名稱和語源

中文的「月」為象形文字，在甲骨文中月像一彎眉月的樣子。東漢許慎在《說文解字》一書中分析月的字型時說：月，闕也。人們經過觀察，發現月圓的時間少，闕（弦月或眉月等）的時間多，於是就照眉月的樣子創造出這個象形字。古稱太陰、玄兔、嬋娟、望舒等。

在英語中月的專有名稱是「」。該名詞源於原始日耳曼語的「mǣnōn」，在725年之前的古英語被稱為""，1135年為「」，大約在1380年變為「」，之後再變成現在的寫法。月球在現代英語的主要形容詞是「」，源自拉丁文的「」。另一個比較不常用的形容詞是「」，則源自古希臘文的「」（），是衍生自字首「」（像是「」）。古希臘塞勒涅（）和古羅馬的狄安娜（）或稱辛西婭（）的女神都是月球的名字。辛西婭和塞勒涅是反映月球處于不同軌道期如遠月點、近月點的專門術名，狄安娜一名連接死亡，意指白天。

以天體位置來看月球也能稱呼為地衛一（，），但以天體位置來稱呼在天文學的術語使用上較為罕見。

形成

有數種機制都認為月球形成於年之前，即大約是太陽系誕生之後的3000萬至5000萬年。這些機制包括分裂說、捕獲說和地月同源說（孿生說）等。分裂說認為月球是由于離心力從地殼分裂出去，但要產生如此大的離心力，需要地球在誕生初始時有超高速的自轉。捕獲說則認為月球是在成型時被地球引力場捕獲的天體，但這種假說需要地球擁有一個有非常大的大氣層來消耗月球通過時的能量，減緩月球運動速度。同源說認為地球和月球形成于同一原生吸積盤，但這種假說無法解釋月球上金屬鐵的匱乏，也不能解釋地月系統的高角動量。

現今主流的地月系統形成理論是大碰撞說：一顆火星大小的天體（被稱為特亞，神話故事中月球女神塞勒涅的母親）與原生地球碰撞，爆裂出的物質進入環繞地球的軌道，經由吸積形成月球。

該假說雖然不是很完美，但也許是最好的解釋。在1984年10月有關月球起源會議召開前的18個月，比爾·哈特曼（Bill Hartmann）、羅傑·菲利普斯（Roger Phillips）和傑夫·泰勒（Jeff Taylor）挑戰月球科學家同事們：「你們有十八個月的時間，下定決心，回到阿波羅數據，回到電腦中，做所有你們必須做的事。不要來參加我們的會議，除非你們有了有關月球誕生的話要說。」1984年夏威夷科納的會議上，大碰撞假說成為最受歡迎的理論。在會議之前，有三種「傳統」理論學派，加上少數開始認真思考大撞擊理論的人，以及為數眾多，認為辯論永遠不能解決問題的中間派。會後，學術界實質上只分為兩派：大碰撞陣營和不可知論者。

大碰撞說認為：在太陽系誕生的早期，巨大的撞擊是很常見的。電腦模擬的大碰撞模型表明，這樣的撞擊後產生的雙星系統具有充分的角動量匹配目前地月系統的軌道參數，而且也可以解釋月球具有相對較小核心的原因。此外，大碰撞說還可以合理解釋地月成分的不同：月球的大部分組成成分都來自撞擊前的天體，而並不是原生的地球。但是這個假說仍然不是很完善，例如對隕石的研究卻顯示內太陽系的其他天體，如火星、灶神星等，其氧和鎢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。一個合理的解釋是導致地月系形成的撞擊混合了地球和月球形成時揮發的物質，有可能導致兩個天體之間同位素的組成變得均衡，但這種解釋仍有爭議。

大碰撞中所釋放的大量能量和之後在地球軌道上再作用的物質會熔化地球的外殼，形成岩漿海。新形成的月球也會產生自己的月球岩漿海，估計它的深度範圍為500公里（310英里）至1737公里(1079英里)，最深相當于月球自身的半徑。

儘管它準確地解釋許多証據，但大撞擊假說很難完全解釋一切，其中大部分涉及月球的組成成分。

另外一種假說則認為大碰撞產生了兩顆在同一軌道上的衛星，一個就是月球，而另外一個較小，直徑只有約1000公里。在數千萬年後，兩個衛星緩慢相撞，最後合二為一。這種假說解釋了月球一面地勢平坦，另一面則地勢起伏不平的原因。

2001年，華盛頓卡耐基研究所的一個研究團隊報告了對月球岩石同位素最精確的測量值，研究小組驚訝地發現，阿波羅計劃所帶回岩石的同位素特徵，與地球岩石相同，而不同于太陽系幾乎所有的天體。這完全出乎于以前認為的進入軌道形成月球的大部分物質都來自于忒伊亞的想法。2007年加州理工大學研究人員宣布，忒伊亞同位素特徵與地球相同的概率低于1%。2012年發表的阿波羅月球樣品的鈦同位素分析同樣表明，月球和地球的組成成分相同，這完全有悖于大碰撞假說預期的月亮形成于遠離地球的軌道或來自忒伊亞。

物理特性

月球是地球的同步自轉衛星，它繞軸自轉的週期與繞地球的公轉周期是相同的，這使得它幾乎永遠以同一面朝向地球。它之前以較快的速度旋轉，在後來由于地球產生潮汐摩擦，讓其自轉速度減慢，直到最後以同一面持續面對地球，即潮汐鎖定。我們將月球朝向地球的一面被稱為正面，而相對的另一面則稱為背面，背面通常也稱為"暗面"，但是事實上它如同正面一樣會被照亮。當月相為新月時，我們看到月球的正面是黑暗的，而月球的背面則被太陽照亮。

