FRANK ZULLO / SCIENCE PHOTO LIBRARY FRANK ZULLO / SCIENCE PHOTO LIBRARY
Crescent Moon over the silhouette of a sahuaro cactus at dawn. The Moon is the Earth's only natural satellite. It measures 3476km in diameter and lies at a mean distance of 384,000km from Earth. Its period of rotation (27.3 days) is the same as the time it takes to complete a single orbit, which is why the Moon keeps the same face towards us at all times. The visible side is characterised by large, dark basins called lunar maria (seas) and numerous craters produced by falling meteoroids. The lunar maria are almost completely crater-free, which suggests they were filled in recent geological times by volcanic material. Photographed in Arizona, USA.
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