FRANK ZULLO / SCIENCE PHOTO LIBRARY FRANK ZULLO / SCIENCE PHOTO LIBRARY
Full moon. A sahuaro cactus appears in silhouette against a rising full moon. A full moon occurs once every 29.5 days, when its disc is completely illuminated by the Sun and out of Earth's shadow. The Moon's period of rotation (27.3 days) is the same as the time it takes to complete a single orbit of the Earth, which is why it keeps the same face towards us at all times. Its 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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