FRANK ZULLO / SCIENCE PHOTO LIBRARY FRANK ZULLO / SCIENCE PHOTO LIBRARY
Amateur astronomers. Time-exposure photograph of a crescent Moon at dusk with silhouetted amateur astronomers setting up equipment in the foreground. The Moon appears as a crescent when it is located between the Earth and the Sun. The visible shape is due to the Moon's shadow blocking sunlight from most of the side facing towards us. The Moon is the Earth's only natural satellite. The mass of the Moon is unevenly distributed and the heavier side always faces towards the Earth. This means that half of the Moon's surface can never be seen from Earth. This unseen portion is called "the dark side of the Moon" regardless of which part is actually illuminated by the Sun.
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