A fat 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 darkness.
The line between them -- the line between day and 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, more than four decades ago.
The Sea of Tranquility covers more than 150,000 square miles -- an area as big as Montana. And like all of the lunar seas, it's a dark volcanic plain. It formed several billion years ago when molten rock bubbled 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 for these features.
If you could zoom in on the Sea of Tranquility, you'd see that it's not as smooth as it seems. Great ripples and mounds undulate across its 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 brilliant companion stands below the Moon: the planet Jupiter.
The Sea of Tranquility is the large dark area near the center of the line that divides lunar night from day -- a line known as the terminator -- which we'll talk more about tomorrow.
Script by Damond Benningfield, Copyright 2010