On the other hand, the thing we call The Moon shouldn't really be considered a moon. The Earth/Moon system should really be classified as two planets in essentially the same orbit around the Sun.
As I think was discussed recently in another thread here, and was pointed out by Isaac Asimov in one of his science essays a long time ago, if you were to plot the paths of the Earth and Moon through space, you'd find they are both approximately 12-sided convex polygons around the Sun, out of phase by pi/12. The Moon's path does not look like something a kid would draw with a Spirograph, contrary to popular opinion.
When you look at moons of other planets, their paths do look like a Spirograph drawing.
If you are having trouble visualizing this, imagine two horses racing around a standard horse racing track. On the first straightaway imagine horse 1 is in the lead. At the first turn, horse 2 gets the inside track and pulls ahead. On the back straightaway, horse 2 leads, but on the second turn, horse 1 gets the inside track and takes the lead back in the turn.
It probably wouldn't even occur to you to think of horse 2 as having done an orbit around horse 1, yet if there were a remote control camera mounted on horse 1, and you were controlling it from the stands and you were trying to keep horse 2 in view at all times, you'd find that you have had to rotate the camera around a full circle. So, from horse 1's frame of reference, horse 2 indeed did orbit it once!
Now imagine the horses on a modified track that instead of two straightaways and two half-circle turns has four straightaways and four quarter-circle turns. Now horse 1 thinks horse 2 circled it twice.
That's essentially what the Earth and Moon are doing, but there are 24 turns in the race course, and the straightaways are not there--as soon as you leave one turn you are starting the next. So, from Earth's point of view it looks like the Moon goes around us 12 times a year. But alien astronomers watching would be like spectators at the horse race--they'd just see two planets orbiting the Sun in nearly the same order, taking turns using the inside track to pull ahead.
Another way to look at it is to consider force ratios. For moons such as those of Mars, or Jupiter, or Saturn, and so on, if you look at the force on the moon from the planet, and the force on it from the Sun, you find the ratio of those two is greater than 1. The planet "pulls harder" than the Sun does.
For the Earth and Moon, the ratio is less than 1. The Sun is pulling harder on the Moon than the Earth is!
That's a rather unsatisfactory way to define moon-hood, because it doesn't depend on any essential features of the two bodies. Replace Earth with something of the same mass but more dense, and the Moon gets promoted to planet, even though its size, shape, and orbit don't change? Yuck!
I believe the center of mass for the earth moon system is something like 3000 km from the center of the earth (in a moonward direction). Just for reference, the radius of the earth is 6300 km.
The density of the earth is ~5.52g/cc, and halving the radius of the earth will increase the density by 8 which gives a density of 44.16 g/cc or 44,160 kg/m^3, which is about twice the density of osmium and a third as dense as the center of the sun.
In other words, the scenario you imagine is not practically possible in a system where one body is much larger than the other (if they're both about the same size, then it a dual planet system).
I don't think this destroys your argument, but the barycenter of the Earth-Moon system is located, according to wikipedia [1], 4670 km from the center of the Earth.
Interestingly, both the Pluto-Charon and the Sun-Jupiter systems have barycenters above the surface of the primary body.
pluto-charon (imo) is a dual dwarf-planet system not planet and moon.
Sun Jupiter is my argument for why a planet shouldn't be defined only by barycenter ___location.
updated density: 13.55g/cc.
this is about the current density of the core of the earth now. It's about halfway between rhodium and mercury at STP, so not as horribly unlikely, but still pretty out there.
> Sun Jupiter is my argument for why a planet shouldn't be defined only by barycenter ___location.
But Jupiter doesn't sustain nuclear fusion so it can't be considered in the same category as the sun. If it was a red/brown dwarf, then we would be in a binary system and that'd be a different story entirely.
i believe the center of mass is inside the sun if you look at each planet-sun pair individually. IOW, 2 body to n-body kind of messes this definition up.
also, by inside the sun do you mean inside the corona?
This rule is for when both bodies are the "same" type.
There are three types:
Black hole -> Sun -> planet/moon
Two suns are a binary sun if they orbit a COM outside both. Same for two planet/moons.
A sun around a black hole is not a binary sun, but we've never named such a thing (except maybe galaxy).
