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Moon and Aldebaran

March 13, 2016

A beautiful crescent Moon hangs high in the sky as darkness falls this evening. The crescent is bathed in sunshine, while the rest of the lunar disk is bathed in earthshine — sunlight reflected off our own planet.

The line between those regions, which divides day from night, is crossing the most famous feature on the lunar surface: the Sea of Tranquility. It’s where astronauts first set foot on the Moon, almost a half century ago.

The Sea of Tranquility covers more than 150,000 square miles — an area as big as Montana. And like all the other lunar seas, it’s a dark volcanic plain. It formed several billion years ago, when molten rock bubbled up to the surface. The rock is rich in iron, magnesium, and other elements that give it the dark color. In fact, that color reminded early lunar mapmakers of the seas here on Earth — hence the “watery” names of these features.

If you could zoom in on the Sea of Tranquility, though, you’d see that it’s not as smooth as it seems. Great ripples and mounds undulate across the surface. And like the rest of the Moon, it’s covered with impact craters — the scars of collisions with space rocks large and small.

Watch the Moon as it drops down the western sky this evening. A bright companion stands to its upper left: Aldebaran, the orange eye of Taurus, the bull. The Moon will move past Aldebaran by tomorrow night, so the star will stand about the same distance to the lower right of the Moon.

Script by Damond Benningfield

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