Bonnie miller, age 13, of milwaukee, wisc.., for her question:
Why isn't the moon considered a planet?
The star of our solar system is the blazing sun. The other members of the family are orbiting bodies of cold minerals. There are nine planets, more than 30 moons, scores of comets, thousands of asteroids and countless lumps of drifting debris all orbiting around our central sun.
The sixth moon outside the rings of saturn is titan, 3000 miles wide. The inner planet of the solar system is mercury, which also happens to be 3000 miles in , diameter. The satellite titan and planet mercury are, thcrefore, the same size. What's more, titan has an atmosphere of gases, and mercury does not. But the status of a planet or a satellite is not decided on its size or atmosphere.
Our moon is 2163 miles in diameter which makes it three quarters the size of mercury or titan. But there is no question that our moon is a satellite. It is more than a quarter as big as the earth, but there is no.question that the earth is the parent planet of the moon.
The matter is settled by the orbit of a heavenly body. Planets, comets, asteroids and satellites all orbit around the sun. But some of them are captives which orbit around other orbiting bodies. Our moon is held captive by the gravity of the planet earth. It cannot escape; and as it travels with the earth around the sun, it makes its own orbit around the planet which holds it in captivity.
In the course of a year, the earth and its satellite, the moon, make one orbit around the sun. In this same period, the moon makes about 13 orbits around the earth. The earth travels in a direct line about its oval orbit. The moon travels back and forth across the orbit of the earth in a series of loops. more ,•
The status of each member of the solar system is decided upon the type of path it travels around the sun. The comets travel in long, narrow paths which often cross the paths of the planets. The swarming asteroids are called planrtoids, or small planets, because each pursues its own orbit around the sun.
Our earth is the only planet with a single satellite of its own. The other planets have two or more moons or none at all. All the satellites are smaller than their parent planets. However, one of the nine moons of saturn is as big as the planet mercury, and two of the moons of giant jupiter are even bigger. But the status of a moon depends upon its orbit. The moon is a satellite, because it is held captive by the earth and orbits around the earth while both of them orbit the sun.