The Earth's only natural satellite is a spectacular sight even with the naked eye. With a small telescope or pair of binoculars, the view is even more amazing. Dark, flat plains called maria, deep craters and bright rays of ejected material pepper the rugged surface.
As the moon orbits Earth, it always keeps one face toward the planet. The permanently hidden part is properly called the "far" side – not the "dark" side. In fact, the part of the moon that is dark changes constantly. The part that is illuminated indicates the moon’s phase. A full cycle of phases requires 29.53 days, or a lunar month.
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In 1500 there were no telescopes, but Leonardo da Vinci was able to observe that the dark part of the crescent moon still has a faint glow. He correctly surmised that this was due to reflected light from Earth.
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As the moon orbits, it rocks back and forth a little, a phenomenon called libration. This allows people to see just a little bit over the edge, into the far side. About 59 percent of the entire lunar surface is visible from Earth.
Today, the moon has been thoroughly mapped by orbiting satellites and walked upon by human visitors. Nevertheless the view of the moon from Earth is still a breathtaking sight.
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