Earth actually has two moons.
news.discovery.com
news.discovery.com
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!
also, isn't the force ratio issue more an issue of the inverse square law than anything else?
That still wouldn't make the Moon a planet though.
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.
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.
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.
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.
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'.
PS, The Earth was the original example of 'flat' and nothing is ever allowed to change as we gain better understanding.
PS: The Earth is not the original definition of flat.
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.
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.
the period and orbital velocity are dependent on the mass of the orbited object and the radius of the orbit.
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).
Interestingly, both the Pluto-Charon and the Sun-Jupiter systems have barycenters above the surface of the primary body.
[1] http://en.wikipedia.org/wiki/Barycentric_coordinates_(astron...
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.
Note: all density comparision made based on info from: http://en.wikipedia.org/wiki/Density#Densities_of_various_ma...
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.
also, by inside the sun do you mean inside the corona?
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.
this wikipedia entry hasa fairly good description: http://en.wikipedia.org/wiki/Dual_planet#Definition_of_a_dou...
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.
[2]: IAU
[3]: http://en.wikipedia.org/wiki/Dual_planet#Definition_of_a_dou...
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.
In any case, we do say that a "sun around a black hole" is a "binary star system". See http://www.sciencedaily.com/releases/2011/03/110325082725.ht... and http://chandra.harvard.edu/photo/category/blackholes.html for two examples of many web pages which use that terminology. From a caption at the chandra.harvard.edu site:
"A binary star system consisting of a black hole and a normal star, located about 11,000 light years from Earth."
>>The center of mass of the solar system is often not inside the Sun.
That paper was just looking at one extreme combination where it's possible. Yes, most of the time it is inside the sun.
I don't think that 3 dimensions make much of a difference because the tilt off the solar plane for Jupiter and Saturn are much: http://en.wikipedia.org/wiki/Invariable_plane
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.
If this is the definition, then lets also report that Saturn now has a gazillion moons.
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?).
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 only exception I can think of at the moment are natural satellites which are part of a greater planetary ring...
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?)
Its pretty much the same as naming our planet the Earth, a pretty generic name for a planet.