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Earth actually has two moons.

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Re: Earth actually has two moons.

#61

Post by johno215 »

joshAg wrote: post #34
tzs wrote: post #24

Earlier quoted context omitted.

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…

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.

[1] http://en.wikipedia.org/wiki/Barycentric_coordinates_(astron...

Re: Earth actually has two moons.

#62

Post by [deleted] »

johno215 wrote: post #61
joshAg wrote: post #34

Earlier quoted context omitted.

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…

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. [1] http://en.wikipedia.org/wiki/Barycentric_coordinates_(astron...

[deleted]

Re: Earth actually has two moons.

#63

Post by joshAg »

johno215 wrote: post #61
joshAg wrote: post #34

Earlier quoted context omitted.

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…

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. [1] http://en.wikipedia.org/wiki/Barycentric_coordinates_(astron...

seems i forgot to multiply by 1.6. whoops.

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...

Re: Earth actually has two moons.

#64

Post by Retric »

keenerd wrote: post #51
Retric wrote: post #44

Earlier quoted context omitted.

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.

Re: Earth actually has two moons.

#65

Post by civilian »

joshAg wrote: post #56
civilian wrote: post #53

Earlier quoted context omitted.

Darn it, you're right. http://mechanicalintegrator.com/2009/solar-system-center-of-...

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.

The parent of my post said:

>>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.

Re: Earth actually has two moons.

#66

Post by dalke »

mturmon wrote: post #31
tzs wrote: post #24

Earlier quoted context omitted.

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!

More on this at: http://en.wikipedia.org/wiki/Double_planet

and http://en.wikipedia.org/wiki/Natural_satellite#The_definitio...

Re: Earth actually has two moons.

#67

Post by dalke »

ars wrote: post #58
wisty wrote: post #52

Earlier quoted context omitted.

So, if a star becomes a black hole, it's planets are no longer planets unless their COM is inside the event horizon?

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…

So which type is a neutron star? :)

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."

Re: Earth actually has two moons.

#68

Post by joshAg »

waqf wrote: post #42
joshAg wrote: post #40

Earlier quoted context omitted.

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.

Ice is made of small molecules like water, methane or carbon dioxide.

is there a concrete cutoff or shades of grey?

Re: Earth actually has two moons.

#69

Post by keenerd »

Retric wrote: post #64
keenerd wrote: post #51

Earlier quoted context omitted.

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…

> 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.

Re: Earth actually has two moons.

#70

Post by maaku »

joshAg wrote: post #68
waqf wrote: post #42

Earlier quoted context omitted.

Ice is made of small molecules like water, methane or carbon dioxide.

is there a concrete cutoff or shades of grey?

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.
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