Full Moon

May 17, 2019

Saturday night is the official full moon, but it’ll pretty much look full all weekend. It rises just after sundown, so it’ll be low on the southeastern horizon in the evening after dark. Often, this can make the moon look bigger, but it’s actually the same size in the sky as if it were straight overhead. A moon low on the horizon can also have a color like yellow, orange, or red. The reason for that is the same reason the sun seems to change color at sunset or sunrise… it’s seen shining through many more layers of atmosphere than if it were higher in the sky. So depending on the conditions, the moon can take on a certain hue.

All that aside, looking at a full moon through a telescope, you notice a lot. The craters, the so-called “seas,” the “Ocean of Storms—“ that’s the big sea… So just what are all those geographic features?

Well, the craters are pretty self-explanatory. They’re impact craters from the eons of the moon’s existence. Some of them are bigger than others, and some have some unique features. Take the Tycho crater. It’s the big one in the southern hemisphere with what looks like lines spiking off of it. That’s impact ejecta. Whatever made the Tycho crater was big enough to spray rock and dust so far across the moon’s surface that we can easily see it from Earth. And we know from samples taken on the Apollo 16 mission that Tycho is a relatively young moon crater, having formed an estimated 108 million years ago. Compare that to the much older “Seas,” which are left over from a time when the moon was geologically active. They’re vast plains of volcanic basalt—basically lava flows.

So take a moment to appreciate the full moon this weekend. Howling is optional.