的。这在地球上造成两处隆起，最明显的是海潮和海平面的升高。由于地球自转的速度大约是月球环绕地球速度的27倍，因此这个隆起在地球表面上被拖曳的速度比月球的移动还快，大约一天绕著地球的转轴旋转一圈。海潮会受到一些影响而增强：水经过海底时的摩擦力与地球自转的耦合，水移动时的惯性，接近陆地的平坦海滩，和不同海洋盆地之间的振荡。太阳的引力对地球海潮的影响大约是月球的一半，它们相互的引力影响造成了大潮和小潮。

月球和靠近月球一侧隆起的重力耦合对地球的自转产生了一个扭矩，从地球的自转中消耗了角动量和转动的动能。反过来，角动量被添加到月球轨道，使月球加速，使得月球升到更高的轨道和有更长的轨道周期。结果是，月球和地球的距离增加，和地球的自转减缓。通过阿波逻任务安装在月球表面上的月球测距仪，测量月球到地球的距离，发现地月距离每年增加38毫米（虽然每年只是月球轨道半径的0.1 ppb）。原子钟也显示地球的自转的一天，每年约减缓15微秒，在UTC的缓慢增加被闰秒加以调整。潮汐拖曳会继续进行，直到地球的自转速度减缓到与月球的轨道周期吻合；然而，在这之前，太阳已经成为红巨星，吞噬掉地球。

月球表面也能体验到周期约27天，振幅约10公分的潮汐，它有两种成分：因为它的同步自转，来自地球的是固定的；和来自太阳的变动。来自地球噵致的量是天秤动，这是月球轨道离心率造成的结果；如果月球轨道是理想的圆，就只会有太阳造成的潮汐。天秤动会改变从地球看见的角度变化，使得从地球可以看见59%的月球表面（但在任何时间看见的都略少于一半）。这些潮汐力累积的应力会造成月震。虽然每次震动可以持续至一小时以上－明显的比地震的时间长－因为缺乏水来阻尼震动的振幅，但月震不如地震的频繁，也比地震微弱。月震的存在是1969年到1972年的阿波罗太空人安放在月球上的地震仪的一个意外发现。

从地球看月球

月相

在满月期间，月球的视亮度约有-12.6等（作为参考，太阳的视亮度为-26.8等。），在夜间最容易察觉得到，但它有时也可在日间看见。（例如上弦月可于下午看见，下弦月可于早上看见。）

地卫一大约每天推迟50分钟从东方升起。但正史中也有一些奇怪的记载，《金史·天文志》记载：「太宗天会十一年（1133年），五月乙丑（6月15日），月忽失行而南，顷之复故。」月球有著异常低的反照率（与煤炭相当）。尽管如此，它仍是天空中继太阳之后第二亮的天体。这一部分是因为对冲效应的增强效果；在弦月时，月球只有十分之一的亮度，而不是满月一半的亮度。此外，由于视觉系统的颜色恒常性重新校准天体的颜色和周围环境的关系，因为周围的天空比较黑暗，会觉得被太阳照射的月球是比较明亮的天体。满月的边缘感觉上会比中心明亮，并没有周边昏暗的效应，这是月球土壤的反射特性，它反射向太阳方向的光多于其它的方向。月亮出现在靠近地平线时会显得比较大，但这纯粹是一种心理上的影响，也就是所谓的月球错觉，最早的叙述出现在西元前7世纪。

月球在天空中最高的高度变化：虽然它有与太阳相同的限制，在一年当中它会随著季节与月相变化，满月在冬天到达最高的位置。18.6年的交点周期也有些影响：当月球的升交点在春分点，月球每个月的纬度可以到达28°。这意味著月球会出现在赤道到纬度28°之间的天顶，反过来 (降交点在春分点)则只有18°。月球的新月方向也取决于观测者的纬度：接近赤道的观测者，可以看见微笑状的新月。

月球的表面是否会随著时间改变，在历史上仍有争议。今天，许多这些主张被认为是虚幻的，是在不同光线条件下观察的结果，不良的视宁度，或不当的绘图。但是，偶尔会出现出气现象，还有小部份的报告可以归因于瞬变月面现象。最近，有人认为月球上一个3公里直径的区域在一百万年前被释放出的气体改变。月球的外观，像太阳一样，也会受到地球大气层的影响：常见的是当月光通过高空的卷层云时，会受到冰晶的折射形成22°的晕环，通过薄云也会有相似的冕环。

食

当地球、太阳和月球在一条直线上时，便会出现蚀。日食发生在朔（有别于新月），当月球介于地球和太阳中间。对照过来，月食发生在满月，当地球介于太阳和月球中间。从地球看月球的角视直径和太阳的角视直径变化的范围是重叠的，因此日食时会有日全食和日环食的可能性。在日全食，月球会将太阳的盘面完全遮蔽掉，因此以肉眼就能看见日冕。由于地球和月球的距离缓慢的在逐渐增加中，月球的角视直径逐渐减小。这意味著在数百万年前的日食，月球都会完全遮蔽掉太阳，而没有发生日环食的可能。同样的，从现在开始大约6亿年之后，月球将不再能够完全遮蔽掉太阳，因此将只会发生日环食。

由于月球环绕地球的轨道相对于地球环绕太阳的轨道有大约5°的倾斜，所以不是每个新月和满月都会发生食。当食发生时，月球必须在两个轨道平面交集的附近。日食和月食复发的周期性，由沙罗周期来描述，其周期大约是18年。

由于月球在天空中总是会遮蔽大约半度直径圆型区域的视野，当一颗亮星或行星经过月球的后方时，就会发生掩星的现象：从视线中隐藏。这样一来，日食只是太阳被掩蔽。由于月球非常接近地球，单独一颗恒星被掩蔽的现象不是在地球上的任何地点都能见到，也无法同时见到。并且因月月球轨道的进动，每年会被掩蔽的恒星也都有所不同。

研究和探测

早期的研究

在天文学发展的早期天文学家已经对月球周期有深刻的理解：如大约在，巴比伦天文学家已经知道月食有大约18年的沙罗周期，印度天文学家已经对月球每个月的距角进行描述，中国天文学家石申确定了一套预测日食月食的公式。之后，月球的天然形状和月光的成因也被了解，古希腊哲学家阿那克萨哥拉推断太阳和月球都是巨大的岩石球体，而且后者通过反射前者的光来发光。虽然中国汉朝时认为月球等同于「气」，他们的「辐射影响」理论解释月球光只是反射自太阳，京房（前77年—前37年）注意到月球是球体。西元499年，印度天文学家阿耶波多（Aryabhata）在他的《Aryabhatiya》记录月球的耀眼光芒是反射阳光的缘故。天文学家兼物理学家海什木发现月球不像镜子那样反射阳光，而是从月球表面每一个方向往所有方向发射出去。中国宋朝的沈括创造一个涂上白色粉末的银球反射阳光，来解释月相的变化，而从侧面看时就能呈现眉月的月相。

亚里士多德的宇宙的描述（On the Heavens），月亮标示出可变元素（土、水、风和火）的球和不朽的恒星（以太）之间的边界，一个有影响力的哲学主导的世纪。然而，在，塞琉西亚的塞琉古的理论认为潮汐是月球引力引起的，因为朝汐的最高点都与月球相对于太阳的位置相对应。阿里斯塔克斯在同一个世纪计算出月球大小和距离，得知地月的距离是地球半径的20倍。托勒密进一步更正这些数值：平均距离是地球半径的58倍，直径是地球的0.29，非常接近现在个别的值60和0.273。阿基米德发明了可计算当时已知行星和月球运动的天象仪。