月球是一個南北極稍扁、赤道稍許隆起的扁球。它的平均極半徑比赤道半徑短500米。南北極區也不對稱，北極區隆起，南極區窪陷約400米。但在一般計算中仍可把月球當作三軸橢圓體看待。物理天平動的研究有助於解決月球形狀問題。通過天平動研究還表明，月球重心和幾何中心並不重合，重心偏向地球2公里。這一結論已為阿波羅登月獲得的資料所證實。

表面地形

科學家曾經使用雷射測高儀和立體影像分析對月球表面的地形進行測量。月球表面最明顯的地形特徵是位于背面的巨大撞擊坑南極-艾托肯盆地，其直徑有2,240公里，是月球上最大的隕石坑，也是太陽系中已知最大的。它的底部是月球上海拔最低的地方，深度達到13公里。而月球海拔最高的地點則正好就在它的東南方，有人認為這個區域是造成南極-艾托肯盆地的撞擊所形成的隆起。月球上的其它大撞擊盆地，如雨海、澄海、危海、史密斯海和東方海等，也都擁有低海拔的區域和高聳的邊緣。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見有黑暗的，相對平坦的平原，我們稱之為月海，這是因為古代的天文學家認為這些地方充滿了水。現在，我們知道這些黑暗部分是古代火山爆發後熔岩漿在窪地凝結成的廣大玄武岩。和地球的玄武岩類似，月海中的玄武岩含有豐富的鐵，而完全缺乏因水流過而出現的礦物。大多數噴發的熔岩漿流入與撞擊盆地相連接的窪地，形成月海。現在科學家已經在月球正面的月海中發現幾個擁有盾狀火山和火山穹頂的地質分區，這些是熔岩漿凝結形成月海的証據。

幾乎所有的月海都位于月球正面，占正面面積的31%，相較之下，在月球背面只有少數的月海，只涵蓋了背面2%的面積。這被認為和通過月球探勘者的伽瑪射線光譜儀所描繪的月球化學圖上所看見在月球正面地殼下的生熱元素的濃縮有關。生熱元素的濃縮會造成地函下的溫度上升，部分熔解，並上升到表面造成噴發。大部分玄武岩的噴發都出現在30至35億年前的雨海紀，但也有少部分樣本的輻射定年顯示其形成于更古老的42億年，也有一些相對年輕的樣品，最年輕的噴發物經由撞擊坑計數測定年限發現其發生在12億年前。

月球上較亮的部分被稱為「高地」，因為它們高於大多數的月海。經由輻射定年測定它們是於44億年前形成的，這意味著這些高地可能是在月球岩漿海形成時的斜長岩堆積所產生的。月球上沒有任何一個主要的山脈被認為由地質構造事件產生的，這和地球的情況剛好相反。

撞擊坑

平均每天月球都會承受超過3噸宇宙物質的撞擊，當中的小行星或彗星撞擊月球表面時會形成撞擊坑，是另一個主要會影響月球表面地形的主要地質事件。現在估計單在月球正面直徑大於1公里的隕石坑就大約有300,000個，其中有些隕石坑以知名的學者、科學家、藝術家和探險家的名字命名。月球地質年代是根據月面上的重大隕石撞擊事件進行分界，包括在酒海、雨海和東方海等的撞擊事件。這些撞擊事件的結構特徵是產生多層物質隆起的環，通常是由數百至數千公里直徑的圍裙狀噴發物沉積形成一個區域性的地層視界。由于月球沒有大氣層、天氣變化，在最近幾十億年也沒有地質活動，大部分環形山都保存得很完好。雖然有幾個多環盆地明顯的已經很久遠，它們還是能用於分派相對的年齡。由於撞擊坑是以恆定的速率累積，計算單位面積內的撞擊坑數目可以用來估計表面的年齡。阿波羅任務收集撞擊熔化的岩石以輻射測定年齡，群集在38億和41億年的年齡：這已被用來解釋撞擊的後期重轟炸期。

覆蓋在月球地殼上的是高度粉碎的（碎裂成更小的顆粒）和撞擊園藝下的表面層稱為風化層，是由撞擊過程形成的。最細微的風化層，是二氧化矽的月球土壤玻璃狀物體，有著像雪一樣的紋理和聞起來像用過的火藥。較老的風化層表面一般比年輕的表面厚；在高地的厚度在10-20米之間，在海的厚度則是3-5米。

在細緻的粉碎風化層下面是「粗風化層（megaregolith）」，厚達數公里高度碎裂的基岩。

表面地質

月球表面的化學元素分布極不均。按照其豐度依次為：氧、硅、鐵、鎂、鈣、鋁、錳、鈦。氧的含量估計為42%（按重量）。碳和氮只有痕跡，似乎只存在于太陽風帶來的微量沉積中，氫主要集中在月球的兩極。