A small sun around a large one with the COM inside the larger one should have a special name, but we never gave that a name either since we've never seen one.
A planet around a black hole is a planet, since it's not the same type as the other one.
the event horizon (schwarzschild radius) of the sun is 3km.
i don't think planethood should be defined based on whether the barycenter of the system is within the orbited object, but I do think that's a pretty good definition for determining whether two objects are a planet moon pair or dual planets.
but in case you con't want to follow, here's a summary:
dual planet system (eg pluto charon): barycenter lies outside of either object
planet with moon (eg earth and moon): barycenter lies within the radius of the planet
planet: "(a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit[1]." [2]
as for why planethood shouldn't be determined by barycenter ___location alone:
"If the definition of a double- or binary-star system is used as a comparison, and it depended only on the ___location of the barycenter, then any revolving body with a barycenter beneath a star's surface would be a planet, and any body with a barycenter lying outside the surface of the star would be another star. In the Solar System, all of the major planets would be planets under this definition except one. The Sun–Jupiter barycenter is the only center of mass that lies outside the surface of the Sun. Therefore, since Jupiter is not a star, the difficulty faced by astronomers to derive a reality-based definition of double planet begins to become clear." [3]
[1]: meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence.
there's a major error in their calculation in that they didn't do that in 3 dimensions, and they assumed that all planets always lie on the same radius from the sun. do the math again with jupiter and saturn on opposite sides and use 3 dimensions, and the CoM should go back into the sun.
Also, Jupiter and Saturn make up most of the mass of the planets (1.8986×10^27 kg and 5.6846×10^26 = 2.46x10^27) where the total mass of the planets is 2.67×10^27 kg.
As long as Jupiter and Saturn are aligned with the sun the barometric center of the solar system will be outside of the sun, and they would align once every 12 years or so, so this does happen from time to time.
Isn't that true for all moons, though, simply because the radius of the orbit around the sun is much larger than the radius of the moon around the planet?
also, isn't the force ratio issue more an issue of the inverse square law than anything else?
From the point of view of the Sun, some satellites with short orbital periods might appear to move back and forth as they run around their planet. Earth's Moon, on the other hand, never moves backward from the point of view of the Sun.
That still wouldn't make the Moon a planet though.
the orbital period of the satellite isn't as important as the orbital velocity of the planet around the sun.
the earth goes around the sun at 29 785.8944 mps, so for a satellite to appear to stand still (from the sun's perspective) it needs that orbital speed (i'm assuming that everything orbits in the same plane to make things easier/cleaner).
using the equation v^2 = GM/R gives an orbital radius of(G = 6.673 x 10^-11 N m^2/kg^2 and M = 5.98x10^24 kg, 1N = 1kg * 1m /1s) 449, 777 m or 449.7 km. Of course, the radius of the earth is 6.3 million meters (6300 km), so this isn't exactly possible.
the only way for a satellite orbiting a planet to appear to move backwards with respect to the thing the planet orbits is for the satellite to have a faster orbital velocity than the planet's orbital velocity.
No. If the Earth's mass were to vanish, the Moon would continue to orbit the Sun. It would be a bit more wobbly, but the Earth is not an essential part of the system.
If the same happened to Jupiter, all of those moons would fly out of of the solar system. (Or at least go cometary.) None of the orbits would survive.
That has to do with the distance between the sun and Jupiter not just Jupiter's mass. http://en.wikipedia.org/wiki/Deimos_%28moon%29 Deimos would continue to orbit the sun in much the same way that the moon would if it's planet disappeared. (Moon: 1.022 km/s, Deimos: 1.3km/s, Callisto's 8.204 km/s. Earth 29.78 km/s, Mars 24.077 km/s, and Jupiter 13.07 km/s)
Edit: http://en.wikipedia.org/wiki/Escape_velocity To escape from the suns orbit at Jupiter's distance from the sun takes 18.5 km/s vs Mars orbit's 34.1 km/s, or the Earth's orbit 42.1 km/s.
PS: The moon was the original example of 'moon' so it's a moon by definition. Any definition that does not include it must be describing something other than a 'moon'.
