Perihelion
Perihelion is the point in an orbit at which an object is closest to the Sun. Its distance and timing depend on the orbit’s shape and orientation.
Elliptical orbit: An orbit shaped like an ellipse, with the central body at one focus. For an elliptical orbit around the Sun, perihelion is one end of the ellipse’s major axis.
Aphelion: The point in an orbit at which an object is farthest from the Sun. It is the opposite endpoint of a Sun-centered orbit’s major axis.
Earth's orbit: The path Earth follows around the Sun, with a period of about one year. Earth reaches perihelion in early January, when it is closest to the Sun.
Apsidal precession: The gradual rotation of an orbit’s line of apsides over time. It shifts the direction of perihelion from one orbit to the next.
Orbital eccentricity: A dimensionless measure of how much an orbit deviates from a circle. Eccentricity determines how far perihelion lies from an orbit’s average distance.
Perigee: The point in an Earth orbit at which an object is closest to Earth. It uses Earth, rather than the Sun, as the central body.
Kepler's laws of planetary motion: Three laws describing planetary orbits, orbital speeds, and the relation between period and distance. The equal-area law predicts that an orbiting body moves fastest near perihelion.
General relativity: Einstein’s theory of gravity, describing gravitation as the curvature of spacetime. Its predictions account for the portion of Mercury’s perihelion advance unexplained by Newtonian gravity.
Semimajor axis: Half the longest diameter of an ellipse, used as a measure of orbital size. Together with eccentricity, it sets the perihelion distance: the semimajor axis times one minus eccentricity.
Apogee: The point in an Earth orbit at which an object is farthest from Earth. It is the Earth-centered counterpart to aphelion, not perihelion.