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 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.
As the perigee of 11 September 2021 will occur close to the time of new moon, the moon will appear as no more than a thin crescent.
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.
On this occasion the Moon will pass within a distance of 368,000 km of the Earth, and appear with an angular diameter of 32.41 arcmin.
The position of the Moon at the moment of perigee will be:
|Object||Right Ascension||Declination||Constellation||Angular Size|
The coordinates above are given in J2000.0.
|The sky on 11 September 2021|
4 days old
All times shown in EDT.
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.
|06 Sep 2021||– New Moon|
|13 Sep 2021||– Moon at First Quarter|
|20 Sep 2021||– Full Moon|
|28 Sep 2021||– Moon at Last Quarter|
Simulated image courtesy of Tom Ruen.