The subsolar point on a

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

is the point at which its sun is perceived to be directly overhead (at the

zenith The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction ( plumb line) opposite to the gravity direction at that location ( nadir). The zenith is the "high ...

); that is, where the sun's rays strike the planet exactly

perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', ⟂. It c ...

to its surface. It can also mean the point closest to the sun on an

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often u ...

, even though the sun might not be visible. To an observer on a planet with an orientation and

rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

similar to those of

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

, the subsolar point will appear to move westward, completing one circuit around the globe each day, approximately moving along the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...

. However, it will also move

north and south North and South may refer to: Literature * ''North and South'' (Gaskell novel), an 1854 novel by Elizabeth Gaskell * ''North and South'' (trilogy), a series of novels by John Jakes (1982–1987) ** ''North and South'' (Jakes novel), first novel ...

between the

tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also refer ...

over the course of a year, so it is spiraling like a helix. The subsolar point contacts the

Tropic of Cancer The Tropic of Cancer, which is also referred to as the Northern Tropic, is the most northerly circle of latitude on Earth at which the Sun can be directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted towa ...

on the

June solstice The June solstice is the solstice on Earth that occurs annually between 20 and 22 June according to the Gregorian calendar. In the Northern Hemisphere, the June solstice is the summer solstice (the day with the longest period of daylight), wh ...

and the

Tropic of Capricorn The Tropic of Capricorn (or the Southern Tropic) is the circle of latitude that contains the subsolar point at the December (or southern) solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reac ...

on the

December solstice The December solstice, also known as the southern solstice, is the solstice that occurs each December – typically on 21 December, but may vary by one day in either direction according to the Gregorian calendar. In the Northern Hemisphere, the ...

. The subsolar point crosses the

Equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...

on the March and

September equinox The September equinox (or southward equinox) is the moment when the Sun appears to cross the celestial equator, heading southward. Because of differences between the calendar year and the tropical year, the September equinox may occur anyt ...

es.

Coordinates of the subsolar point

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:

\phi_ = \delta,

\lambda_ = -15(T_\mathrm-12+E_\mathrm/60).

where *

\phi_

is the latitude of the subsolar point in degrees, *

\lambda_

is the longitude of the subsolar point in degrees, *

\delta

is the

declination of the Sun The position of the Sun in the sky is a function of both the time and the geographic location of observation on Earth's surface. As Earth orbits the Sun over the course of a year, the Sun appears to move with respect to the fixed stars on th ...

in degrees, *

T_\mathrm

is the Greenwich Mean Time or UTC, *

E_\mathrm

is the equation of time in minutes.

Observation in specific locations

* Qibla observation by shadows, when the subsolar point passes through the Ka'bah in Saudi Arabia, allowing the Muslim sacred direction to be found by observing shadows. * When the point passes through

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...

, which is the only U.S. state in which this happens, the event is known as Lahaina Noon.

References

External links

Day and Night World Map
(shows location of subsolar point for any user-specified time) {{DEFAULTSORT:Subsolar Point Spherical astronomy Earth Astronomical coordinate systems