Relevant video from QI: How many moons does the earth have? http://www.youtube.com/watch?v=D1zuAQAhhMI
FYI Fry was incorrect as it is not a mooon, but instead orbits the sun: http://en.wikipedia.org/wiki/3753_Cruithne
Earth actually has two moons.
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Re: Earth actually has two moons.
#22Link to referenced MIT Technology Review post:
Re: Earth actually has two moons.
#23On 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 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.
The Moon is closer to spherical than the Earth is. The Moon's flattening is something like 0.001, as opposed to 0.003 for Earth.
Re: Earth actually has two moons.
#24On 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.
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!
Re: Earth actually has two moons.
#25If 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?
Re: Earth actually has two moons.
#26How many moons does the Sun have??
Re: Earth actually has two moons.
#27This article should be called "Astronomers actually redefine the term moon" If this is the definition, then lets also report that Saturn now has a gazillion moons.
We do. See http://en.wikipedia.org/wiki/Moons_of_Saturn
I think he's referring to Saturn's rings, which is largly composed of objects like the “moon” in this article.
Re: Earth actually has two moons.
#28Earlier 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?
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.
Re: Earth actually has two moons.
#29Earlier quoted context omitted.
We do. See http://en.wikipedia.org/wiki/Moons_of_Saturn
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.
Re: Earth actually has two moons.
#30If 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?
Most likely because the Moon was given its name long before we knew that other planets had moons.
Its pretty much the same as naming our planet the Earth, a pretty generic name for a planet.