Argument of Perigee
An orbital element that defines the orientation of an elliptical orbit within its plane by measuring the angle from the ascending node to perigee.
Explanation
The argument of perigee (ω, omega) is one of the six Keplerian orbital elements. It describes where the low point of an orbit (perigee) is located relative to the equatorial crossing point (ascending node). An argument of perigee of 0 degrees means perigee occurs at the ascending node (equatorial crossing heading north), while 90 degrees means perigee occurs at the northernmost point of the orbit. This element is particularly important for highly elliptical orbits like Molniya orbits, where the argument of perigee controls the latitude at which the satellite spends most of its time at apogee. For Molniya communications satellites, the argument of perigee is set to 270 degrees so apogee occurs over the Northern Hemisphere. The argument of perigee precesses over time due to Earth's J2 gravitational perturbation, requiring periodic correction for missions that need a specific ground track.
Why It Matters
The argument of perigee determines where in an orbit a satellite is fastest (perigee) and slowest (apogee), which controls coverage patterns for elliptical orbits and affects ground-track evolution.
Concept Map
How Argument of Perigee connects to other glossary terms:
Frequently Asked Questions
Why does argument of perigee matter for Molniya orbits?
A Molniya orbit with argument of perigee at 270 degrees places apogee over the Northern Hemisphere, where the satellite lingers for hours providing coverage.
Does argument of perigee change over time?
Yes. Earth's J2 perturbation causes argument of perigee to precess. The rate depends on altitude, inclination, and eccentricity.
Sources
Last updated: July 1, 2026