Your Deimos (and Moon) velocities are off, as those are velocities relative to Mars and Earth comparatively. The real question is, does 24.1km/s +- 1.3km/s keep Deimos in orbit without Mars. 5% change in velocity will probably knock it a bit out of whack, but I am not doing the perigee/apogee calculations right now.
PS, The Earth was the original example of 'flat' and nothing is ever allowed to change as we gain better understanding.
Everything's orbital velocity was taken relative to what they orbit. My point is if you take Jupiter and +/- the moon's orbital velocity you get a more extreme orbits than you do with Mars +/- Deimos. So the new orbits say just as much about how far they are from the sun vs what they orbit.
PS: The Earth is not the original definition of flat.
No, you took the Moon's velocity relative to the Earth. The Moon orbits the Sun and its velocity relative to Earth is only useful for astrology purposes.
If you want to use the deepest gravity well then the Moon orbits the black hole in the center of the Milky Way Galaxy (plus the matter closer the the center). Orbital velocity 216 kilometers per second around Milky way vs 42.1 +/- 1 km/s around the sun and earth.
However, when you look at the actual accelerations involved the moon is much more attracted to the earth (by over 100x) than it is to the sun or the center of the Galaxy. Which is why the moon is tidally locked with the earth and not the sun.
PS: All of these still don't add up to the 583km/s velocity relative to the CBR.
> However, when you look at the actual accelerations involved the moon is much more attracted to the earth (by over 100x) than it is to the sun
Your "over 100x" figure is entirely made up. The correct answer is 0.46.
Depth of the well does not matter, it is the steepness of the well. In other words, what is pulling the hardest on the Moon. The Sun pulls twice as hard as the Earth, so there is a compelling argument that the Sun is the Moon's primary.
The black hole at the center of the Milky Way is (rounding up for your benefit) 4 million solar masses and 27000 ly away. But that inverse square law really hurts and the Sun's gravitational force is 733e15 times stronger than the black hole's.
Let's step it up and include all 10 billion solar masses in the center. The Sun is still ahead by a factor of 290 trillion. The galactic core has almost no effect on the solar system, so it is silly to claim that any planetary body orbits the core.
Regarding tidal lock, the force of tidal lock is (more or less) proportionate to gravitational force * angular velocity. While the Earth's gravitational force on the Moon is half as strong as the Sun's, the relative angular velocity is 12 times faster. So the Earth's tidal forces on the Moon are six times stronger than those of the Sun. Naturally, the Moon is tidal locked to the Earth.
If Jupiter were to orbit the sun at the same distance of the earth, what would happen to the orbits of the moons if its mass suddenly disappeared? Would they still be cometary (at the least) or would they settle into a planetary orbit?
At 1AU, orbital velocity is 42.3 km/s. Io does 17.3 km/s relative to Jupiter. Io would be gone. Calliso does 8.2 km/s and I highly suspect that would be too fast to maintain a planetary orbit at 1AU.
The orbital velocity of Earth is the escape velocity of the solar system, from Earth (42.1 km/s). Fun fact, this means that if we ever wanted to dispose of nuclear waste by dumping it in a star, we'd need much less rocket fuel to hit Alpha Centauri than the Sun.
Jupiter has an orbital velocity of only 13.7 km/s, so even if Io is going the right direction and all the elliptical planes are parallel, isn't that still just a max orbital velocity of ~40 km/s, which is slow enough for orbit at 1AU?
It'd be interesting to see what happens at Jupiter's orbit, thouh. Does Io always leave? What about Callisto? It seems like there would be a large part of their orbit that would send them off into cometary orbits (or worse) depending on what direction they are going when scotty beams jupiter aboard the enterprise.
Can you explain (or link to) "Fun fact, this means that if we ever wanted to dispose of nuclear waste by dumping it in a star, we'd need much less rocket fuel to hit Alpha Centauri than the Sun."
It's not making much sense to my tired mind today.
It takes less energy to accelerate an object away from both the Earth and sun (assuming your launch pad is on an appropriate spot on the Earth surface), than to accelerate an object away from the Earth but into the sun without just getting the object stuck in an orbit like Venus or Mercury.
I don't see how this and Kepler's laws fit together. The moon's mass is significantly different from the earth. How could the moon have the same orbit as the earth if the earth wasn't there?
