In ancient times, it was believed that Earth is stable and Sun is orbiting the Earth. Afterwards along with the advancement of science and technology, it is proved wrong. Renaissance-era polymath Nicolaus Copernicus was first to prove this. In his 'Theory of Universe', he corroborated that the Sun rather than Earth at the center of the universe. Later on Italian astronomer Galileo Galilei also supported this concept.


Perigee and Apogee

There are always two apsides or extreme points (i.e., nearest and farthest) of any elliptical orbit. Each point is named with an appropriate prefix (for instance, "peri-", "ap-", "apo-", etc.), joining with a reference suffix of the host body (like "-gee", "-helion", etc.).

The terminologies 'Perigee' and 'Apogee' are used in the context of two apsides of the orbit of moon around the earth. The lunar orbit is elliptical with one side closer to earth than the other. So the distance between moon and earth varies throughout the months of a year. The average lunar distance is 3,84,402 km. (2,38,856 mi.).
Lunar Perigee and Apogee
An Illustration of Lunar Perigee and Apogee
The nearest point of the lunar orbit to the Earth is called 'Perigee', whereas the farthest point is known as 'Apogee'. Orbital distance in Perigee is around 3,62,600 km. and for Apogee it is 4,05,400 km. 27 days 7 hours 43 minutes 11.6 seconds is taken for the moon to travel from perigee to perigee. This is known as 'Anomalistic Month'.

Supermoon and Micromoon

Perigee and Apogee
A comparison between Supermoon and Micromoon
Moon passes through the apogee, or perigee about once a month. Thus, when a full moon occurs around its perigee position, then it is called 'Supermoon', because moon comes to the closest distance to earth and it looks biggest than all other times. In other hand, when full moon occurs near about its apogee position, it looks smallest for being in the farthest distance from earth. It is called 'Micromoon'.

Related Terminologies

In the context of orbital apsides of other celestial objects, the terminology changes, as the suffix varies for each celestial bodies.
  • For a non-specified primary object, the terms 'pericenter' and 'apocenter' are used to denote respectively nearest and farthest point of orbit around it.
  • If a celestial object (e.g., Earth, Jupiter, Halley's Comet, etc.) orbits the sun, then the extreme points of the orbit are termed as 'perihelion' (closest distance) and 'aphelion' (farthest distance).
  • Likewise for any other star rather than sun, the terms become 'periastron' and 'apastron'.
  • Moon has no natural satellites. For the case of artificial satellites or man-made objects that are launched from earth into the lunar orbit, the least distance is called 'pericynthion', and the greatest distance is called 'apocynthion'. Sometimes these are also called 'perilune' and 'apolune'.
  • The apsides of the orbit of Ganymede around Jupiter, the extreme points are known as 'perijove' and 'apojove".

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