31–40 of 71 posts
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.
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!
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 a…
The Moon is a moon because the center of mass of the system is inside the Earth. It's as simple as that.
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.
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!
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.
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!
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).
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 a…
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?
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.
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.
also, by inside the sun do you mean inside the corona?
Earlier quoted context omitted.
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?
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.
Earlier quoted context omitted.
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?
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.
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'.
Earlier quoted context omitted.
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?
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.
Earlier quoted context omitted.
I think he's referring to Saturn's rings, which is largly composed of objects like the “moon” in this article.
yea but saturn's rings are made of ice, moons and planets are made of rock just like asteroids not ice like comets.