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The subsolar point on a planet or a moon 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.
A 2021 publication presents a method that uses a solar azimuth formula based on the subsolar point and the atan2 function, as defined in Fortran 90, that gives an unambiguous solution without the need for circumstantial treatment. [7] The subsolar point is the point on the surface of the Earth where the Sun is overhead.
At the June solstice the subsolar point is further north than any other time: at latitude 23.44° north, known as the Tropic of Cancer. Similarly at the December solstice the subsolar point is further south than any other time: at latitude 23.44° south, known as the Tropic of Capricorn. The subsolar point will cross every latitude between ...
The subsolar point travels through the tropics. Hawaii is the only US state in the tropics and thus the only one to experience Lāhainā Noon. [ 5 ] In 2022 and 2023, the phenomenon occurred in Honolulu on May 26 and July 16. [ 4 ]
This means that the tropical zone includes everywhere on Earth which is a subsolar point at least once during the solar year. Thus the maximum latitudes of the tropics have equal distances from the equator on either side. Likewise, they approximate the angle of the Earth's axial tilt.
The term meridian comes from the Latin meridies, meaning "midday"; the subsolar point passes through a given meridian at solar noon, midway between the times of sunrise and sunset on that meridian. [3] Likewise, the Sun crosses the celestial meridian at the same time.
Systematically observing the sunrise, people discovered that it occurs between two extreme locations at the horizon and eventually noted the midpoint between the two. Later it was realized that this happens on a day when the duration of the day and the night are practically equal and the word "equinox" comes from Latin aequus, meaning "equal", and nox, meaning "night".
While the formula can be derived by applying the cosine law to the zenith-pole-Sun spherical triangle, the spherical trigonometry is a relatively esoteric subject.. By introducing the coordinates of the subsolar point and using vector analysis, the formula can be obtained straightforward without incurring the use of spherical trigonometry.