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 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 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 14%, between 356,500 km at perigee (closest approach to the Earth) and 406,700 km at apogee (furthest from the Earth).

Its angular size also varies by the same factor, between 29.4 arcmin and 33.5 arcmin and its brightness also changes, though this is hard to detect in practice since the Moon's phases are changing at the same time.

The chart below shows this variation in the Moon's angular size to scale. The change is small enough that it is virtually impossible to perceive except by comparing photographs:

The Moon
Full Moon
at perigee
The Moon
Full Moon
at apogee

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.

This perigee will coincide closely with the time of month when the Moon is at full phase, so the full moon this month will appear fractionally larger and brighter than usual.

The Moon illusion

Although the angular size of the Moon only changes by a very modest amount in reality, a very common optical illusion is that the Moon appears very much larger than it really is when it is close to the horizon. This is called the Moon illusion – and is nothing more than an optical illusion. Any photograph will reveal that the Moon is exactly the same size regardless of whether it appears on the horizon or directly overhead.

The reason why we perceive this optical illusion is hotly debated. However, it may explain why some people are convinced that the Moon appears larger on some nights than others, despite the actual changes in its true size being so small.

Celestial coordinates

On this occasion the Moon will pass within a distance of 357,000 km of the Earth, and appear with an angular diameter of 33.41 arcmin.

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

Object Right Ascension Declination Constellation Angular Size
The Moon 18h01m50s 26°40'S Sagittarius 33'24"

The coordinates above are given in J2000.0.

The sky on 14 June 2022
Sunrise
05:19
Sunset
20:27
Twilight ends
22:35
Twilight begins
03:11

15-day old moon
Waning Gibbous

98%

15 days old

Planets
Rise Culm. Set
Mercury 04:15 11:19 18:22
Venus 03:35 10:36 17:37
Moon 21:12 00:34 05:07
Mars 02:03 08:21 14:39
Jupiter 01:39 07:45 13:52
Saturn 00:02 05:14 10:26
All times shown in EDT.

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

14 Jun 2022  –  Full Moon
20 Jun 2022  –  Moon at Last Quarter
28 Jun 2022  –  New Moon
06 Jul 2022  –  Moon at First Quarter

Image credit

Simulated image courtesy of Tom Ruen.

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41.14°N
73.26°W
EDT

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