Subsolar point: where the Sun is directly overhead

Motion and geometry

Because a typical terrestrial planet rotates, the subsolar point travels westward relative to the surface, completing a circuit once per solar day. Its latitude equals the star’s declination (on Earth the Sun’s declination varies through the year because the axis is tilted), and its longitude follows the planet’s rotation so that local solar noon occurs at the longitude containing the subsolar point. On Earth the latitude oscillates between the Tropic of Cancer and the Tropic of Capricorn as seasons change, while at equinoxes it lies at the equator.

Calculation and practical notes

Finding the subsolar point requires the star's apparent declination and the planet's current rotation angle. For many applications you can approximate the subsolar longitude by converting the time of day to degrees of rotation (about 15° per hour on Earth). The latitude is equal to the star’s declination, which depends on orbital geometry and axial tilt.

Importance and uses

• Solar energy: locations near the subsolar point receive maximal instantaneous insolation and are useful for high-efficiency design considerations.
• Climate and meteorology: the migrating band of highest solar heating helps drive Hadley circulation and seasonal weather patterns.
• Navigation and astronomy: knowing the subsolar point relates to local solar noon and can aid timekeeping or calibration of observations.
• Planetary science: mapping subsolar motion helps model surface temperatures on planets, moons and exoplanets.

Variations on other worlds and notable facts

On tidally locked planets or moons the analogue — often called the substellar point — may remain nearly fixed above one surface location, producing a permanent day side and night side; atmospheric and oceanic flows then redistribute heat from that point. The subsolar point on bodies with complex rotations or librations (for example some moons) can trace unusual paths. For general reference see discussions of how the subsolar point circles a globe once per day and how the concept connects to local solar noon and the apparent position of the central star.

Quick facts: the subsolar point marks maximum direct sunlight; it moves westward on a rotating world; on Earth it migrates seasonally between about 23.4° N and 23.4° S; on synchronously rotating bodies it may be stationary.