The subsolar point on a
planet is the point at which its
Sun is perceived to be directly overhead (at the
zenith);[1] that is, where the Sun's
rays strike the planet exactly
perpendicular to its surface. It can also mean the point closest to the Sun on an
astronomical object, even though the Sun might not be visible.
To an observer on a planet with an
orientation and
rotation similar to those of
Earth, the subsolar point will appear to move westward with a speed of 1600 km/h, completing one circuit around the
globe each day, approximately moving along the
equator. However, it will also move
north and south between the
tropics over the course of a year, so will appear to spiral like a
helix.
The subsolar point moves constantly on the surface of the Earth, but for any given time, its coordinates, or
latitude and
longitude, can be calculated as follows:[2]
where
is the latitude of the subsolar point in degrees,
is the longitude of the subsolar point in degrees,