The Moon, a Planet?

We all grew up thinking that there were nine planets circling the Sun. Now astronomers tell us there are only eight: tiny Pluto has been demoted. It's just a small rock, they say (well, about two thousand miles in diameter - it's not nothing.) These astronomers attempt to soothe us by telling us that there may be other planets out there, but so far away they could only be seen in the largest telescopes.

Well, there's another candidate for planethood, and you don't need a telescope to see it. It's our own Moon.

There are good reasons for thinking the Moon is really a planet, not a satellite. First of all, it circles the Earth, not around the equator the way satellites do around the other planets, but in the plane where all the planets move: the ecliptic.

Second, other planets in the solar system have satellites about the same size as our Moon. But for each of them, these satellites are dwarfed by their parent planets. Jupiter, for example, has a diameter forty times that of its largest satellites. The diameter of the Earth is only four times that of the moon.

Finally, for each of the other planets, these planets pull on their satellites hundreds or thousands of times more than the Sun. For each of them, the planet is really in charge.

But in the case of our Moon, things are the other way around. The Sun's pull on our Moon is actually greater than the Earth's. The Sun is in charge here, and the Moon, like any other planet, is really circling the Sun.

It's only our self-centeredness that makes us say that the Moon is rotating around us. From the point of view of the Moon, the Earth circles it. Like a dumbbell, they circle each other as they endlessly orbit the Sun -- a double planet!

So the answer to all three questions -- where are you going?, how big are you?, and who's in charge? -- say that the Moon is not really ours. She's a sister planet.

(10/02/09)