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What is an Apogee?

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  • Written By: Brendan McGuigan
  • Edited By: Niki Foster
  • Last Modified Date: 10 November 2016
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The apogee is the point at which a body is at its furthest orbit from the Earth. The opposite of the apogee, the point at which an orbiting body is nearest to the earth, is the perigee. Apogee is an Earth-specific term, and other terms exist for different celestial bodies.

The apsis, for example, is a generic term used to describe the point at which an orbiting body is furthest from or closest to its orbital center. The specific generic term for the furthest point is apoapsis, and the term for the nearest point is the periapsis. For objects orbiting the sun, the term aphelion is used to describe the furthest point, while perihelion describes the nearest.

The word apogee comes from the Greek prefix apo- meaning 'away' and the word gaia meaning 'earth'. Apogee is a French derivation of the Latin term apogaeum from the Ptolemaic Greek apogaion.

When a body is at apogee, it is also at its minimal orbital velocity. If you view an orbit as similar to a pendulum, this becomes easy to visualize. As the object goes away from the center of orbit, its velocity slows until it reaches apogee and reverses its course. At apogee, its kinetic energy is at its minimal point and its potential energy is at its maximum. Conversely, when an object is at perigee it has its greatest kinetic energy and least potential energy, and is at its peak velocity.

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The apogee of many bodies is not a fixed number, though it tends to be very close at each orbit. When the apogee of the moon, for example, occurs during a new or full moon, the distance is greater than at other times. Peak apogees for the moon include August 4th, 2005, when the moon's apogee was 406,628km (252,667 miles), and September 22nd, 2006, when the moon's apogee is 406,499km (252,587 miles). By contrast, perigees of the moon fall in the range of 356,410km (221,460 miles).

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bluedolphin
Post 3

It's actually possible to guess when the moon is at an apogee. In the full moon state at apogee, the moon looks smaller than usual. And the perigee is apparent too because the full moon looks larger.

Another way to know apogee and perigee times is to check tides. When the moon is at apogee, tides will be lower. When the moon is farther away, it has less gravitational pull on the earth. When the moon is at perigee, the tides will be higher because of its proximity to earth and its gravitational pull.

It's all very cool to observe for those interested in these things.

burcinc
Post 2

@discographer-- Well since the orbit pattern of the moon around the earth is known, astronomers just need to use a formula to calculate the apogee and perigee. It's not at all difficult.

Calculating apogees for moons of other planets is probably more complex.

discographer
Post 1

The difference between the moon's apogee and perigee is not that great. That's kind of surprising. I had expected it to be more. How do scientists measure the apogee and perigee. Does the moon stay at its apogee point for a considerable time?

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