月岩

月球表面主要有四類岩石：月海玄武岩、高地岩石（主要包括斜長岩與富鎂的結晶岩套）、克里普岩和角礫岩，且分布不均。

月壤

月壤是月球表面的細微表岩屑，主要是玄武岩和斜長岩物理崩解的結果，由多年來持續的流星撞擊和太陽及星際帶電粒子轟擊所造成。

內部構造

月球是一個已經分異的天體，即它擁有地殼、地函、和核心。月球的內核富含固態鐵，半徑大約為240公里，此外還有一個流體的外核，主要成分是液態鐵，半徑大約為300公里。核心周圍是部分熔融的邊界層，約有500公里的寬度邊界層結構是在45億年前月球形成不久之後，由月球岩漿海通過分離結晶形成的。岩漿海的結晶可以經由沉澱形成由鎂鐵質和沉積的橄欖石、斜輝石和斜方輝石等礦物組成的地函。四分之三的岩漿海結晶之後，可能形成密度較低的斜長石並浮在地殼的頂部。最後才由液體結晶的部分會被夾在地殼和地函之間，並且含有大量不相容和發熱的元素和之相符的是從月球軌道上遙感繪製的月球地質化學圖也顯示其地殼幾乎都是由斜長岩組成。通過對部分熔融的地函噴發出的熔岩流冷凝下來的月岩樣本的研究，科學家確認地函含有比地球更豐富的鐵，其主要成分是鎂鐵質。通過地球物理技術發現月球地殼的平均厚度約為50公里左右。

月球是太陽系內密度第二高的衛星，僅次于木衛一埃歐。但是月球的內核並不大，半徑大約是350公里甚至更小，只佔月球大小的約20%，相較之下，其它地球型天體的比例約為50%。它的組成尚不是完全清楚，可能是由金屬鐵組成，同時含有少量硫和鎳。對月球隨著時間變化轉動的分析顯示月球核心至少仍有部分是熔融的。

重力和磁場

月球表面的引力約為地球的六分之一。月球的重力場已經通過圍繞月球旋轉的探測器發射無線電信號的zh-hans:多普勒效应;zh-hk:多普勒效應;zh-tw:都卜勒效應;所測量的。月球重力場主要的特徵是擁有質量瘤，即在一些巨大的撞擊盆地卻反而出現較重的重力分布，這可能與組成這些盆地的玄武岩熔岩流密度較大有關係，這些異常對環繞月球軌道的太空船有極大的影響，如果經月球這些地域時，假如太空船與月面距離足夠低，而且軌道不加修正的話，那麼太空船會在數個月或數年間在月球表面墜毀。但令人困惑的是，熔岩流密度本身不足以完全解釋重力異常，有一些質量瘤的存在明顯和月海中的火山作用形成的熔岩流無關。

月球擁有一個外在磁場，其強度不到地球磁場百分之一，範圍在1至數百特士拉之間。已發現月球上有類似質量瘤的異常的磁場區。這些磁場區有明顯不同于其他地方的磁場強度。天體液體金屬核心可以生成的全球性雙極性磁場，但現在月球的磁場並不是由液體金屬核心產生的，而可能是在月球演化的歷史早期被磁化而一直保留至今的地殼磁場，月球磁場另一種可能來源是在大碰撞事件期間生成的瞬態磁場殘餘的磁化，通過撞擊產生的電漿雲包圍，擴大了磁場的範圍，這種說法受到最大的地殼磁場撞擊盆地對面出現對蹠點的支持。

大氣層

月球有一個非常稀薄、接近真空的大氣層，總質量低於10公噸。如此小的大氣質量在月球表面產生的壓力大約是3atm（0.3nPa），數值隨著月球一天的時間不同而改變。月球大氣的來源包括出氣和濺射，如太陽風的離子轟極月球表面釋放出的原子。過往曾經檢測到由濺射產生的原子包括鈉和鉀，相同的情況也曾在水星和木衛一埃歐的大氣中發現過。月球大氣的氦-4來自太陽風，氬-40、氡-222和釙-210則來自月球地函相關元素放射性衰變後的濺射。但月球大氣中缺乏存在於月球表岩屑的氧、氮、碳、氫和鎂等自然元素的原子或分子，目前原因尚不清楚。月船1號已經在月球大氣中發現水蒸氣的存在，其含量隨著月球緯度的不同而改變，大約在緯度為60-70度時水蒸氣的含量最高。這些水蒸氣可能是由月球表面表岩屑的水冰升華而生成的。月球大氣層的氣體有些被月球的重力吸引回到表岩屑，有些由于太陽的輻射壓，或者被太陽風的電離後逃逸到太空中。

季節

月球的轉軸傾角只有1.54°，遠小於地球的23.44°。由於這個緣故，太陽照射對月球季節變化的影響很小，反而是月球表面地形對季節變化有重要作用。在2004年，約翰·霍普金斯大學的Ben Bussey博士率領的小組研究克萊芒蒂娜探測器在1994年獲得的影像，發現位于月球北極的皮爾斯環形山邊緣有4個區域在整個月球日中都被陽光所照亮，形成永晝峰，而在月球南極地區沒有類似的區域。而在極區的許多環形山底部是永久黑暗的，沒有受到陽光照射。這些黑暗的環形山底部是極低溫的：月球勘測軌道飛行器在夏天的南極環形山底部測得的最低溫度是35K（−238 °C），而在接近冬至時在北極測得埃爾米特環形山的溫度只有26K（−247 °C）。這個溫度比冥王星的表面溫度還要低，是太空船在太陽系中所測得的最低溫度。

