Expert: Courtney Seligman - 12/14/2008

Question
Hello. I was trying to figure out what classifies the moon as a satellite, and not a planet. I found my answer on that, but I was reading something about if you do the math of Newtons law of gravity, then the moon is more attracted to the sun, not the earth. Also, if the Earth was non-existent, then the moon would still be there orbiting the sun. But for now, since the earth does exist, the moon orbits around the earth (which orbits around the sun). So if the moon would still be here orbiting the sun without the earths presence, then why isn't it considered a planet?

I know my question looks all tongue tied, so if you need me to reword my question, let me know. Thank you!

Answer
(Please forgive the delay in answering this question. It was sent to the Question Pool, presumably because whoever you sent it to was on vacation or rejected it, and I just discovered its existence.)

To be classified as a planet, an object has to go around the Sun, and if it also goes around something else, it must be significantly larger than that secondary object. When, as in the case of the Moon, the other object (the Earth) is much larger, the object is considered a satellite of the larger object, not a planet.

If the Earth did not exist, and the Moon was following its current path all on its own, it would almost certainly be classified as a planet. Whether it would be considered a major or dwarf planet would be a matter of debate, but it would be called a planet of some sort. But since the Earth does exist, and is much larger than the Moon, the Moon cannot be considered a planet. It is, however, a "world" in its own right, and of great interest to astronomers and planetary geologists. In fact, because of its solid surface and interior, it is of more interest in some ways than the gaseous/liquid planets, such as Jupiter and Saturn.

Courtney Seligman
Professor of Astronomy
Long Beach City College