在中世纪望远镜发明之前，已经有越来越多人认识到月球是一个球体，但许多人却认为它的表面是非常平滑的。1609年，伽利略在《星际信使》中使用第一架伸缩望远镜描绘的月球，注意到它并不是光滑的，有著环形山和山。望远镜描绘出如下的月球：乔瓦尼·巴蒂斯塔·里乔利（Giovanni Battista Riccioli）和弗朗切斯科·马里亚·格里马尔迪（Francesco Maria Grimaldi）在17世纪后期的努力产生现今使用的月球命名系统。威罕·皮尔（Wilhelm Beer）和梅德勒（Johann Heinrich Madler）在1834-6年间发展出更精确的，并且在1837年出版相关的书，第一次用三角法准确的研究月球特徵，包括一千多座山的高度、并引导对月球研究的精确度可能如同地球的地理。最先注意到的月球环形山科学家是伽利略，一直被认为是火山。直到1870年代，理查德·波达（Richard Proctor）才提出这是由撞击形成的假设。这种观点在1892年获得地质学家葛洛夫·吉伯特（Grove Karl Gilbert）的实验支持，1920年至1940年的比较研究引导月球地层学发展，在1950年代成为天体地质学的一个崭新且持续发展的分支。

第一次直接探测：1959–1976

在冷战期间，美国和苏联一直希望在太空科技领先对方。这场太空竞赛在1969年7月20日，美国阿波罗11号的指挥官尼尔·阿姆斯壮登陆月球时达到高峰，他是登陆月球的第一人，而目前最近一次登陆过月球的人是尤金·塞尔南，他是1972年12月阿波罗17号任务的成员。

苏联的任务

冷战刺激了苏联和美国的太空竞赛，令人类加速了对月球的探测。一旦发射器有足够的能力，这些国家就发射无人探测器进行飞越和撞击或登陆的任务。来自苏联的月球计画太空船最先完成多项目标：于1958年进行了三次未赋予名称的失败任务之后，第一个脱离地球的引力，并且飞越过月球的人造物体是月球1号；第一个撞击月球表面的人造物体是月球2号；第一个拍摄到通常是被遮蔽而看不见的月球背面影像的是月球3号，这全都发生在1959年。

第一艘成功执行在月球软著陆的是月球9号，第一艘环绕月球的无人太空船是月球10号，两者均在1966年完成任务。将月球的岩石和土壤标本带回地球的标本返回任务（月球16号、月球20号和月球24号）总共带回0.38公斤的月岩。两个先锋的机器人太空船在1970年和1973年登陆月球，是苏联的月球步行者计画的一部分。

在美苏的登月竞赛中苏联使用了N1火箭，尝试将其用于搭载载人登月航天器，但因机件故障造成四次试射失败，最终以输家身份结束这场:太空竞赛。

美国的任务

美国的月球探测始于机器人任务的发展，旨在实现载人登陆月球的最终目标：喷射推进实验室的测量员计画，在月球9号发射后4个月，发射第一艘登陆月球的太空船。NASA载人的阿波罗计画也在同时发展；经过无人的阿波罗太空船在地球轨道上一系列的测试之后，和苏联月球飞行能力的刺激，阿波罗8号于1968年首度执行载人环绕月球轨道的任务。在1969年人类首次登陆月球，与后续多次的登陆月球，使很多人认为这是太空竞赛的最高峰。尼尔·阿姆斯壮是美国阿波罗11号任务的指挥官，他在1969年7月21日02:56（世界时）踏上月球表面，成为第一位在月球漫步的人。从阿波罗11号到17号（除了阿波罗13号中止了登陆月球的任务）的任务，总共带回382公斤、共2,196块月球岩石和土壤标本。美国登陆月球和返回使1960年代初期在的技术获得长足的进步与发展，特别是在烧蚀化学、软体工程和重返大气层技术，和高阶巨大计画整合管理等领域。

在整个阿波罗任务中，许多科学仪器被建置在月球表面。能长期工作的仪器站，包括热流量探测器、地震仪、磁强计，它们是阿波罗12号、14、15、16和17设置的。它们将资料直接传送回地球，直到1977年才因为预算的原因而停止，但是工作站的月球雷射测距回向反射器阵列是被动式的仪器，它们仍在使用中。从地球例行测量的测站到月球基地的距离精确范围在几公分之内，并且从这些资料可以对月球核心的大小有所理解。

目前的时代：1990–现在

阿波罗计划之后，更多的国家已经直接参与月球的探测。在1990年，日本将太空船Hiten送到月球，成为第三个拥有环绕月球轨道卫星的国家。这艘太空船在月球轨道上释放了一个小探测器Hagoromo，但是发射失败，妨碍了进一步的科学应用任务。

在1994年，美国国防部和NASA联合发射了克莱芒蒂娜至月球轨道。这个任务首度获得几乎整个月球的全球地形图，和第一份月球表面全球的多光谱影像。此后在1998年又派遣了月球探勘者任务，仪器显示在月球的极区有过量的氢，这可能是存在于永久阴暗的环形山内部风化层表层数公尺处的水冰。

欧洲太空船智能1号，第二艘使用离子推进的太空船，从2004年11月15日进入月球轨道直到2006年9月3日，并且第一次对月球表面的化学元素做了详细的调查。

中国的中国探月工程也发射了第一艘进入月球轨道的太空船，嫦娥一号，从2007年11月5日直到2009年3月1日撞击月球。在6个月的任务期间，获得月球表面完整的影像图。嫦娥二号于2010年10月1日发射升空，最主要任务是为嫦娥三号预定著陆的虹湾拍照，而其分辨解析力为约1米。嫦娥三号携带月球车于2013年12月2日发射升空，并于12月14日著陆月球表面。2018年12月8日嫦娥四号著陆器、玉兔二号探测车由长征三号乙改进Ⅲ型运载火箭发射升空，2019年1月3日成功在预选的著陆区月球背面南极-艾特肯盆地（South Pole-Aitken，SPA）内的冯·卡门撞击坑（Von Kármán）著陆。2020年1月2日嫦娥四号著陆器和「玉兔二号」月球车按地面指令完成月夜模式设置，顺利进入月夜休眠。2020年11月24日嫦娥五号于海南文昌发射场发射升空，完成月球表面自动采样任务后，于12月17日凌晨1时59分在内蒙古四子王旗著陆场。

在2007年10月4日至2009年6月10日之间，日本宇宙航空研究开发机构的「月亮女神（Selene）」任务，携带了一架高明晰度电视摄影机，和两个小的无线电发射卫星，获得许多月球地理的资料和从地球轨道之外高明晰的影片。

印度的第一次月球任务，月船1号，从2008年11月8日起环绕月球，直到2009年8月27日，创建了月球表面高解析的化学、矿物学和照片地质地图，并确认月球土壤中存在著水分子。印度太空研究组织计画在2013年发射月船2号，携带俄罗斯的月球漫游车。印度也曾表示希望在2020年能够送人上月球。月船2号最终于2019年7月22日发射成功。然而著陆器于当年9月7日因硬著陆而撞毁，轨道器仍然在轨道上继续执行科考任务。

其他即将进行的月球探测任务包括俄罗斯的月球-团块——以它们的火星探测器福布斯－土壤的轨道器为基础的一种无人登陆器，架设地震仪，预计在2012年发射。

在2007年9月13日宣布的Google月球X大奖，鼓励私人资助的月球探索计画，将提供2,000万美元给任何让机器人登上月球且合乎其他指定标准的人。

美国发射的「月球勘测轨道飞行器」（LRO）和「LCROSS」撞击器于2009年6月18日进入轨道；随后与轨道上的飞行器，在2009年10月9日一起在计画内与计画外广泛的观测LCROSS撞击Cabeus，完成他的使命，之后，LRO仍然继续运作，以月球高度测量术获得高解析度的影像。

在美国总统乔治·沃克·布希在2004年1月14日宣布在2020年重返月球之后，NASA开始恢复载人任务计画。星座计画开始资助与测试载人太空船和发射器，并且研究和设计月球基地。但是，2011年的政府预算已经取消了对NASA星座计画的挹注，这将迫使NASA取消在太空技术上的推行以及高推力火箭的研究。

