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Now they actually know the distance to the moon to within a few inches. Apollo 11 astronauts left an array of corner mirrors on the moon. A corner mirror is three mirrors arranged like a corner of a box, all at right angles to each other. Such a mirror reflects light directly back toward the source of the light. Scientists shine a laser at the moon, and measure the time it takes to reflect back. They can measure this time to within a tiny fraction of a second. So they deduce the distance to the moon (at any given time) to within a few inches.