水的存在

月球的表面不存在液態水，因為太陽輻射會使水被光解並快速逸入太空。但從1960年代以來，科學家假設由彗星撞擊所帶來的水、或者來自太陽風的氫和含氧豐富的月岩反應所產生的水，都可能以冰的型態沉積下來，並在月球兩極撞擊坑低溫的永久陰影區留下可以追蹤得到的痕跡。電腦模擬月面的永久陰影區約有14,000平方公里。在月球上可用水的數量是一個重要的因素，可以決定建設一個月球適居區計畫的成本效益，因為從地球運水到月球的費用極為昂貴。

近年來，已經在月球表面發現水的特徵。在1994年，安裝在克萊門汀號太空船的雙向雷達實驗，顯示有少量、冰凍的水存在接近表面的凹穴內。但是，後續使用阿雷西博天文台的雷達觀測，又認為此一發現可能是由新撞擊坑中的岩石近被撞擊的岩石噴出的。在1998年，月球勘探者攜帶的中子能譜計顯示，在極地附近深度1米的風化層存在著高濃度的氫。在2008年，對一顆由阿波羅15號帶回的熔岩珠的分析，顯示有微量的水存在於球狀硅酸鹽玻璃內。

在2008年，印度的月船1號太空船使用在載月球礦物繪圖儀確認表面有水冰的存在。分光計觀測在反射的陽光中偵測到羥基的通用吸收譜線，提供了有大量水冰在月球表面的證據。太空船顯示濃度可能高達1000PPM。在2009年，月球坑觀測和傳感衛星送了一個2,300公斤的撞擊器到極區永久陰暗的環形山，並且從噴出的羽狀物質中至少檢測到100公斤的水。LCROSS另一個實驗的數據顯示偵測到的水量，更靠近155公斤（± 12公斤）。

與地球的關係

軌道

月球相對於固定的恆星以27.32天的週期完整地繞行軌道一周。更正確的說，月球的平恆星週期是27.321661天，和平回歸週期（從分點至分點）是27.321582天（「天文曆書的補充解釋」, 1961, at p.107）（它的恆星週期）。然而，因為地球間同時間也繞著太陽轉，它對地球呈現相同相位的時間就會較長，大約是29.53天（它的會合週期）。與其他行星大多數的衛星不同，月球的軌道比較接近黃道平面，而不是地球的赤道平面。月球的軌道受到太陽和地球而有許多小、複雜並且相互影響而難解的攝動，例如月球軌道平面的漸進轉動，這影響到月球其它的運動狀態。卡西尼定律以數學敘述出後續的影響。

其中主要的軌道變化有：偏心率變化、軌道傾角變化、拱線運動、交點西退、中心差。

偏心率變化

月球軌道偏心率變化在1/15到1/23的範圍內，偏心率的平均值為0.0549，接近1/18。

嚴格來說，地球與月球圍繞共同質心運轉，共同質心距地心4,671公里（即地球半徑的2/3處）。由於共同質心在地球表面以下，地球圍繞共同質心的運動好像是在「晃動」一般。從地球北極上空觀看，地球和月球均以逆時針方向自轉；而且月球也是以逆時針繞地運行；甚至地球也是以逆時針繞日公轉的。

很多人不明白為什麼月球軌道傾角和月球自轉軸傾角的數值會有這麼大的變化。其實，軌道傾角是相對於中心天體（即地球）而言的，而自轉軸傾角則相對於衛星（即月球）本身的軌道面。這個定義習慣很適合一般情況（例如人造衛星的軌道）而且數值是相當固定的，但月球卻非如此。

拱線運動

月球圍繞地球的橢圓軌道，在它自己的平面上也不是固定的，其橢圓的拱線（近地點和遠地點的連線）沿月球公轉方向向前移動，每8.85年移動一周。中國早在東漢，賈逵就提出月球視運動的最疾點每九年運動一周，這實際上正是拱線運動的結果。

軌道傾角變化

月球軌道（白道）對地球軌道（黃道）的交角（黃白交角）變化在4°57～5°19之間，平均值為5°09。

月球的軌道平面（白道面）與黃道面（地球的公轉軌道平面）保持著5.145 396°的夾角，而月球自轉軸則與黃道面的法線成1.5424°的夾角。因為地球並非完美球形，而是在赤道較為隆起，因此白道面在不斷進動（即與黃道的交點在順時針轉動），每6793.5天（18.5966年）完成一周。期間，白道面相對於地球赤道面（地球赤道面以23.45°傾斜於黃道面）的夾角會由28.60°（即23.45°+ 5.15°）至18.30°（即23.45°- 5.15°）之間變化。同樣地，月球自轉軸與白道面的夾角亦會介乎6.69°（即5.15° + 1.54°）及3.60°（即5.15° - 1.54°）。月球軌道這些變化又會反過來影響地球自轉軸的傾角，使它出現±0.002 56°的擺動，稱為章動。

交點西退

白道與黃道的交線，其空間位置並不固定，而是不斷地向西運動，每18.6年運行一周。這一現象早在東漢末年就為劉洪發現，並用於月食預報計算中。

中心差

由於月球軌道是橢圓而不是圓形，月球公轉速度並不均勻。月球運動同均勻的圓周運動比較，時而超前，時而落後，其半振幅為6°.29，週期為27.55455日。

幾何天秤動

由於月球軌道為橢圓形，當月球處於近地點時，它的自轉速度便追不上公轉速度，因此我們可見月面東部達東經98度的地區，相反，當月處於遠地點時，自轉速度比公轉速度快，因此我們可見月面西部達西經98度的地區。這種現象稱為經天秤動。又由於月球的自轉軸傾斜於公轉軌道平面（白道面），而白道與黃道又有約5度的交角，因此月球繞地球公轉一周時，極區會作約7度的晃動，這種現象稱為緯天秤動。再者，由於月球距離地球只有60地球半徑之遙，若觀測者從月出觀測至月落，觀測點便有了一個地球直徑的位移，可多見月面經度1度的地區。這種現象稱為周日天秤動。