法律地位

虽然月球号系列探测器将苏联的旗帜散布在月面各处，美国国旗也象徵性地插在阿波罗太空人的登陆点，但目前没有任何一个国家宣称月球表面的任何一部分是他们的领土。根据苏联和美国在1967年签署的外太空条约，月球和外太空是「全人类所共有的地方」。这份条约也限制了月球只能供和平目地的使用，明确禁止军事设施和大规模毁灭性武器的设置。1979年的月球协定限制单一国家对月球资源的创建、开发与利用，但是至2020年1月没有任何一个拥有载人航天能力的国家签署。虽然有一些个人曾经宣称拥有月球的全部或部分，但这些没有一件是真实的。

文化

月球规则的相位变化是一个很好的计时器，周期性增长和衰减的形式成为许多古老历法的基础。2万至3万年前骨制计数棒上的缺口被认为是月相的标记。阴历的一个月大约是30天。英语中的名词month和日耳曼语系与其它同源的语系来自原始日耳曼语的*mǣnṓth-，这又连结到前述原始日耳曼语的*mǣnōn，显示德国民间在使用阳历之前是使用阴历。

月球已经给予艺术和文学作品无数的灵感，它是许多视觉艺术、表演艺术、诗歌、散文和音乐艺术的主题。有5,000年历史的爱尔兰Knowth石刻，可能是被发现、最早的代表月球的描绘。月球上明亮的高地和黑暗的海的对比，在不同的文化和民族中创造出不同的形象，像是月球上的人、兔子、野牛、嫦娥、玉兔、螃蟹和其它的等等。在许多史前和古代的文化中，月球化身为月神，或其它超自然的现象和占星图的月亮，到今天仍然被继续传播。

• 在中国有嫦娥奔月的神话。

　• 中国历代以来，在诗歌文学中对于月亮，有许多不同的雅称：

　　• 和满月形状有关：白玉盘、半轮、宝镜、冰镜、冰轮、冰盘、蟾盘、飞镜、飞轮、挂镜、金镜、金盆、明镜、瑶台镜、银盘、玉镜、玉轮、玉盘、玉盆、圆影、月轮。

　　• 和新月形状有关：悬钩、玉弓、玉钩、蛾眉。

　　• 和月亮光芒有关：蟾光、方晖、金波、清光、夜光、幽阳。

　　• 和神话有关：白兔、蟾蜍、蟾宫、嫦娥、顾菟、广寒、桂宫、桂魄、姮娥、琼阙、素娥、兔影、银阕珠宫、玉蟾、玉京、玉栏、玉兔、圆蟾、月桂、清虚、望舒。

　　• 其他：冰壶、冰鉴、冰魄、婵娟、秋影、太阴。

• 在希腊神话中，月亮女神叫做阿耳忒弥斯，月球的天文符号就像一弯新月，也象徵阿耳忒弥斯的神弓。

• 在北欧神话中，玛尼是驾驶月车的神明。

文学作品

• 法国科幻小说作家儒勒·凡尔纳的小说《从地球到月球》，利用巨型大炮将人发射到月球上去。

• 中国古代唐朝诗人李白的唐诗《静夜思》和《月下独酌》

• 中国古代宋朝文学家苏轼的词作《水调歌头·明月几时有》

音乐作品

精神病的联想

西方文化中，月球长久以来也与精神错乱和非理性相关联；精神错乱（lunacy）和疯癫（loony）这两个字都源自拉丁文的月亮「Luna」。哲学家亚里斯多德和老普林尼都辩称满月容易影响个人，甚至导致精神错乱。他认为主要由水构成的大脑，一定会受到月球和潮汐的影响；但是月球的引力太微弱，不会影响到任何一个人。即使在今天，虽然没有科学的依据，依然有人坚称精神科病患的数量、交通事故、杀人或自杀的事件，在一轮满月的期间会增加。

Names and etymology

The English proper name for Earth's natural satellite is typically written as Moon, with a capital M. The noun moon is derived from Old English , which stems from Proto-Germanic *mēnōn, which in turn comes from Proto-Indo-European *mēnsis 'month' (from earlier *mēnōt, genitive *mēneses) which may be related to the verb 'measure' (of time).

Occasionally, the name Luna is used in scientific writing and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon. Cynthia is a rare poetic name for the Moon personified as a goddess, while Selene (literally 'Moon') is the Greek goddess of the Moon.

The English adjective pertaining to the Moon is lunar, derived from the Latin word for the Moon, . Selenian is an adjective used to describe the Moon as a world, rather than as a celestial object, but its use is rare. It is derived from , the Greek word for the Moon, and its cognate selenic was originally a rare synonym but now nearly always refers to the chemical element selenium. The element name selenium and the prefix seleno- (as in selenography, the study of the physical features of the Moon) come from this Greek word.

Artemis, the Greek goddess of the wilderness and the hunt, also came to be identified with Selene, and was sometimes called Cynthia after her birthplace on Mount Cynthus. Her Roman equivalent is Diana. The names Luna, Cynthia, and Selene are reflected in technical terms for lunar orbits such as apolune, pericynthion and selenocentric.

The astronomical symbols for the Moon are the crescent and decrescent , for example in M☾ 'lunar mass'.

Natural history

Lunar geologic timescale

The lunar geological periods are named after their characteristic features, from most impact craters outside the dark mare, to the mare and later craters, and finally the young, still bright and therefore readily visible craters with ray systems like Copernicus or Tycho.

Formation

Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System. Historically, several formation mechanisms have been proposed, but none satisfactorily explains the features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force would require too great an initial rotation rate of Earth. Gravitational capture of a pre-formed Moon depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon. A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon. None of these hypotheses can account for the high angular momentum of the Earth–Moon system.

The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body (named Theia) with the proto-Earth. The oblique impact blasted material into orbit about the Earth and the material accreted and formed the Moon just beyond the Earth's Roche limit of ~.

Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of the Moon derived from the impactor, rather than the proto-Earth. However, models from 2007 and later suggest a larger fraction of the Moon derived from the proto-Earth. Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth–Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, although this is debated.

The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system. The newly formed Moon would have had its own magma ocean; its depth is estimated from about to .

While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. Models that have the Moon acquiring a significant amount of the proto-earth are more difficult to reconcile with geochemical data for the isotopes of zirconium, oxygen, silicon, and other elements. A study published in 2022, using high-resolution simulations (up to particles), found that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits.

On November 1, 2023, scientists reported that, according to computer simulations, remnants of Theia could still be present inside the Earth.

Natural development

The newly formed Moon settled into a much closer Earth orbit than it has today. Each body therefore appeared much larger in the sky of the other, eclipses were more frequent, and tidal effects were stronger.

Due to tidal acceleration, the Moon's orbit around Earth has become significantly larger, with a longer period.

Following formation, the Moon has cooled and most of its atmosphere has been stripped. The lunar surface has since been shaped by large impact events and many small ones, forming a landscape featuring craters of all ages.

The Moon was volcanically active until 1.2 billion years ago, which laid down the prominent lunar maria. Most of the mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some are as young as 1.2 billion years and some as old as 4.2 billion years. There are differing explanations for the eruption of mare basalts, particularly their uneven occurrence which mainly appear on the near-side. Causes of the distribution of the lunar highlands on the far side are also not well understood. Topological measurements show the near side crust is thinner than the far side. One possible scenario then is that large impacts on the near side may have made it easier for lava to flow onto the surface.

Physical characteristics

The Moon is a very slightly scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to restore hydrostatic equilibrium at its current orbital distance.

Size and mass

The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizable as one of its planetary-mass moons, making it a satellite planet under the geophysical definitions of the term. It is smaller than Mercury and considerably larger than the largest dwarf planet of the Solar System, Pluto. The Moon is the largest natural satellite in the Solar System relative to its primary planet.

The Moon's diameter is about 3,500 km, more than one-quarter of Earth's, with the face of the Moon comparable to the width of either mainland Australia, Europe or the contiguous United States. The whole surface area of the Moon is about 38 million square kilometers, comparable to that of the Americas.

The Moon's mass is of Earth's, being the second densest among the planetary moons, and having the second highest surface gravity, after Io, at and an escape velocity of .

Structure

The Moon is a differentiated body that was initially in hydrostatic equilibrium but has since departed from this condition. It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as and a fluid outer core primarily made of liquid iron with a radius of roughly . Around the core is a partially molten boundary layer with a radius of about . This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago.

Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallized, lower-density plagioclase minerals could form and float into a crust atop. The final liquids to crystallize would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly anorthosite. The Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth. The crust is on average about thick.

The Moon is the second-densest satellite in the Solar System, after Io. However, the inner core of the Moon is small, with a radius of about or less, around 20% of the radius of the Moon. Its composition is not well understood but is probably metallic iron alloyed with a small amount of sulfur and nickel analyses of the Moon's time-variable rotation suggest that it is at least partly molten. The pressure at the lunar core is estimated to be .

Gravitational field

On average the Moon's surface gravity is (; ), about half of the surface gravity of Mars and about a sixth of Earth's.

The Moon's gravitational field is not uniform. The details of the gravitational field have been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism.

Magnetic field

The Moon has an external magnetic field of less than 0.2 nanoteslas, or less than one hundred thousandth that of Earth. The Moon does not have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating. Early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. This early dynamo field apparently expired by about one billion years ago, after the lunar core had crystallized. Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the ___location of the largest crustal magnetizations situated near the antipodes of the giant impact basins.

Atmosphere

The Moon has an atmosphere consisting of only an exosphere, which is so tenuous as to be nearly vacuum, with a total mass of less than . The surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa); it varies with the lunar day. Its sources include outgassing and sputtering, a product of the bombardment of lunar soil by solar wind ions. Elements that have been detected include sodium and potassium, produced by sputtering (also found in the atmospheres of Mercury and Io); helium-4 and neon from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood. Water vapor has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith. These gases either return into the regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field.

Studies of Moon magma samples retrieved by the Apollo missions demonstrate that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space.

A permanent Moon dust cloud exists around the Moon, generated by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising up to 100 kilometers above the surface. Dust counts made by LADEE's Lunar Dust EXperiment (LDEX) found particle counts peaked during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being denser near the boundary between the Moon's dayside and nightside.

Surface conditions

Ionizing radiation from cosmic rays, their resulting neutron radiation, and the Sun results in an average radiation level of 1.369 millisieverts per day during lunar daytime, which is about 2.6 times more than the level on the International Space Station, 510 times more than the level during a trans-Atlantic flight, and 200 times more than the level on Earth's surface. For further comparison, radiation levels average about 1.84 millisieverts per day on a flight to Mars and about 0.64 millisieverts per day on Mars itself, with some locations on Mars possibly having levels as low as 0.342 millisieverts per day.

Solar radiation also electrically charges the highly abrasive lunar dust and makes it levitate. This effect contributes to the easy spread of the sticky, lung- and gear-damaging lunar dust.

The Moon's axial tilt with respect to the ecliptic is only 1.5427°, much less than the 23.44° of Earth. This small axial tilt means that the Moon's solar illumination varies much less with season than Earth's, and it also allows for the existence of some peaks of eternal light at the Moon's north pole, at the rim of the crater Peary.

The lunar surface is exposed to drastic temperature differences ranging from to depending on the solar irradiance.

Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow, making topographical details play a decisive role on local surface temperatures.

Parts of many craters, particularly the bottoms of many polar craters, are permanently shadowed. These craters of eternal darkness have extremely low temperatures. The Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at and just close to the winter solstice in the north polar crater Hermite. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto.

Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened mostly gray surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder. The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from in the highlands and in the maria. Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick.

These extreme conditions are considered to make it unlikely for spacecraft to harbor bacterial spores at the Moon for longer than just one lunar orbit.

Surface features

The topography of the Moon has been measured with laser altimetry and stereo image analysis. Its most extensive topographic feature is the giant far-side South Pole–Aitken basin, some in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. At deep, its floor is the lowest point on the surface of the Moon, reaching at in a crater within Antoniadi crater. The highest elevations of the Moon's surface, with the so-called Selenean summit at , are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin. Other large impact basins such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale possess regionally low elevations and elevated rims. The far side of the lunar surface is on average about higher than that of the near side.

The discovery of fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. Similar shrinkage features exist on Mercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon does not have tectonic plates, its tectonic activity is slow, and cracks develop as it loses heat.

Scientists have confirmed the presence of a cave on the Moon near the Sea of Tranquillity, not far from the 1969 Apollo 11 landing site. The cave, identified as an entry point to a collapsed lava tube, is roughly 45 meters wide and up to 80 m long. This discovery marks the first confirmed entry point to a lunar cave. The analysis was based on photos taken in 2010 by NASA's Lunar Reconnaissance Orbiter. The cave's stable temperature of around could provide a hospitable environment for future astronauts, protecting them from extreme temperatures, solar radiation, and micrometeorites. However, challenges include accessibility and risks of avalanches and cave-ins. This discovery offers potential for future lunar bases or emergency shelters.

Volcanic features

The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains called maria (singular mare; Latin for "seas", as they were once believed to be filled with water) are vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. The majority of these lava deposits erupted or flowed into the depressions associated with impact basins, though the Moon's largest expanse of basalt flooding, Oceanus Procellarum, does not correspond to an obvious impact basin. Different episodes of lava flow in maria can often be recognized by variations in surface albedo and distinct flow margins.

As the maria formed, cooling and contraction of the basaltic lava created wrinkle ridges in some areas. These low, sinuous ridges can extend for hundreds of kilometers and often outline buried structures within the mare. Another result of maria formation is the creation of concentric depressions along the edges, known as arcuate rilles. These features occur as the mare basalts sink inward under their own weight, causing the edges to fracture and separate.

In addition to the visible maria, the Moon has mare deposits covered by ejecta from impacts. Called cryptomares, these hidden mares are likely older than the exposed ones. Conversely, mare lava has obscured many impact melt sheets and pools. Impact melts are formed when intense shock pressures from collisions vaporize and melt zones around the impact site. Where still exposed, impact melt can be distinguished from mare lava by its distribution, albedo, and texture.

Sinuous rilles, found in and around maria, are likely extinct lava channels or collapsed lava tubes. They typically originate from volcanic vents, meandering and sometimes branching as they progress. The largest examples, such as Schroter's Valley and Rima Hadley, are significantly longer, wider, and deeper than terrestrial lava channels, sometimes featuring bends and sharp turns that again, are uncommon on Earth.

Mare volcanism has altered impact craters in various ways, including filling them to varying degrees, and raising and fracturing their floors from uplift of mare material beneath their interiors. Examples of such craters include Taruntius and Gassendi. Some craters, such as Hyginus, are of wholly volcanic origin, forming as calderas or collapse pits. Such craters are relatively rare and tend to be smaller (typically a few kilometers wide), shallower, and more irregularly shaped than impact craters. They also lack the upturned rims characteristic of impact craters.

Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side maria. There are also some regions of pyroclastic deposits, scoria cones and non-basaltic domes made of particularly high viscosity lava.

Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side compared with 2% of the far side. This is likely due to a concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. Most of the Moon's mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some being as young as 1.2 billion years and as old as 4.2 billion years.

In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old. Moonquakes and releases of gas indicate continued lunar activity. Evidence of recent lunar volcanism has been identified at 70 irregular mare patches, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements. Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell, inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities on the far side in the Orientale basin.

The lighter-colored regions of the Moon are called terrae, or more commonly highlands, because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago and may represent plagioclase cumulates of the lunar magma ocean. In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events.

The concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after the Moon's formation. Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth.

Impact craters

A major geologic process that has affected the Moon's surface is impact cratering, with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than on the Moon's near side. Lunar craters exhibit a variety of forms, depending on their size. In order of increasing diameter, the basic types are simple craters with smooth bowl shaped interiors and upturned rims, complex craters with flat floors, terraced walls and central peaks, peak ring basins, and multi-ring basins with two or more concentric rings of peaks. The vast majority of impact craters are circular, but some, like Cantor and Janssen, have more polygonal outlines, possibly guided by underlying faults and joints. Others, such as the Messier pair, Schiller, and Daniell, are elongated. Such elongation can result from highly oblique impacts, binary asteroid impacts, fragmentation of impactors before surface strike, or closely spaced secondary impacts.

