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Argument of periapsis

one of the orbital elements used to specify the orbit of an object in space
A diagram of Keplerian orbital elements.

In astronomy, the argument of periapsis (ω) is a way of talking about the orbit of a planet, asteroid or comet. It is also known as the argument of perihelion or the argument of perifocus. It is the angle (starting from the center of the orbit) between an orbiting body's periapsis and its ascending node. Periapsis is the point when the orbiting object comes the closest to the thing it is orbiting around; for example, the moon is at periapsis when it is closest to the Earth. The ascending node is one of two places where an orbiting object passes through the reference plane, an imaginary flat surface which runs through the object being orbited around. The size of the angle depends on which way the object is orbiting.

The angle is measured in the orbital plane and in the direction of motion. For specific types of orbits, words such as "perihelion" (for Sun-centered orbits), "perigee" (for Earth-centered orbits), "pericenter" (general), etc. may replace the word "periapsis".

An argument of periapsis of 0° means that the orbiting body will be at its closest approach to the central body at the same moment that it crosses the plane of reference from south to north. An argument of periapsis of 90° means that the orbiting body will reach periapsis at its northmost distance from the plane of reference.

Argument of perihelion and Longitude of ascending node of planets.

Planet Argument of periapsis (ω) in degrees Longitude of Ascending node (Ω) in degrees
Mercury 29.022 48.378
Venus 54.780 76.785
Earth 85.901 18.272
Mars 286.231 49.667
Jupiter 273.442 100.398
Saturn 336.178 113.759
Uranus 98.862 73.967
Neptune 256.932 131.823
Pluto 113.175 110.088