如同絕大多數天體運行，月球繞地球的長期軌道痕跡是一個甜甜圈，月球軌道遠離的現象會到目前軌道的大約1.4倍為止，然後再慢慢繞回來。

相對大小

月球相對於地球的大小是最大的：直徑略大于地球的四分之一，質量約為1/81。就衛星與行星的相對大小比例來說，它是太陽系最大的衛星（雖然冥衛一凱倫與矮行星冥王星相對來說更大）。

然而，地球和月球仍然被認為是一種行星-衛星系統，而不是雙行星系統，因為它們的質心，一般所謂的質量中心，位於地球表面之下約1,700公里處。

潮汐效應

地球上的潮汐主要是來自月球牽引地球兩側引力強度的漸進變化的潮汐力造成的。這在地球上造成兩處隆起，最明顯的是海潮和海平面的升高。由於地球自轉的速度大約是月球環繞地球速度的27倍，因此這個隆起在地球表面上被拖曳的速度比月球的移動還快，大約一天繞著地球的轉軸旋轉一圈。海潮會受到一些影響而增強：水經過海底時的摩擦力與地球自轉的耦合，水移動時的慣性，接近陸地的平坦海灘，和不同海洋盆地之間的振盪。太陽的引力對地球海潮的影響大約是月球的一半，它們相互的引力影響造成了大潮和小潮。

月球和靠近月球一側隆起的重力耦合對地球的自轉產生了一個扭矩，從地球的自轉中消耗了角動量和轉動的動能。反過來，角動量被添加到月球軌道，使月球加速，使得月球升到更高的軌道和有更長的軌道週期。結果是，月球和地球的距離增加，和地球的自轉減緩。通過阿波邏任務安裝在月球表面上的月球測距儀，測量月球到地球的距離，發現地月距離每年增加38毫米（雖然每年只是月球軌道半徑的0.1 ppb）。原子鐘也顯示地球的自轉的一天，每年約減緩15微秒，在UTC的緩慢增加被閏秒加以調整。潮汐拖曳會繼續進行，直到地球的自轉速度減緩到與月球的軌道週期吻合；然而，在這之前，太陽已經成為紅巨星，吞噬掉地球。

月球表面也能體驗到周期約27天，振幅約10公分的潮汐，它有兩種成分：因為它的同步自轉，來自地球的是固定的；和來自太陽的變動。來自地球噵致的量是天秤動，這是月球軌道離心率造成的結果；如果月球軌道是理想的圓，就只會有太陽造成的潮汐。天秤動會改變從地球看見的角度變化，使得從地球可以看見59%的月球表面（但在任何時間看見的都略少於一半）。這些潮汐力累積的應力會造成月震。雖然每次震動可以持續至一小時以上－明顯的比地震的時間長－因為缺乏水來阻尼震動的振幅，但月震不如地震的頻繁，也比地震微弱。月震的存在是1969年到1972年的阿波羅太空人安放在月球上的地震儀的一個意外發現。

從地球看月球

月相

在滿月期間，月球的視亮度約有-12.6等（作為參考，太陽的視亮度為-26.8等。），在夜間最容易察覺得到，但它有時也可在日間看見。（例如上弦月可於下午看見，下弦月可於早上看見。）

地衛一大約每天推遲50分鐘從東方升起。但正史中也有一些奇怪的記載，《金史·天文志》記載：「太宗天會十一年（1133年），五月乙丑（6月15日），月忽失行而南，頃之複故。」月球有著異常低的反照率（與煤炭相當）。儘管如此，它仍是天空中繼太陽之後第二亮的天體。這一部分是因為對沖效應的增強效果；在弦月時，月球只有十分之一的亮度，而不是滿月一半的亮度。此外，由於視覺系統的顏色恆常性重新校準天體的顏色和周圍環境的關係，因為周圍的天空比較黑暗，會覺得被太陽照射的月球是比較明亮的天體。滿月的邊緣感覺上會比中心明亮，並沒有周邊昏暗的效應，這是月球土壤的反射特性，它反射向太陽方向的光多於其它的方向。月亮出現在靠近地平線時會顯得比較大，但這純粹是一種心理上的影響，也就是所謂的月球錯覺，最早的敘述出現在西元前7世紀。

月球在天空中最高的高度變化：雖然它有與太陽相同的限制，在一年當中它會隨著季節與月相變化，滿月在冬天到達最高的位置。18.6年的交點週期也有些影響：當月球的升交點在春分點，月球每個月的緯度可以到達28°。這意味著月球會出現在赤道到緯度28°之間的天頂，反過來 (降交點在春分點)則只有18°。月球的新月方向也取決於觀測者的緯度：接近赤道的觀測者，可以看見微笑狀的新月。

