The celestial equator intersects the ecliptic in two points. The vernal equinox is the one at which the Sun in its apparent annual motion crosses from South to North of the equator. The vernal equinox can be used as a reference point to define the celestial longitude locating the Earth along its orbit during the year. The longitude of the perihelion (Ï–) represents the angular position of the perihelion of the orbit from the vernal equinox. This angle gives the position of the Earth on its orbit at the spring equinox. Knowing the longitude of the perihelion, it is possible to determine the Earth-Sun distance at any time in the year. Climatic precession (e sin Ï–) is usually used instead of the longitude of the perihelion. It is the main factor driving the 24 h-mean irradiance.
Astronomers of old civilizations kept records of the direction of the Earth’s axis of rotation in the sky. Five thousand years ago, stars circled around α-Draco’s in the night sky, while it is now around αUrsa...
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Loutre, MF. (2009). Precession, Climatic. In: Gornitz, V. (eds) Encyclopedia of Paleoclimatology and Ancient Environments. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-4411-3_194
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