What is the difference between perigee and apogee?
The Moon does not remain at one fixed distance from Earth. Its orbit is slightly elliptical and is also perturbed by the gravity of the Sun and planets. Perigee is the nearest point in a particular orbit; apogee is the farthest.
Typical distances are roughly 363,000 kilometers near perigee and 405,000 kilometers near apogee, measured from the centers of Earth and Moon, though individual values vary.
The comparison is simple:
- Perigee: Closest to Earth, with a somewhat larger apparent disk and stronger lunar tidal influence.
- Apogee: Farthest from Earth, with a somewhat smaller apparent disk and weaker lunar tidal influence.
- Timing: Each occurs about once per orbit, regardless of the Moon’s phase.
What changes for an observer
At perigee, the Moon subtends a larger angle and appears somewhat larger and brighter. At apogee, it appears smaller. The changing angular size is enough to affect solar eclipses. A new Moon relatively close to Earth may fully cover the Sun and create totality. A more distant Moon may leave a bright ring and produce an annular eclipse.
Distance also influences tidal force. A perigean new or Full Moon can enhance the already stronger spring-tide range, although coastline shape, water depth, weather, and air pressure remain essential to actual water levels.
Perigee is not always a supermoon
Perigee occurs once each orbit, including at crescent, quarter, and new phases. The popular supermoon label applies only when a Full Moon happens near perigee. Likewise, apogee alone is not a micromoon unless it nearly coincides with fullness under the chosen definition.
