Perigee is the point in an orbit closest to Earth and apogee is the farthest; a satellite moves fastest at perigee and slowest at apogee.
Most orbits are slightly oval, not perfect circles. So every orbit has a closest point and a farthest point.
For an orbit around Earth these are the perigee (closest) and apogee (farthest). The general words are periapsis and apoapsis. Around the Moon they become perilune and apolune; around the Sun, perihelion and aphelion. Together they are often just written as one pair: apogee and perigee.
Catalogues and trackers usually list heights above Earth's surface. NVS-02, the Indian navigation satellite stuck in its transfer orbit, is listed at 214 × 37,251 km. That means its perigee is 214 km above the ground and its apogee 37,251 km.
It is at its fastest near perigee and slowest near apogee. Worked out from those heights, it races through perigee at about 10.2 km/s but crawls through apogee at about 1.5 km/s.
A low perigee dips a satellite into thin traces of air on every lap. Drag acts mostly there, so over time it slowly pulls the apogee down. Below about 120 to 160 km, drag brings a satellite down within days.
A high apogee means long, slow stretches far from Earth. NVS-02 spends most of each 11-hour lap high above the planet and sweeps through its low point in a short time.
The Moon's own orbit has a perigee of about 363,300 km and an apogee of about 405,500 km from Earth.
When Chandrayaan-3 arrived at the Moon on 5 August 2023, ISRO reported its first lunar orbit as 164 × 18,074 km. By 16 August it had been lowered to 153 × 163 km, nearly circular. Before landing, the lander dropped its perilune to just 25 km, in a 25 × 134 km orbit.
India's Mars Orbiter Mission entered Mars orbit on 24 September 2014 with its closest point (periapsis) 421.7 km above Mars and its farthest (apoapsis) 76,993.6 km. One lap took 72 hours 51 minutes 51 seconds.
Watch NVS-02's estimated altitude: it spends hours climbing slowly to apogee, then drops quickly through perigee. Track NVS-02
For an ellipse with semi-major axis a and eccentricity e, measured from the centre of the body being orbited:
r_p = a (1 − e), r_a = a (1 + e), so a = (r_p + r_a)/2 and e = (r_a − r_p)/(r_a + r_p).
Catalogue heights must be converted to radii first by adding the planet's radius (6,378 km for Earth).
Worked numbers for NVS-02:
Check with conservation of angular momentum: at the two ends of the orbit the velocity is at right angles to the radius, so r_p·v_p = r_a·v_a. Here 6,592 × 10.25 = 67,570 and 43,629 × 1.549 = 67,580 — equal, as they must be. The speed ratio is just the inverse of the distance ratio: v_p / v_a = r_a / r_p ≈ 6.6.
This is also why a satellite spends most of its time near apogee: it is moving slowly there. For NVS-02, the half of the orbit farther from Earth takes up most of each 660-minute lap.
Perigee is the point of a satellite's orbit nearest to Earth; apogee is the point farthest from Earth.
At perigee. Its angular momentum r × v stays constant, so where r is smallest v must be largest.
Add Earth's radius to each height (6,592 and 43,629 km) and average them: a ≈ 25,111 km.