Argument of Perigee

The argument of perigee (ω) is one of the six classical Keplerian elements. It measures the angle in the orbital plane from the ascending node (where the satellite crosses the equator northbound) to the point of closest approach to Earth (perigee). For circular orbits where perigee is undefined, it is conventionally set to 0°. The critical inclination of 63.4° prevents the argument of perigee from precessing, which is why Molniya orbits use that inclination — it keeps perigee locked over the southern hemisphere.