11 May 2012

Our Super Moon

Posted by Ryan Anderson

The "Supermoon" of March 19, 2011 (right), compared to a rather "average" moon of December 20, 2010 (left): note the size difference. Images by Marco Langbroek.

You may have heard all the excitement last weekend about the so-called “supermoon”. The gist of it is that the moon’s orbit is not perfectly circular, so its distance from the earth varies slightly. When it is at the closest point in its orbit, it looks slightly larger (and therefore slightly brighter) in the sky. It’s not really a big deal, and all the talk of the SuperMoon got many astronomers worked up in much the same way we get worked up about theĀ Mars Hoax or theĀ 2012 doomsday.

But all the talk of the “SuperMoon” got me thinking and I realized that we were missing a teachable moment. No, the moon being at perihelion is not a big deal, but our Moon is pretty “super”. Let me show you why:

When you look at the mass of the largest moons in the solar system relative to their planets, our moon really stands out. In fact, most of the bars on this chart aren’t even visible because the moons are so tiny compared to their planets. To see them, we have to switch to a logarithmic scale (meaning each step on the vertical axis is 10 times larger than the previous step).

There, now we can at least see some of the other large moons. Phobos and Deimos are still off the chart because they are just tiny captured asteroids, dwarfed even by the relatively small mass of Mars. Likewise, other than Neptune’s giant moon Triton (which may actually be a captured Kuiper Belt object) its other moons are tiny.

Of course, our moon is physically smaller than the big moons in the outer solar system, but because the gas giants are so large, their moons are smaller in a relative sense. This difference points to a difference in how the moons formed. Whereas most of the big moons around the outer planets formed out of the accretion disks for their planets, our Moon formed when the proto-Earth was whacked by a giant impactor about the size of Mars, ripping off a huge chunk of material that ended up coalescing as our Moon.

So, out of all the planets, our moon is the largest relative to its planet. Of course, if Pluto was still counted as a planet, its moon Charon would hold that title. Charon is so big that the center of mass of the Pluto-Charon system is outside of Pluto, leading some people to call Pluto-Charon a “double planet”.

Now I’m looking forward to next year’s “super moon” so I can (a) tell them that the moon being a little closer isn’t that big a deal, and (b) I can tell them why we really do have a super moon.