Perihelion

Perihelion is the point in the orbit of a celestial body around the Sun where the body is nearest to the Sun. It applies to planets, dwarf planets, comets, asteroids, and spacecraft in solar orbits. The term is specific to Sun-centered (heliocentric) orbits; the general term for the closest approach to any focus is periapsis, with aphelion or apoapsis as the farthest point.

In orbital mechanics, the distance at perihelion, q, relates to the orbit’s semi-major axis a and eccentricity e by q = a(1 − e). The eccentricity measures how elongated the orbit is (e = 0 for a circle, e approaching 1 for a highly elongated ellipse). The orbital speed is highest at perihelion, as described by the vis-viva equation: v^2 = μ(2/r − 1/a), so at r = q the speed is greater than elsewhere in the orbit.

Examples include Earth, whose perihelion occurs in early January at about 147 million kilometers from the Sun (near 1 AU), with aphelion in early July at about 152 million kilometers. Other bodies exhibit a wide range of perihelion distances; Halley’s Comet, for instance, has a perihelion near 0.6 AU during its return. Perihelion timing and distance influence solar radiation received, though seasons are mainly governed by axial tilt rather than distance alone. Mercury’s perihelion precession provided important tests of general relativity and planetary perturbations.