月球的表面是否會隨著時間改變，在歷史上仍有爭議。今天，許多這些主張被認為是虛幻的，是在不同光線條件下觀察的結果，不良的視寧度，或不當的繪圖。但是，偶爾會出現出氣現象，還有小部份的報告可以歸因於瞬變月面現象。最近，有人認為月球上一個3公里直徑的區域在一百萬年前被釋放出的氣體改變。月球的外觀，像太陽一樣，也會受到地球大氣層的影響：常見的是當月光通過高空的卷層雲時，會受到冰晶的折射形成22°的暈環，通過薄雲也會有相似的冕環。

食

當地球、太陽和月球在一條直線上時，便會出現蝕。日食發生在朔（有別於新月），當月球介於地球和太陽中間。對照過來，月食發生在滿月，當地球介於太陽和月球中間。從地球看月球的角視直徑和太陽的角視直徑變化的範圍是重疊的，因此日食時會有日全食和日環食的可能性。在日全食，月球會將太陽的盤面完全遮蔽掉，因此以肉眼就能看見日冕。由於地球和月球的距離緩慢的在逐漸增加中，月球的角視直徑逐漸減小。這意味著在數百萬年前的日食，月球都會完全遮蔽掉太陽，而沒有發生日環食的可能。同樣的，從現在開始大約6億年之後，月球將不再能夠完全遮蔽掉太陽，因此將只會發生日環食。

由於月球環繞地球的軌道相對於地球環繞太陽的軌道有大約5°的傾斜，所以不是每個新月和滿月都會發生食。當食發生時，月球必須在兩個軌道平面交集的附近。日食和月食復發的週期性，由沙羅週期來描述，其周期大約是18年。

由於月球在天空中總是會遮蔽大約半度直徑圓型區域的視野，當一顆亮星或行星經過月球的後方時，就會發生掩星的現象：從視線中隱藏。這樣一來，日食只是太陽被掩蔽。由於月球非常接近地球，單獨一顆恆星被掩蔽的現象不是在地球上的任何地點都能見到，也無法同時見到。並且因月月球軌道的進動，每年會被掩蔽的恆星也都有所不同。

研究和探測

早期的研究

在天文學發展的早期天文學家已經對月球週期有深刻的理解：如大約在，巴比倫天文學家已經知道月食有大約18年的沙羅週期，印度天文學家已經對月球每個月的距角進行描述，中國天文學家石申確定了一套預測日食月食的公式。之後，月球的天然形狀和月光的成因也被了解，古希臘哲學家阿那克薩哥拉推斷太陽和月球都是巨大的岩石球體，而且後者通過反射前者的光來發光。雖然中國漢朝時認為月球等同於「氣」，他們的「輻射影響」理論解釋月球光只是反射自太陽，京房（前77年—前37年）注意到月球是球體。西元499年，印度天文學家阿耶波多（Aryabhata）在他的《Aryabhatiya》記錄月球的耀眼光芒是反射陽光的緣故。天文學家兼物理學家海什木發現月球不像鏡子那樣反射陽光，而是從月球表面每一個方向往所有方向發射出去。中國宋朝的沈括創造一個塗上白色粉末的銀球反射陽光，來解釋月相的變化，而從側面看時就能呈現眉月的月相。

亞里士多德的宇宙的描述（On the Heavens），月亮標示出可變元素（土、水、風和火）的球和不朽的恆星（以太）之間的邊界，一個有影響力的哲學主導的世紀。然而，在，塞琉西亞的塞琉古的理論認為潮汐是月球引力引起的，因為朝汐的最高點都與月球相對於太陽的位置相對應。阿里斯塔克斯在同一個世紀計算出月球大小和距離，得知地月的距離是地球半徑的20倍。托勒密進一步更正這些數值：平均距離是地球半徑的58倍，直徑是地球的0.29，非常接近現在個別的值60和0.273。阿基米德發明了可計算當時已知行星和月球運動的天象儀。

在中世紀望遠鏡發明之前，已經有越來越多人認識到月球是一個球體，但許多人卻認為它的表面是非常平滑的。1609年，伽利略在《星際信使》中使用第一架伸縮望遠鏡描繪的月球，注意到它並不是光滑的，有著環形山和山。望遠鏡描繪出如下的月球：喬瓦尼·巴蒂斯塔·里喬利（Giovanni Battista Riccioli）和弗朗切斯科·馬里亞·格里馬爾迪（Francesco Maria Grimaldi）在17世紀後期的努力產生現今使用的月球命名系統。威罕·皮爾（Wilhelm Beer）和梅德勒（Johann Heinrich Madler）在1834-6年間發展出更精確的，並且在1837年出版相關的書，第一次用三角法準確的研究月球特徵，包括一千多座山的高度、並引導對月球研究的精確度可能如同地球的地理。最先注意到的月球環形山科學家是伽利略，一直被認為是火山。直到1870年代，理查德·波達（Richard Proctor）才提出這是由撞擊形成的假設。這種觀點在1892年獲得地質學家葛洛夫·吉伯特（Grove Karl Gilbert）的實驗支持，1920年至1940年的比較研究引導月球地層學發展，在1950年代成為天體地質學的一個嶄新且持續發展的分支。

第一次直接探測：1959–1976

在冷戰期間，美國和蘇聯一直希望在太空科技領先對方。這場太空競賽在1969年7月20日，美國阿波羅11號的指揮官尼爾·阿姆斯壯登陸月球時達到高峰，他是登陸月球的第一人，而目前最近一次登陸過月球的人是尤金·塞爾南，他是1972年12月阿波羅17號任務的成員。

