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Earth actually has two moons.

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Re: Earth actually has two moons.

#51

Post by keenerd »

Retric wrote: post #44
keenerd wrote: post #43

Earlier quoted context omitted.

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 bett…

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.

Re: Earth actually has two moons.

#52

Post by wisty »

joshAg wrote: post #36
digitalsushi wrote: post #32

Earlier quoted context omitted.

I am so confused now. The center of mass of the solar system is often not inside the Sun.

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?

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

Re: Earth actually has two moons.

#53

Post by civilian »

digitalsushi wrote: post #32
ars wrote: post #16

Earlier quoted context omitted.

The Moon is a moon because the center of mass of the system is inside the Earth. It's as simple as that.

I am so confused now. The center of mass of the solar system is often not inside the Sun.

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

Re: Earth actually has two moons.

#54

Post by [deleted] »

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…

[deleted]

Re: Earth actually has two moons.

#55

Post by akkartik »

te_platt wrote: post #48
akkartik wrote: post #46

Does this finding increase the odds of a Tunguska-size impact?

No, It doesn't change the estimate of the number of asteroids near Earth. It just points out that more get caught in orbit than previously thought.

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?)

Re: Earth actually has two moons.

#56

Post by joshAg »

civilian wrote: post #53
digitalsushi wrote: post #32

Earlier quoted context omitted.

I am so confused now. The center of mass of the solar system is often not inside the Sun.

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.

Re: Earth actually has two moons.

#57

Post by joshAg »

wisty wrote: post #52
joshAg wrote: post #36

Earlier quoted context omitted.

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?

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

nope, not at all.

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...

Re: Earth actually has two moons.

#58

Post by ars »

wisty wrote: post #52
joshAg wrote: post #36

Earlier quoted context omitted.

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?

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 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.

Re: Earth actually has two moons.

#59

Post by whatusername »

keenerd wrote: post #45
joshAg wrote: post #37

Earlier quoted context omitted.

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 rocke…

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.

Re: Earth actually has two moons.

#60

Post by gujk »

whatusername wrote: post #59
keenerd wrote: post #45

Earlier quoted context omitted.

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 rocke…

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.
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