The lunar geologic timescale is based on the most prominent impact events, such as multi-ring formations like Nectaris, Imbrium, and Orientale that are between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon. The lack of an atmosphere, weather, and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface.

However care needs to be exercised with the crater counting technique due to the potential presence of secondary craters. Ejecta from impacts can create secondary craters that often appear in clusters or chains but can also occur as isolated formations at a considerable distance from the impact. These can resemble primary craters, and may even dominate small crater populations, so their unidentified presence can distort age estimates.

The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment period of increased impacts.

High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a contemporary crater-production rate significantly higher than was previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years. This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts.

Lunar swirls

Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and often have a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface magnetic fields and many are located at the antipodal point of major impacts. Well known swirls include the Reiner Gamma feature and Mare Ingenii. They are hypothesized to be areas that have been partially shielded from the solar wind, resulting in slower space weathering.

Presence of water

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon. Computer simulations suggest that up to of the surface may be in permanent shadow. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.

In years since, signatures of water have been found to exist on the lunar surface. In 1994, the bistatic radar experiment located on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters. In 1998, the neutron spectrometer on the Lunar Prospector spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions. Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior.

The 2008 Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000 ppm. Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018. In 2009, LCROSS sent a impactor into a permanently shadowed polar crater, and detected at least of water in a plume of ejected material. Another examination of the LCROSS data showed the amount of detected water to be closer to .

In May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220 was reported, the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's upper mantle. Although of considerable selenological interest, this insight does not mean that water is easily available since the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.

Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time "definitive evidence" for water-ice on the lunar surface. The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and crevices, allowing it to persist as ice on the surface since they are shielded from the sun.

In October 2020, astronomers reported detecting molecular water on the sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (SOFIA).

Earth–Moon system

Orbit

The Earth and the Moon form the Earth–Moon satellite system with a shared center of mass, or barycenter. This barycenter is (about a quarter of Earth's radius) beneath the Earth's surface.

The Moon's orbit is slightly elliptical, with an orbital eccentricity of 0.055.

The semi-major axis of the geocentric lunar orbit, called the lunar distance, is approximately 400,000 km (250,000 miles or 1.28 light-seconds), comparable to going around Earth 9.5 times.

The Moon makes a complete orbit around Earth with respect to the fixed stars, its sidereal period, about once every 27.3 days. However, because the Earth–Moon system moves at the same time in its orbit around the Sun, it takes slightly longer, 29.5 days, to return to the same lunar phase, completing a full cycle, as seen from Earth. This synodic period or synodic month is commonly known as the lunar month and is equal to the length of the solar day on the Moon.

Due to tidal locking, the Moon has a 1:1 spin–orbit resonance. This rotation–orbit ratio makes the Moon's orbital periods around Earth equal to its corresponding rotation periods. This is the reason for only one side of the Moon, its so-called near side, being visible from Earth. That said, while the movement of the Moon is in resonance, it still is not without nuances such as libration, resulting in slightly changing perspectives, making over time and ___location on Earth about 59% of the Moon's surface visible from Earth.

Unlike most satellites of other planets, the Moon's orbital plane is closer to the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbit gradually rotates once every 18.61years, which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws.

Tidal effects

The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides closest to each other, resulting in tidal forces. Ocean tides are the most widely experienced result of this, but tidal forces also considerably affect other mechanics of Earth, as well as the Moon and their system.

The lunar solid crust experiences tides of around amplitude over 27 days, with three components: a fixed one due to Earth, because they are in synchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun. The Earth-induced variable component arises from changing distance and libration, a result of the Moon's orbital eccentricity and inclination (if the Moon's orbit were perfectly circular and un-inclined, there would only be solar tides). According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining Earth's magnetic field.

The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972.

The most commonly known effect of tidal forces is elevated sea levels called ocean tides. While the Moon exerts most of the tidal forces, the Sun also exerts tidal forces and therefore contributes to the tides as much as 40% of the Moon's tidal force; producing in interplay the spring and neap tides.

The tides are two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. The tide under the Moon is explained by the Moon's gravity being stronger on the water close to it. The tide on the opposite side can be explained either by the centrifugal force as the Earth orbits the barycenter or by the water's inertia as the Moon's gravity is stronger on the solid Earth close to it and it is pull away from the farther water.

Thus, there are two high tides, and two low tides in about 24 hours. Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon's time to orbit the Earth.

If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects:

• the frictional coupling of water to Earth's rotation through the ocean floors

• the inertia of water's movement

• ocean basins that grow shallower near land

• the sloshing of water between different ocean basins

As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory.

System evolution

Delays in the tidal peaks of both ocean and solid-body tides cause torque in opposition to the Earth's rotation. This "drains" angular momentum and rotational kinetic energy from Earth's rotation, slowing the Earth's rotation. That angular momentum, lost from the Earth, is transferred to the Moon in a process known as tidal acceleration, which lifts the Moon into a higher orbit while lowering orbital speed around the Earth.

Thus the distance between Earth and Moon is increasing, and the Earth's rotation is slowing in reaction. Measurements from laser reflectors left during the Apollo missions (lunar ranging experiments) have found that the Moon's distance increases by per year (roughly the rate at which human fingernails grow).

Atomic clocks show that Earth's Day lengthens by about 17 microseconds every year, slowly increasing the rate at which UTC is adjusted by leap seconds.

This tidal drag makes the rotation of the Earth, and the orbital period of the Moon very slowly match. This matching first results in tidally locking the lighter body of the orbital system, as is already the case with the Moon. Theoretically, in 50 billion years, the Earth's rotation will have slowed to the point of matching the Moon's orbital period, causing the Earth to always present the same side to the Moon. However, the Sun will become a red giant, most likely engulfing the Earth–Moon system long before then.

If the Earth–Moon system isn't engulfed by the enlarged Sun, the drag from the solar atmosphere can cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of , it will cross Earth's Roche limit, meaning that tidal interaction with Earth would break apart the Moon, turning it into a ring system. Most of the orbiting rings will begin to decay, and the debris will impact Earth. Hence, even if the Sun does not swallow up Earth, the planet may be left moonless.

Position and appearance

The Moon's highest altitude at culmination varies by its lunar phase, or more correctly its orbital position, and time of the year, or more correctly the position of the Earth's axis. The full moon is highest in the sky during winter and lowest during summer (for each hemisphere respectively), with its altitude changing towards dark moon to the opposite.

At the North and South Poles the Moon is 24 hours above the horizon for two weeks every tropical month (about 27.3 days), comparable to the polar day of the tropical year. Zooplankton in the Arctic use moonlight when the Sun is below the horizon for months on end.

The apparent orientation of the Moon depends on its position in the sky and the hemisphere of the Earth from which it is being viewed. In the northern hemisphere it appears upside down compared to the view from the southern hemisphere. Sometimes the "horns" of a crescent moon appear to be pointing more upwards than sideways. This phenomenon is called a wet moon and occurs more frequently in the tropics.

The distance between the Moon and Earth varies from around (perigee) to (apogee), making the Moon's distance and apparent size fluctuate up to 14%. On average the Moon's angular diameter is about 0.52°, roughly the same apparent size as the Sun (see ). In addition, a purely psychological effect, known as the Moon illusion, makes the Moon appear larger when close to the horizon.

Rotation

The tidally locked synchronous rotation of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. The side of the Moon that faces Earth is called the near side, and the opposite the far side. The far side is often inaccurately called the "dark side", but it is in fact illuminated as often as the near side: once every 29.5 Earth days. During dark moon to new moon, the near side is dark.

The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by Earth. With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998–99 NASA Lunar Prospector mission found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth.

Illumination and phases

Half of the Moon's surface is always illuminated by the Sun (except during a lunar eclipse). Earth also reflects light onto the Moon, observable at times as Earthlight when it is reflected back to Earth from areas of the near side of the Moon that are not illuminated by the Sun.

