Simulated image courtesy of Tom Ruen.

The Moon at perigee

Dominic Ford, Editor
From the Moon feed

Objects: The Moon
Please wait
Loading 0/4
Click and drag to rotate
Mouse wheel to zoom in/out
Touch with mouse to dismiss
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 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.

The perigee of 7 September 2022 will occur when the Moon is close to full phase, and so it 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 364,000 km of the Earth, and appear with an angular diameter of 32.77 arcmin.

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

Object Right Ascension Declination Constellation Angular Size
The Moon 20h49m50s 22°57'S Capricornus 32'46"

The coordinates above are given in J2000.0.

The sky on 07 September 2022
Sunrise
06:16
Sunset
19:12
Twilight ends
20:48
Twilight begins
04:36

11-day old moon
Waxing Gibbous

93%

11 days old

Planets
Rise Culm. Set
Mercury 08:23 14:01 19:40
Venus 05:11 11:58 18:44
Moon 18:07 22:50 02:19
Mars 22:50 06:14 13:39
Jupiter 19:58 02:04 08:10
Saturn 18:08 23:10 04:12
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

03 Sep 2022  –  Moon at First Quarter
10 Sep 2022  –  Full Moon
17 Sep 2022  –  Moon at Last Quarter
25 Sep 2022  –  New Moon

Image credit

Simulated image courtesy of Tom Ruen.

Share

Follow

Cambridge

Latitude:
Longitude:
Timezone:

42.38°N
71.11°W
EDT

Color scheme