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The substellar point of a planet (or moon) is the point on its surface such that some particular star of interest is at the zenith, which is (roughly) the surface-location on the line between the center of the planet and the center of the star. A planet's substellar point for a particular star is generally constantly shifting across its surface. The term subsolar point indicates the surface-location at which the Sun is at the zenith. On solar system bodies, the term subsolar point is commonly used regarding the effects of the Sun or regarding navigation. For navigation across Earth, such as past ocean navigation, the substellar point of other stars have been of interest. References to an extra-solar planet's substellar point generally refer to that regarding its host star.