For this week's APOD, I chose the picture called Lunar Nearside. It is a picture of the lunar nearside of the moon, updated on March 3, 2011. It is compiled from 1,300 different images from the Lunar Reconnaissance Orbiter spacecraft's wide angle camera. These LRO pictures were all recorded over a two week period last December.
The lunar nearside exists because the Moon rotates on its axis and orbits the Earth at the same rate, which would be about once every 28 days. The rotation always appears to keep one side, called the nearside, facing towards Earth. The Moon has this synchronous rotation because of Earth's gravitational field, which raises solid-body ties on the Moon. It is because of all this that this high resolution compilation image of the moon, lunar maria (which are dark and smooth, and are actually lava-flooded impact basins), and rugged highlands, are so well known to astronomers and sky gazers here on Earth. For this image, one can slide the cursor from a computer over the picture, to see all of the mares and main craters labeled.
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