A quick glimpse at the first-quarter Moon tonight reveals a large round dark patch toward the top of the lunar disk, right along the line that divides night from day. The patch is the Sea of Serenity -- a volcanic plain that covers an area as big as New Mexico.
To the eye alone, it looks smooth and unblemished. But even a modest telescope reveals that the Sea of Serenity is more complex than that. When the Sun is at a low angle, as it is now, you can see long ridges and wrinkles that can stretch for dozens of miles. These features formed over hundreds of millions of years, as lava filled and refilled a circular basin.
Lava first flowed into the basin almost four billion years ago. As it did so, its weight probably mashed down the center of the basin, forming a bowl in the middle. A few hundred million years later, the bowl was filled with fresh lava. It, too, pushed downward, forming another bowl that was filled a few hundred million years after that.
The first flooding is still visible as a dark ring around the edge of the Sea of Serenity. The second flooding is somewhat lighter, with the final flooding lighter still. These three episodes give the Sea of Serenity a complicated story -- a story told in its dark volcanic rocks.
The Moon is well up in the southwest as night falls. Regulus, the brightest star of Leo, the lion, hovers a little to its upper left. They set in the wee hours of the morning.
Script by Damond Benningfield, Copyright 2011
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