Since the Moon's axial tilt with respect to the ecliptic is 1.5427°, in every draconic year (346.62 days) the Sun moves from being 1.5427° north of the lunar equator to being 1.5427° south of it and then back, just as on Earth the Sun moves from the Tropic of Cancer to the Tropic of Capricorn and back once every tropical year. The poles of the Moon are therefore in the dark for half a draconic year (or with only part of the Sun visible) and then lit for half a draconic year. The amount of sunlight falling on horizontal areas near the poles depends on the altitude angle of the Sun. But these "seasons" have little effect in more equatorial areas.

With the different positions of the Moon, different areas of it are illuminated by the Sun. This illumination of different lunar areas, as viewed from Earth, produces the different lunar phases during the synodic month. The phase is equal to the area of the visible lunar sphere that is illuminated by the Sun. This area or degree of illumination is given by (1-\cos e)/2=\sin^2(e/2), where e is the elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).

Brightness and apparent size of the Moon changes also due to its elliptic orbit around Earth. At perigee (closest), since the Moon is up to 14% closer to Earth than at apogee (most distant), it subtends a solid angle which is up to 30% larger. Consequently, given the same phase, the Moon's brightness also varies by up to 30% between apogee and perigee. A full (or new) moon at such a position is called a supermoon.

Observational phenomena

There has been historical controversy over whether observed features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly diameter region of the lunar surface was modified by a gas release event about a million years ago.

Albedo and color

The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun. This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon. Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the center, without limb darkening, because of the reflective properties of lunar soil, which retroreflects light more towards the Sun than in other directions. The Moon's color depends on the light the Moon reflects, which in turn depends on the Moon's surface and its features, having for example large darker regions. In general, the lunar surface reflects a brown-tinged gray light.

At times, the Moon can appear red or blue.

It may appear red during a lunar eclipse, because of the red spectrum of the Sun's light being refracted onto the Moon by Earth's atmosphere. Because of this red color, lunar eclipses are also sometimes called blood moons. The Moon can also seem red when it appears at low angles and through a thick atmosphere.

The Moon may appear blue depending on the presence of certain particles in the air, such as volcanic particles, in which case it can be called a blue moon.

Because the words "red moon" and "blue moon" can also be used to refer to specific full moons of the year, they do not always refer to the presence of red or blue moonlight.

Eclipses

Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon, but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye.

Because the distance between the Moon and Earth is very slowly increasing over time, the angular diameter of the Moon is decreasing. As it evolves toward becoming a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing. The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur.

As the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years.

Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky, the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.

History of exploration and human presence

Pre-telescopic observation (before 1609)

It is believed by some that the oldest cave paintings from up to 40,000 BP of bulls and geometric shapes, or 20–30,000 year old tally sticks were used to observe the phases of the Moon, keeping time using the waxing and waning of the Moon's phases.

Aspects of the Moon were identified and aggregated in lunar deities from prehistoric times and were eventually documented and put into symbols from the very first instances of writing in the 4th millennium BC. One of the earliest-discovered possible depictions of the Moon is a 3,000 BCE rock carving Orthostat 47 at Knowth, Ireland. The crescent depicting the Moon as with the lunar deity Nanna/Sin have been found from the 3rd millennium BCE.

The oldest named astronomer and poet Enheduanna, Akkadian high priestess to the lunar deity Nanna/Sin and pricess, daughter of Sargon the Great ( – BCE), had the Moon tracked in her chambers. The oldest found and identified depiction of the Moon in an astronomical relation to other astronomical features is the Nebra sky disc from , depicting features like the Pleiades next to the Moon.

The ancient Greek philosopher Anaxagoras reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. Elsewhere in the to , Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, and Indian astronomers had described the Moon's monthly elongation. The Chinese astronomer Shi Shen gave instructions for predicting solar and lunar eclipses.

In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries. Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. In the , Seleucus of Seleucia correctly thought that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun. In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance.

The Chinese of the Han dynasty believed the Moon to be energy equated to qi and their 'radiating influence' theory recognized that the light of the Moon was merely a reflection of the Sun; Jing Fang (78–37 BC) noted the sphericity of the Moon. Ptolemy (90–168 AD) greatly improved on the numbers of Aristarchus, calculating a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters, close to the correct values of about 60 and 0.273 respectively. In the 2nd century AD, Lucian wrote the novel A True Story, in which the heroes travel to the Moon and meet its inhabitants. In 510 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. The astronomer and physicist Ibn al-Haytham (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions. Shen Kuo (1031–1095) of the Song dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent. During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognized as a sphere, though many believed that it was "perfectly smooth".

Telescopic exploration (1609–1959)

In 1609, Galileo Galilei used an early telescope to make drawings of the Moon for his book , and deduced that it was not smooth but had mountains and craters. Thomas Harriot had made but not published such drawings a few months earlier.

Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834–1836 of Wilhelm Beer and Johann Heinrich von Mädler, and their associated 1837 book , the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography. Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions. This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s, leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology.

First missions to the Moon (1959–1976)

After World War II the first launch systems were developed and by the end of the 1950s they reached capabilities that allowed the Soviet Union and the United States to launch spacecraft into space. The Cold War fueled a closely followed development of launch systems by the two states, resulting in the so-called Space Race and its later phase the Moon Race, accelerating efforts and interest in exploration of the Moon.

After the first spaceflight of Sputnik 1 in 1957 during International Geophysical Year the spacecraft of the Soviet Union's Luna program were the first to accomplish a number of goals. Following three unnamed failed missions in 1958, the first human-made object Luna 1 escaped Earth's gravity and passed near the Moon in 1959. Later that year the first human-made object Luna 2 reached the Moon's surface by intentionally impacting. By the end of the year Luna 3 reached as the first human-made object the normally occluded far side of the Moon, taking the first photographs of it.

The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first vehicle to orbit the Moon was Luna 10, both in 1966.

Following President John F. Kennedy's 1961 commitment to a crewed Moon landing before the end of the decade, the United States, under NASA leadership, launched a series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the Jet Propulsion Laboratory's Ranger program, the Lunar Orbiter program and the Surveyor program. The crewed Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing, in 1968 Apollo 8 made the first human mission to lunar orbit (the first Earthlings, two tortoises, had circled the Moon three months earlier on the Soviet Union's Zond 5, followed by turtles on Zond 6).

The first time a person landed on the Moon and any extraterrestrial body was when Neil Armstrong, the commander of the American mission Apollo 11, set foot on the Moon at 02:56 UTC on July 21, 1969. Considered the culmination of the Space Race, an estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time. While at the same time another mission, the robotic sample return mission Luna 15 by the Soviet Union had been in orbit around the Moon, becoming together with Apollo 11 the first ever case of two extraterrestrial missions being conducted at the same time.

The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) removed of lunar rock and soil in 2,196 separate samples.

Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations, but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used.

Apollo 17 in 1972 remains the last crewed mission to the Moon. Explorer 49 in 1973 was the last dedicated U.S. probe to the Moon until the 1990s.

The Soviet Union continued sending robotic missions to the Moon until 1976, deploying in 1970 with Luna 17 the first remote controlled rover Lunokhod 1 on an extraterrestrial surface, and collecting and returning 0.3 kg of rock and soil samples with three Luna sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976).

Moon Treaty and explorational absence (1976–1990)

Following the last Soviet mission to the Moon of 1976, there was little further lunar exploration for fourteen years. Astronautics had shifted its focus towards the exploration of the inner (e.g. Venera program) and outer (e.g. Pioneer 10, 1972) Solar System planets, but also towards Earth orbit, developing and continuously operating, beside communication satellites, Earth observation satellites (e.g. Landsat program, 1972), space telescopes and particularly space stations (e.g. Salyut program, 1971).

Negotiation in 1979 of Moon treaty, and its subsequent ratification in 1984 was the only major activity regarding the Moon until 1990.

Renewed exploration (1990–present)

In 1990 Hiten – Hagoromo, the first dedicated lunar mission since 1976, reached the Moon. Sent by Japan, it became the first mission that was not a Soviet Union or U.S. mission to the Moon.

In 1994, the U.S. dedicated a mission to fly a spacecraft (Clementine) to the Moon again for the first time since 1973. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. In 1998, this was followed by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.