蘇聯的任務

冷戰刺激了蘇聯和美國的太空競賽，令人類加速了對月球的探測。一旦發射器有足夠的能力，這些國家就發射無人探測器進行飛越和撞擊或登陸的任務。來自蘇聯的月球計畫太空船最先完成多項目標：於1958年進行了三次未賦予名稱的失敗任務之後，第一個脫離地球的引力，並且飛越過月球的人造物體是月球1號；第一個撞擊月球表面的人造物體是月球2號；第一個拍攝到通常是被遮蔽而看不見的月球背面影像的是月球3號，這全都發生在1959年。

第一艘成功執行在月球軟著陸的是月球9號，第一艘環繞月球的無人太空船是月球10號，兩者均在1966年完成任務。將月球的岩石和土壤標本帶回地球的標本返回任務（月球16號、月球20號和月球24號）總共帶回0.38公斤的月岩。兩個先鋒的機器人太空船在1970年和1973年登陸月球，是蘇聯的月球步行者計畫的一部分。

在美蘇的登月競賽中蘇聯使用了N1火箭，嘗試將其用于搭載載人登月航天器，但因機件故障造成四次試射失敗，最終以輸家身份結束這場:太空競賽。

美國的任務

美國的月球探測始於機器人任務的發展，旨在實現載人登陸月球的最終目標：噴射推進實驗室的測量員計畫，在月球9號發射後4個月，發射第一艘登陸月球的太空船。NASA載人的阿波羅計畫也在同時發展；經過無人的阿波羅太空船在地球軌道上一系列的測試之後，和蘇聯月球飛行能力的刺激，阿波羅8號於1968年首度執行載人環繞月球軌道的任務。在1969年人類首次登陸月球，與後續多次的登陸月球，使很多人認為這是太空競賽的最高峰。尼爾·阿姆斯壯是美國阿波羅11號任務的指揮官，他在1969年7月21日02:56（世界時）踏上月球表面，成為第一位在月球漫步的人。從阿波羅11號到17號（除了阿波羅13號中止了登陸月球的任務）的任務，總共帶回382公斤、共2,196塊月球岩石和土壤標本。美國登陸月球和返回使1960年代初期在的技術獲得長足的進步與發展，特別是在燒蝕化學、軟體工程和重返大氣層技術，和高階巨大計畫整合管理等領域。

在整個阿波羅任務中，許多科學儀器被建置在月球表面。能長期工作的儀器站，包括熱流量探測器、地震儀、磁強計，它們是阿波羅12號、14、15、16和17設置的。它們將資料直接傳送回地球，直到1977年才因為預算的原因而停止，但是工作站的月球雷射測距回向反射器陣列是被動式的儀器，它們仍在使用中。從地球例行測量的測站到月球基地的距離精確範圍在幾公分之內，並且從這些資料可以對月球核心的大小有所理解。

目前的時代：1990–現在

阿波羅計劃之後，更多的國家已經直接參與月球的探測。在1990年，日本將太空船Hiten送到月球，成為第三個擁有環繞月球軌道衛星的國家。這艘太空船在月球軌道上釋放了一個小探測器Hagoromo，但是發射失敗，妨礙了進一步的科學應用任務。

在1994年，美國國防部和NASA聯合發射了克萊芒蒂娜至月球軌道。這個任務首度獲得幾乎整個月球的全球地形圖，和第一份月球表面全球的多光譜影像。此後在1998年又派遣了月球探勘者任務，儀器顯示在月球的極區有過量的氫，這可能是存在於永久陰暗的環形山內部風化層表層數公尺處的水冰。

歐洲太空船智能1號，第二艘使用離子推進的太空船，從2004年11月15日進入月球軌道直到2006年9月3日，並且第一次對月球表面的化學元素做了詳細的調查。

中國的中國探月工程也發射了第一艘進入月球軌道的太空船，嫦娥一號，從2007年11月5日直到2009年3月1日撞擊月球。在6個月的任務期間，獲得月球表面完整的影像圖。嫦娥二號於2010年10月1日發射升空，最主要任務是為嫦娥三號預定著陸的虹灣拍照，而其分辨解析力為約1米。嫦娥三號攜帶月球車于2013年12月2日發射升空，並于12月14日著陸月球表面。2018年12月8日嫦娥四號著陸器、玉兔二號探測車由長征三號乙改進Ⅲ型運載火箭發射升空，2019年1月3日成功在預選的著陸區月球背面南極-艾特肯盆地（South Pole-Aitken，SPA）內的馮·卡門撞擊坑（Von Kármán）著陸。2020年1月2日嫦娥四號著陸器和「玉兔二號」月球車按地面指令完成月夜模式設置，順利進入月夜休眠。2020年11月24日嫦娥五號於海南文昌發射場發射升空，完成月球表面自動採樣任務後，於12月17日凌晨1時59分在內蒙古四子王旗著陸場。

在2007年10月4日至2009年6月10日之間，日本宇宙航空研究開發機構的「月亮女神（Selene）」任務，攜帶了一架高明晰度電視攝影機，和兩個小的無線電發射衛星，獲得許多月球地理的資料和從地球軌道之外高明晰的影片。

印度的第一次月球任務，月船1號，從2008年11月8日起環繞月球，直到2009年8月27日，創建了月球表面高解析的化學、礦物學和照片地質地圖，並確認月球土壤中存在著水分子。印度太空研究組織計畫在2013年發射月船2號，攜帶俄羅斯的月球漫遊車。印度也曾表示希望在2020年能夠送人上月球。月船2號最終于2019年7月22日發射成功。然而著陸器于當年9月7日因硬著陸而撞毀，軌道器仍然在軌道上繼續執行科考任務。

