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 The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

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 "highest" ...

); that is, where the sun's

rays Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gra ...

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 can ...

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 us ...

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

orientation Orientation may refer to: Positioning in physical space * Map orientation, the relationship between directions on a map and compass directions * Orientation (housing), the position of a building with respect to the sun, a concept in building de ...

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 surfa ...

, the subsolar point will appear to move westward, completing one circuit around the

globe A globe is a spherical model of Earth, of some other celestial body, or of the celestial sphere. Globes serve purposes similar to maps, but unlike maps, they do not distort the surface that they portray except to scale it down. A model 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 als ...

. 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 referred to ...

over the course of a year, so it is spiraling like a

helix A helix () is a shape like a corkscrew or spiral staircase. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as two intertwined helic ...

. 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 toward ...

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), whil ...

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 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 als ...

on the

March March is the third month of the year in both the Julian and Gregorian calendars. It is the second of seven months to have a length of 31 days. In the Northern Hemisphere, the meteorological beginning of spring occurs on the first day of Marc ...

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 anytime ...

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 in degrees, *

T_\mathrm

is the Greenwich Mean Time or UTC, *

E_\mathrm

is the

equation of time In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in F ...

in minutes.

Observation in specific locations

Qibla observation by shadows Twice every year, the Sun culminates at the zenith of the ''Kaaba'' in Mecca, the holiest site in Islam, at local solar noon, allowing the qibla (the direction towards the ''Kaaba'') to be ascertained in other parts of the world by observing the ...

, 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 the United States, a state is a constituent political entity, of which there are 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory where it shares its sove ...

in which this happens, the event is known as

Lahaina Noon Lāhainā Noon is a semi-annual tropical solar phenomenon when the Sun culminates at the zenith at solar noon, passing directly overhead (above the subsolar point). The term ''Lāhainā Noon'' was coined by the Bishop Museum in Hawai'i. Detai ...

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