The next years saw a row of first missions to the Moon by a new group of states actively exploring the Moon.

Between 2004 and 2006 the first spacecraft by the European Space Agency (ESA) (SMART-1) reached the Moon, recording the first detailed survey of chemical elements on the lunar surface.

The Chinese Lunar Exploration Program reached the Moon for the first time with the orbiter Chang'e 1 (2007–2009), obtaining a full image map of the Moon.

India reached, orbited and impacted the Moon in 2008 for the first time with its Chandrayaan-1 and Moon Impact Probe, becoming the fifth and sixth state to do so, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil.

The U.S. launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor on June 18, 2009. LCROSS completed its mission by making a planned and widely observed impact in the crater Cabeus on October 9, 2009, whereas LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery.

China continued its lunar program in 2010 with Chang'e 2, mapping the surface at a higher resolution over an eight-month period, and in 2013 with Chang'e 3, a lunar lander along with a lunar rover named Yutu (c=玉兔|l=Jade Rabbit). This was the first lunar rover mission since Lunokhod 2 in 1973 and the first lunar soft landing since Luna 24 in 1976, making China the third country to achieve this.

In 2014 the first privately funded probe, the Manfred Memorial Moon Mission, reached the Moon.

Another Chinese rover mission, Chang'e 4, achieved the first landing on the Moon's far side in early 2019.

Also in 2019, India successfully sent its second probe, Chandrayaan-2 to the Moon.

In 2020, China carried out its first robotic sample return mission (Chang'e 5), bringing back 1,731 grams of lunar material to Earth.

The U.S. developed plans for returning to the Moon beginning in 2004, and with the signing of the U.S.-led Artemis Accords in 2020, the Artemis program aims to return the astronauts to the Moon in the 2020s. The Accords have been joined by a growing number of countries. The introduction of the Artemis Accords has fueled a renewed discussion about the international framework and cooperation of lunar activity, building on the Moon Treaty and the ESA-led Moon Village concept.

2022 South Korea lauched Danuri successfully, its first mission to the Moon, launched from the US.

2023 and 2024 India and Japan became the fourth and fifth country to soft land a spacecraft on the Moon, following the Soviet Union and United States in the 1960s, and China in the 2010s. Notably, Japan's spacecraft, the Smart Lander for Investigating Moon, survived 3 lunar nights. The IM-1 lander became the first commercially built lander to land on the Moon in 2024.

China launched the Chang'e 6 on May 3, 2024, which conducted another lunar sample return from the far side of the Moon. It also carried a Chinese rover to conduct infrared spectroscopy of lunar surface. Pakistan sent a lunar orbiter called ICUBE-Q along with Chang'e 6.

Nova-C 2, iSpace Lander and Blue Ghost were all launched to the Moon in 2024.

Future

Beside the progressing Artemis program and supporting Commercial Lunar Payload Services, leading an international and commercial crewed opening up of the Moon and sending the first woman, person of color and non-US citizen to the Moon in the 2020s, China is continuing its ambitious Chang'e program, having announced with Russia's struggling Luna-Glob program joint missions. Both the Chinese and US lunar programs have the goal to establish in the 2030s a lunar base with their international partners, though the US and its partners will first establish an orbital Lunar Gateway station in the 2020s, from which Artemis missions will land the Human Landing System to set up temporary surface camps.

While the Apollo missions were explorational in nature, the Artemis program plans to establish a more permanent presence. To this end, NASA is partnering with industry leaders to establish key elements such as modern communication infrastructure. A 4G connectivity demonstration is to be launched aboard an Intuitive Machines Nova-C lander in 2024. Another focus is on in situ resource utilization, which is a key part of the DARPA lunar programs. DARPA has requested that industry partners develop a 10–year lunar architecture plan to enable the beginning of a lunar economy.

Human presence

In 1959 the first extraterrestrial probes reached the Moon (Luna program), just a year into the space age, after the first ever orbital flight. Since then, humans have sent a range of probes and people to the Moon. The first stay of people on the Moon was conducted in 1969, in a series of crewed exploration missions (the Apollo Program), the last having taken place in 1972.

Uninterrupted presence has been the case through the remains of impactors, landings and lunar orbiters. Some landings and orbiters have maintained a small lunar infrastructure, providing continuous observation and communication at the Moon.

Increasing human activity in cislunar space as well as on the Moon's surface, particularly missions at the far side of the Moon or the lunar north and south polar regions, are in need for a lunar infrastructure. For that purpose, orbiters in orbits around the Moon or the Earth–Moon Lagrange points, have since 2006 been operated. With highly eccentric orbits providing continuous communication, as with the orbit of Queqiao and Queqiao-2 relay satellite or the planned first extraterrestrial space station, the Lunar Gateway.

Human impact

While the Moon has the lowest planetary protection target-categorization, its degradation as a pristine body and scientific place has been discussed. If there is astronomy performed from the Moon, it will need to be free from any physical and radio pollution. While the Moon has no significant atmosphere, traffic and impacts on the Moon causes clouds of dust that can spread far and possibly contaminate the original state of the Moon and its special scientific content. Scholar Alice Gorman asserts that, although the Moon is inhospitable, it is not dead, and that sustainable human activity would require treating the Moon's ecology as a co-participant.

The so-called "Tardigrade affair" of the 2019 crashed Beresheet lander and its carrying of tardigrades has been discussed as an example for lacking measures and lacking international regulation for planetary protection.

Space debris beyond Earth around the Moon has been considered as a future challenge with increasing numbers of missions to the Moon, particularly as a danger for such missions. As such lunar waste management has been raised as an issue which future lunar missions, particularly on the surface, need to tackle.

Human remains have been transported to the Moon, including by private companies such as Celestis and Elysium Space. Because the Moon has been sacred or significant to many cultures, the practice of space burials have attracted criticism from indigenous peoples leaders. For example, thenNavajo Nation president Albert Hale criticized NASA for sending the cremated ashes of scientist Eugene Shoemaker to the Moon in 1998.

Beside the remains of human activity on the Moon, there have been some intended permanent installations like the Moon Museum art piece, Apollo 11 goodwill messages, six lunar plaques, the Fallen Astronaut memorial, and other artifacts.

Longterm missions continuing to be active are some orbiters such as the 2009-launched Lunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launched Chang'e 3 with its Lunar Ultraviolet Telescope still operational.

Five retroreflectors have been installed on the Moon since the 1970s and since used for accurate measurements of the physical librations through laser ranging to the Moon.

There are several missions by different agencies and companies planned to establish a long-term human presence on the Moon, with the Lunar Gateway as the currently most advanced project as part of the Artemis program.

Astronomy from the Moon

The Moon has been used as a site for astronomical and Earth observations. The Earth appears in the Moon's sky with an apparent size of 1° 48 to 2°, three to four times the size of the Moon or Sun in Earth's sky, or about the apparent width of two little fingers at an arm's length away. Observations from the Moon started as early as 1966 with the first images of Earth from the Moon, taken by Lunar Orbiter 1. Of particular cultural significance is the 1968 photograph called Earthrise, taken by Bill Anders of Apollo 8 in 1968. In April 1972 the Apollo 16 mission set up the first dedicated telescope, the Far Ultraviolet Camera/Spectrograph, recording various astronomical photos and spectra.

The Moon is recognized as an excellent site for telescopes. It is relatively nearby; certain craters near the poles are permanently dark and cold and especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth. The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies and employed in the construction of mirrors up to 50 meters in diameter. A lunar zenith telescope can be made cheaply with an ionic liquid.

Living on the Moon

The only instances of humans living on the Moon have taken place in an Apollo Lunar Module for several days at a time (for example, during the Apollo 17 mission). One challenge to astronauts during their stay on the surface is that lunar dust sticks to their suits and is carried into their quarters. Astronauts could taste and smell the dust, which smells like gunpowder and was called the "Apollo aroma". This fine lunar dust can cause health issues.

In 2019, at least one plant seed sprouted in an experiment on the Chang'e 4 lander. It was carried from Earth along with other small life in its Lunar Micro Ecosystem.

