Argument of periapsis

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 or perigee" (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 Periapsis and Longitude of Ascending Node of Various Planet Classes

Argument of Periapsis and Longitude of Ascending Node of Planets

Planet	Argument of periapsis (ω) in degrees	Longitude of Ascending node (☊) 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

Argument of Periapsis and Longitude of Ascending Node of Dwarf Planets

Dwarf Planet	Argument of Periapsis (ω) in Degrees	Longitude of Ascending Node (☊) in Degrees
Ceres	73.1	80.7
Pluto	113.175	110.088
Haumea	238.779	122.1628
Makemake	294.835	79.6194
Eris	151.643	35.95086

References

1. Willman, Alexander J. “Sol Planetary System Data.” Princeton University, The Trustees of Princeton University, 1996, https://www.princeton.edu/~willman/planetary_systems/Sol/.