Simulated image courtesy of Tom Ruen.

The Moon at perigee

Dominic Ford, Editor
From the Moon feed

Objects: The Moon
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The sky at

The Moon will reach the closest point along its orbit to the Earth and will appear slightly larger than at other times.

The Moon's distance from the Earth varies because its orbit is not perfectly circular – it is instead slightly oval-shaped, tracing out a path called an ellipse.

As the Moon traverses this elliptical path around the Earth each month, its distance varies by around 10%, between 363,000 km and 405,000 km. Its angular size also varies by the same factor, and its brightness also changes, though this is hard to detect in practice, given the Moon's phases are changing at the same time.

The exact period of the Moon's cycle between perigee (closest approach), apogee (furthest recess) and back again is 27.555 days – a period of time called an anomalistic month. This is very close to the Moon's orbital period (27.322 days), but slightly longer. For more information on why these periods don't exactly match, see In-The-Sky.org's glossary article for the term month.

The perigee of 1 January 2010 will occur when the Moon is close to full phase, and so it will appear fractionally larger and brighter than usual.

On this occasion the Moon will pass within a distance of 358,000 km of the Earth, and appear with an angular diameter of 33.30 arcmin. This may be compared to its average size of 31.07 arcmin.

The position of the Moon at the moment of perigee will be:

Object Right Ascension Declination Constellation Angular Size
The Moon 07h51m30s +20°29' Gemini 33'17"

The coordinates above are given in J2000.0.

The Moon illusion

The Moon's cycle between perigee and apogee is a genuine variation in the Moon's angular size. This should not be confused with the Moon illusion – an optical illusion that makes the Moon appear much larger than it really is when it is close to the horizon. The reason why we experience this optical illusion is still hotly debated.

The sky on 28 October 2020
Sunrise
07:48
Sunset
17:58
Twilight ends
19:41
Twilight begins
06:04

12-day old moon
Waxing Gibbous

91%

12 days old

Planets
Rise Culm. Set
Mercury 07:13 12:27 17:42
Venus 04:35 10:44 16:53
Moon 17:22 23:24 04:20
Mars 17:18 23:42 06:11
Jupiter 13:50 18:07 22:24
Saturn 14:07 18:30 22:53
All times shown in PDT.

Source

The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Related news

01 Jan 2010  –  The Moon at perigee
07 Jan 2010  –  Moon at Last Quarter
14 Jan 2010  –  The Moon at perihelion
14 Jan 2010  –  New Moon

Image credit

Simulated image courtesy of Tom Ruen.

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