An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time

THE CRATERED FACE OF THE MOON

On its way to Jupiter, the Galileo spacecraft captured this view of the north polar region of the Moon in 1992. You can see craters of all sizes, each made when a chunk of rock (or occasionally, ice) hit the Moon and exploded from the violence of the impact. (Courtesy of Jet Propulsion Laboratory/NASA)

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