Isn't the criteria for planethood having sufficient mass to be largely spherical? I thought the moon was oblong due to tidal locking with the Earth. I realize that the Earth isn't a sphere, but in that case it's due to the centripetal force of its rotation on its own mass, which I think gives it a pass.
Saturn's rings are made of the exact same stuff as its moons, because that's what they are. One or more of Saturn's moons broke up millions of years ago under gravitational/tidal forces, and over time the debree spread out into a ring.
And BTW, there are plenty of examples of moons made of ice, and at the temperatures of the outer solar system ice can be harder than rock (and BTW, Neptune and Uranus are called "ice giants"--care to guess why?).
what's the difference between rock and ice? is there a boundary where if the melting point is below a certain value then the substance is ice, and if it's above that value it's rock? it's not like rock can't melt in the right circumstances.
I think what the GP is getting at is the chemical difference of ionic minerals (rock) vs amorphous collections of molecular compounds (i.e. water or methane ice). There is a categorical difference between the two chemically, although as an aside their properties in micro-gravity and the cold vacuum of the outer solar system are quite different from what you might have learned in your chemistry class.
The article states that these 'moons' typically stay for around 10 months. Do they escape from the Earth's gravity or are they pulled closer to the Earth and eventually break up in the atmosphere?
2006 RH120 is a tiny near-Earth asteroid with a diameter of about five metres, which ordinarily orbits the Sun but makes close approaches to the Earth–Moon system every twenty years or so. Occasionally the object temporarily enters Earth orbit through temporary satellite capture (TSC).
The irony is that the real headline is more exciting, which is that there is a specimen orbiting earth, just waiting to be brought back for inspection.
If more get caught in orbit than we thought, then are we gradually learning that the cloud of asteroids around us may not be uniformly distributed anymore?
As an extreme example, if the cloud of asteroids was entirely concentrated in the earth's orbit that would increase the odds of a collision. So wouldn't discovering that there's always one increase the odds (by a lot less)?
The number of pieces on a chessboard matters very little. You could be winning or losing based on their arrangement.
Shit, this could be alarming. I'm gonna research this more. (Anybody here see Melancholia?)
If moon = natural satellite and if the "moons" of other planets have names, what is the name of earth's moon? Why do they have to use it so interchangeably?
As I think was discussed recently in another thread here, and was pointed out by Isaac Asimov in one of his science essays a long time ago, if you were to plot the paths of the Earth and Moon through space, you'd find they are both approximately 12-sided convex polygons around the Sun, out of phase by pi/12. The Moon's path does not look like something a kid would draw with a Spirograph, contrary to popular opinion.
When you look at moons of other planets, their paths do look like a Spirograph drawing.
If you are having trouble visualizing this, imagine two horses racing around a standard horse racing track. On the first straightaway imagine horse 1 is in the lead. At the first turn, horse 2 gets the inside track and pulls ahead. On the back straightaway, horse 2 leads, but on the second turn, horse 1 gets the inside track and takes the lead back in the turn.
It probably wouldn't even occur to you to think of horse 2 as having done an orbit around horse 1, yet if there were a remote control camera mounted on horse 1, and you were controlling it from the stands and you were trying to keep horse 2 in view at all times, you'd find that you have had to rotate the camera around a full circle. So, from horse 1's frame of reference, horse 2 indeed did orbit it once!
Now imagine the horses on a modified track that instead of two straightaways and two half-circle turns has four straightaways and four quarter-circle turns. Now horse 1 thinks horse 2 circled it twice.
That's essentially what the Earth and Moon are doing, but there are 24 turns in the race course, and the straightaways are not there--as soon as you leave one turn you are starting the next. So, from Earth's point of view it looks like the Moon goes around us 12 times a year. But alien astronomers watching would be like spectators at the horse race--they'd just see two planets orbiting the Sun in nearly the same order, taking turns using the inside track to pull ahead.
Another way to look at it is to consider force ratios. For moons such as those of Mars, or Jupiter, or Saturn, and so on, if you look at the force on the moon from the planet, and the force on it from the Sun, you find the ratio of those two is greater than 1. The planet "pulls harder" than the Sun does.
For the Earth and Moon, the ratio is less than 1. The Sun is pulling harder on the Moon than the Earth is!