periheliin

Periheliin, often rendered in English as perihelion, is the point in the orbit of a body around the Sun where the body is closest to the Sun. It is a fundamental concept in celestial mechanics and orbital dynamics. The term derives from Greek peri- meaning near and Helios, the Sun; variants of the term exist in several languages, including the form periheliin in some language traditions.

In an elliptical orbit with semi-major axis a and eccentricity e, the periheliin distance q is q = a(1 - e). The corresponding aphelion distance is Q = a(1 + e). The radial distance as a function of true anomaly ν is r(ν) = a(1 - e^2)/(1 + e cos ν). If e = 0, the orbit is circular and q = a. Periheliin is one of the two classical orbital extrema, the other being aphelion, the farthest point from the Sun.

Earth’s periheliin occurs around early January, at about 0.983 astronomical units from the Sun. Mercury’s periheliin lies near 0.307 AU. Periheliin affects solar irradiance only modestly, but the resulting variation in orbital speed follows Kepler’s laws. The concept is essential for accurate modeling of planetary motions, planning interplanetary missions, and understanding the long-term dynamics of comets and asteroids.