The most general of these terms is ‘periapse’; this signifies the point in an orbit nearest to the focus of gravitational attraction. (The line of apsides is the long axis of an elliptical orbit; see Apsides, apsis). The reverse of this condition is the point of greatest distance from the focus, the ‘apoapse’ (see also Aphelion ).
The perihelion is the position in the elliptical orbit of a planet, asteroid, periodic comet, or spacecraft around the Sun, where the object reaches its closest approach to the Sun. Also, in a non-periodic parabolic orbit of a comet, it is the position at the instant when the comet passes closest to the Sun. From the Greek prefix peri- (about or near) plus helios (the Sun); in other cases, plus gea (Earth), and so on. Due to the motion of the apsides (q.v.) the date of Earth's perihelion drifts through time. It is now about 2–4 January; in the 19th century it was in late December.
The perigee is an analogous parameter referred to the Earth; it is the position...
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Wood, F.J., Fairbridge, R.W. (1997). Periapse, perihelion, perigee, peribac.
In: Encyclopedia of Planetary Science. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4520-4_298
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