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Star patterns like constellations are very helpful for navigating a spacecraft. Most spacecraft have steered by the stars, or at least checked the stars once in a while to make sure the spacecraft was still on course and pointed in the right direction. The star tracker, like a camera, takes a picture of the star patterns in its view and compares the picture with its built-in star maps. This is how it can tell the spacecraft exactly which way it is pointed. This capability is particularly useful for TESS, which must view the same area of sky for nearly a month in order to reveal any brightness changes of stars that could indicate orbiting planets.