其他即將進行的月球探測任務包括俄羅斯的月球-團塊——以它們的火星探測器福布斯－土壤的軌道器為基礎的一種無人登陸器，架設地震儀，預計在2012年發射。

在2007年9月13日宣布的Google月球X大獎，鼓勵私人資助的月球探索計畫，將提供2,000萬美元給任何讓機器人登上月球且合乎其他指定標準的人。

美國發射的「月球勘測軌道飛行器」（LRO）和「LCROSS」撞擊器於2009年6月18日進入軌道；隨後與軌道上的飛行器，在2009年10月9日一起在計畫內與計畫外廣泛的觀測LCROSS撞擊Cabeus，完成他的使命，之後，LRO仍然繼續運作，以月球高度測量術獲得高解析度的影像。

在美國總統喬治·沃克·布希在2004年1月14日宣布在2020年重返月球之後，NASA開始恢復載人任務計畫。星座計畫開始資助與測試載人太空船和發射器，並且研究和設計月球基地。但是，2011年的政府預算已經取消了對NASA星座計畫的挹注，這將迫使NASA取消在太空技術上的推行以及高推力火箭的研究。

法律地位

雖然月球號系列探測器將蘇聯的旗幟散布在月面各處，美國國旗也象徵性地插在阿波羅太空人的登陸點，但目前沒有任何一個國家宣稱月球表面的任何一部分是他們的領土。根據蘇聯和美國在1967年簽署的外太空條約，月球和外太空是「全人類所共有的地方」。這份條約也限制了月球只能供和平目地的使用，明確禁止軍事設施和大規模毀滅性武器的設置。1979年的月球協定限制單一國家對月球資源的創建、開發與利用，但是至2020年1月沒有任何一個擁有載人航天能力的國家簽署。雖然有一些個人曾經宣稱擁有月球的全部或部分，但這些沒有一件是真實的。

文化

月球規則的相位變化是一個很好的計時器，周期性增長和衰減的形式成為許多古老曆法的基礎。2萬至3萬年前骨製計數棒上的缺口被認為是月相的標記。陰曆的一個月大約是30天。英語中的名詞month和日耳曼語系與其它同源的語系來自原始日耳曼語的*mǣnṓth-，這又連結到前述原始日耳曼語的*mǣnōn，顯示德國民間在使用陽曆之前是使用陰曆。

月球已經給予藝術和文學作品無數的靈感，它是許多視覺藝術、表演藝術、詩歌、散文和音樂藝術的主題。有5,000年歷史的愛爾蘭Knowth石刻，可能是被發現、最早的代表月球的描繪。月球上明亮的高地和黑暗的海的對比，在不同的文化和民族中創造出不同的形象，像是月球上的人、兔子、野牛、嫦娥、玉兔、螃蟹和其它的等等。在許多史前和古代的文化中，月球化身為月神，或其它超自然的現象和占星圖的月亮，到今天仍然被繼續傳播。

• 在中國有嫦娥奔月的神話。

　• 中國歷代以來，在詩歌文學中對於月亮，有許多不同的雅稱：

　　• 和滿月形狀有關：白玉盤、半輪、寶鏡、冰鏡、冰輪、冰盤、蟾盤、飛鏡、飛輪、挂鏡、金鏡、金盆、明鏡、瑤台鏡、銀盤、玉鏡、玉輪、玉盤、玉盆、圓影、月輪。

　　• 和新月形狀有關：懸鉤、玉弓、玉鉤、蛾眉。

　　• 和月亮光芒有關：蟾光、方暉、金波、清光、夜光、幽陽。

　　• 和神話有關：白兔、蟾蜍、蟾宮、嫦娥、顧菟、廣寒、桂宮、桂魄、姮娥、瓊闕、素娥、兔影、銀闋珠宮、玉蟾、玉京、玉欄、玉兔、圓蟾、月桂、清虛、望舒。

　　• 其他：冰壺、冰鑒、冰魄、嬋娟、秋影、太陰。

• 在希臘神話中，月亮女神叫做阿耳忒彌斯，月球的天文符號就像一彎新月，也象徵阿耳忒彌斯的神弓。

• 在北歐神話中，瑪尼是駕駛月車的神明。

文學作品

• 法國科幻小說作家儒勒·凡爾納的小說《從地球到月球》，利用巨型大炮將人發射到月球上去。

• 中國古代唐朝詩人李白的唐詩《靜夜思》和《月下獨酌》

• 中國古代宋朝文學家蘇軾的詞作《水調歌頭·明月幾時有》

音樂作品

精神病的聯想

西方文化中，月球長久以來也與精神錯亂和非理性相關聯；精神錯亂（lunacy）和瘋癲（loony）這兩個字都源自拉丁文的月亮「Luna」。哲學家亞里斯多德和老普林尼都辯稱滿月容易影響個人，甚至導致精神錯亂。他認為主要由水構成的大腦，一定會受到月球和潮汐的影響；但是月球的引力太微弱，不會影響到任何一個人。即使在今天，雖然沒有科學的依據，依然有人堅稱精神科病患的數量、交通事故、殺人或自殺的事件，在一輪滿月的期間會增加。

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 𓇹, 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.