Subsolar

The subsolar point is the point on a planet’s surface where the Sun is at the zenith, i.e., directly overhead. The concept applies to any body with a surface that receives light from a star.

As a planet rotates and orbits its star, the subsolar point traces a path across the surface. On Earth, its latitude equals the solar declination, which varies throughout the year between about +23.44° and −23.44°, corresponding to the Tropics of Cancer and Capricorn. The longitude changes with the planet’s rotation, so the subsolar point moves west to east in the planet’s reference frame and marks local solar noon at any given location. At the equinoxes the subsolar point lies on the equator; at the solstices it reaches the tropics.

The subsolar point is a fundamental reference for the distribution of insolation, the amount of solar radiation reaching a surface, and therefore for climate and weather modeling, daylight duration, and solar-energy planning. It helps explain seasonal temperature patterns and energy balance on a planetary scale.

In other bodies, the same idea applies. A rapidly rotating planet with axial tilt exhibits a roaming subsolar latitude, while a tidally locked body has a fixed subsolar point on its dayside. In all cases, the subsolar point indicates where sunlight is most direct and noon occurs.