Astronomy Magazine
Apogee and perigee
The high and low points of an orbit around Earth.
Every orbit is an ellipse, so unless it is perfectly circular there is a furthest point from Earth and a nearest one. The names change with the body being orbited — apoapsis and periapsis generally, apolune and perilune at the Moon. They matter operationally because a burn at perigee is the most efficient way to raise the far side of an orbit, which is the whole basis of the transfer manoeuvres that move spacecraft between orbits. The efficiency comes from the Oberth effect: a burn does more for you where you are already moving fastest, which is perigee. It is why a spacecraft leaving for another planet drops close to Earth first and lights the engine there, and why an escape burn from a high orbit costs more than the same burn from a low one despite starting nearer the exit.
Where it has come up lately
Related terms
Sun-synchronous orbit
A near-polar orbit that crosses the equator at the same local time every day.
Geostationary orbit
The one orbit where a satellite appears to hang motionless over a fixed point.
Kessler syndrome
The runaway scenario where collisions in orbit generate debris that causes more collisions.
Low Earth orbit
Everything from about 160 to 2,000 km up, where almost all spacecraft and all people are.
Questions
What is Apogee and perigee?
The high and low points of an orbit around Earth.
Why does it matter?
The efficiency comes from the Oberth effect: a burn does more for you where you are already moving fastest, which is perigee. It is why a spacecraft leaving for another planet drops close to Earth first and lights the engine there, and why an escape burn from a high orbit costs more than the same burn from a low one despite starting nearer the exit.