Legal status

Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation claims ownership of any part of the Moon's surface. Likewise no private ownership of parts of the Moon, or as a whole, is considered credible.

The 1967 Outer Space Treaty defines the Moon and all outer space as the "province of all mankind". It restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. A majority of countries are parties of this treaty.

The 1979 Moon Agreement was created to elaborate, and restrict the exploitation of the Moon's resources by any single nation, leaving it to a yet unspecified international regulatory regime. As of January 2020, it has been signed and ratified by 18 nations, none of which have human spaceflight capabilities.

Since 2020, countries have joined the U.S. in their Artemis Accords, which are challenging the treaty. The U.S. has furthermore emphasized in a presidential executive order ("Encouraging International Support for the Recovery and Use of Space Resources.") that "the United States does not view outer space as a 'global commons and calls the Moon Agreement "a failed attempt at constraining free enterprise."

With Australia signing and ratifying both the Moon Treaty in 1986 as well as the Artemis Accords in 2020, there has been a discussion if they can be harmonized. In this light an Implementation Agreement for the Moon Treaty has been advocated for, as a way to compensate for the shortcomings of the Moon Treaty and to harmonize it with other laws and agreements such as the Artemis Accords, allowing it to be more widely accepted.

In the face of such increasing commercial and national interest, particularly prospecting territories, U.S. lawmakers have introduced in late 2020 specific regulation for the conservation of historic landing sites and interest groups have argued for making such sites World Heritage Sites and zones of scientific value protected zones, all of which add to the legal availability and territorialization of the Moon.

In 2021, the Declaration of the Rights of the Moon was created by a group of "lawyers, space archaeologists and concerned citizens", drawing on precedents in the Rights of Nature movement and the concept of legal personality for non-human entities in space.

Coordination and regulation

Increasing human activity at the Moon has raised the need for coordination to safeguard international and commercial lunar activity. Issues from cooperation to mere coordination, through for example the development of a shared Lunar time, have been raised.

In particular the establishment of an international or United Nations regulatory regime for lunar human activity has been called for by the Moon Treaty and suggested through an Implementation Agreement, but remains contentious. Current lunar programs are multilateral, with the US-led Artemis program and the China-led International Lunar Research Station. For broader international cooperation and coordination, the International Lunar Exploration Working Group (ILEWG), the Moon Village Association (MVA) and more generally the International Space Exploration Coordination Group (ISECG) has been established.

In culture and life

Timekeeping

Since pre-historic times people have taken note of the Moon's phases and its waxing and waning cycle and used it to keep record of time. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. The counting of the days between the Moon's phases eventually gave rise to generalized time periods of lunar cycles as months, and possibly of its phases as weeks.

The words for the month in a range of different languages carry this relation between the period of the month and the Moon etymologically. The English month as well as moon, and its cognates in other Indo-European languages (e.g. the Latin and Ancient Greek (meis) or (mēn), meaning "month") stem from the Proto-Indo-European (PIE) root of moon, *méh1nōt, derived from the PIE verbal root *meh1-, "to measure", "indicating a functional conception of the Moon, i.e. marker of the month" (cf. the English words measure and menstrual). To give another example from a different language family, the Chinese language uses the same word for moon as for month, which furthermore can be found in the symbols for the word week.

This lunar timekeeping gave rise to the historically dominant, but varied, lunisolar calendars. The 7th-century Islamic calendar is an example of a purely lunar calendar, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon.

Of particular significance has been the occasion of full moon, highlighted and celebrated in a range of calendars and cultures, an example being the Buddhist Vesak. The full moon around the southern or northern autumnal equinox is often called the harvest moon and is celebrated with festivities such as the Harvest Moon Festival of the Chinese lunar calendar, its second most important celebration after the Chinese lunisolar Lunar New Year.

Furthermore, association of time with the Moon can also be found in religion, such as the ancient Egyptian temporal and lunar deity Khonsu.

Cultural representation

Humans have not only observed the Moon since prehistoric times, but have also developed intricate perceptions of the Moon. Over time the Moon has been characterized and associated in many different ways, from having a spirit or being a deity, and an aspect thereof or an aspect in astrology, being made an important part of many cosmologies.

This rich history of humans viewing the Moon has been evidenced starting with depictions from 40,000 BP and in written form from the 4th millennium BCE in the earliest cases of writing. The oldest named astronomer and poet Enheduanna, Akkadian high priestess to the lunar deity Nanna/Sin and pricess, daughter of Sargon the Great ( – BCE), tracked the Moon and wrote poems about her divine Moon.

Crescent

For the representation of the Moon, especially its lunar phases, the crescent (����) has been a recurring symbol in a range of cultures since at least 3,000 BCE or possibly earlier with bull horns dating to the earliest cave paintings at 40,000 BP. In writing systems such as Chinese the crescent has developed into the symbol , the word for Moon, and in ancient Egyptian it was the symbol ����, meaning Moon and spelled like the ancient Egyptian lunar deity Iah, which the other ancient Egyptian lunar deities Khonsu and Thoth were associated with.

Iconographically the crescent was used in Mesopotamia as the primary symbol of Nanna/Sîn, the ancient Sumerian lunar deity, who was the father of Inanna/Ishtar, the goddess of the planet Venus (symbolized as the eight pointed Star of Ishtar), and Utu/Shamash, the god of the Sun (symbolized as a disc, optionally with eight rays), all three often depicted next to each other. Nanna/Sîn is, like some other lunar deities, for example Iah and Khonsu of ancient Egypt, Mene/Selene of ancient Greece and Luna of ancient Rome, depicted as a horned deity, featuring crescent shaped headgears or crowns.

The particular arrangement of the crescent with a star known as the star and crescent (☪️) goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and the planet Venus, in combination. It came to represent the selene goddess Artemis, and via the patronage of Hecate, which as triple deity under the epithet trimorphos/trivia included aspects of Artemis/Diana, came to be used as a symbol of Byzantium, with Virgin Mary (Queen of Heaven) later taking her place, becoming depicted in Marian veneration on a crescent and adorned with stars. Since then the heraldric use of the star and crescent proliferated, Byzantium's symbolism possibly influencing the development of the Ottoman flag, specifically the combination of the Turkish crescent with a star, and becoming a popular symbol for Islam (as the hilal of the Islamic calendar) and for a range of nations.

Other association

The features of the Moon, the contrasting brighter highlands and darker maria, have been seen by different cultures forming abstract shapes. Such shapes are among others the Man in the Moon (e.g. Coyolxāuhqui) or the Moon Rabbit (e.g. the Chinese Tu'er Ye or in Indigenous American mythologies the aspect of the Mayan Moon goddess, from which possibly Awilix is derived, or of Metztli/Tēcciztēcatl).

Occasionally some lunar deities have been also depicted driving a chariot across the sky, such as the Hindu Chandra/Soma, the Greek Artemis, which is associated with Selene, or Luna, Selene's ancient Roman equivalent.

Color and material wise the Moon has been associated in Western alchemy with silver, while gold is associated with the Sun.

Through a miracle, the so-called splitting of the Moon in Islam, association with the Moon applies also to Muhammad.

Representation in modern culture

The perception of the Moon in the modern era has been informed by telescope-enabled modern astronomy and later by spaceflight which enabled actual human activity at the Moon, particularly the culturally impactful lunar landings. These new insights inspired cultural references, connecting romantic reflections about the Moon and speculative fiction such as science-fiction dealing with the Moon.

Contemporarily the Moon has been seen as a place for economic expansion into space, with missions prospecting for lunar resources. This has been accompanied with renewed public and critical reflection on humanity's cultural and legal relation to the celestial body, especially regarding colonialism, as in the 1970 poem "Whitey on the Moon". In this light the Moon's nature has been invoked, particularly for lunar conservation and as a common.

In 2021 20 July, the date of the first crewed Moon landing, became the annual International Moon Day.

Lunar effect

The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans. The Moon has long been associated with insanity and irrationality; the words lunacy and lunatic are derived from the Latin name for the Moon, Luna. Philosophers Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person. Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims.