Perigee is when an object orbiting Earth is closest, while apogee is when it’s farthest. The moon’s distance affects tides and weather, and it appears larger in perigee. Apsis encompasses both points, and it’s important for satellite companies to calculate to maintain orbit.
When an object orbiting the Earth is at its closest point to the Earth in its orbit, this phase is referred to as perigee. Conversely, when the same object is as far in its orbit as possible, it is said to be in “apogee.” This term was first used to refer to the moon, which moves regularly through perigee and apogee as it orbits the Earth, although it can also be applied to the many artificial satellites that orbit the Earth.
The orbits are elliptical, rather than circular, which explains why satellites can be at varying distances from the Earth as they orbit. More generally, astronomers use the term “apsis” to encompass both the closest and furthest point in an orbit, with periapsis being closest and apoapsis being farthest away. When talking about things that specifically orbit the Earth, astronomers refer to apogee and perigee, and there are specialized terms for objects that also orbit other major celestial bodies, such as objects that move around the sun (perihelion and aphelion).
The moon’s different distance from Earth plays a role in weather and tides. When the moon is at perigee, it exerts more force on the Earth, which can cause more extreme tides. Weather cycles can also vary cyclically with the moon’s orbit, and as a result, meteorologists and tidal forecasters rely on astronomical projections of the moon’s distance when preparing forecasts and reports. At any moment, astronomers can calculate how far the moon is from Earth and how many days it will take for the moon to reach perigee.
Occasionally, people look at the moon and notice that it appears to be larger than usual, which doesn’t seem possible. The answer to this mystery lies in the fact that when the moon is in perigee, it actually appears slightly larger than when it’s in apogee, because it’s closer. People interested in performing a little experiment could take a picture of the full moon when the moon is at apogee and repeat the conditions when the full moon is near perigee to see an illustration of the difference in size.
Being able to calculate apogee and perigee is also important for companies that launch and maintain satellites. It is important for an object to find the right orbit so that it stays in orbit without being pulled towards Earth, and perigee can be a dangerous point in a satellite’s orbit if it is too close to Earth.
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