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.
As the perigee of 13 September 2017 will occur close to the time of new moon, the moon will appear as no more than a thin crescent.
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 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 13 September 2017|
All times shown in EDT.
The circumstances of this event were computed using the DE405 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.
|13 Sep 2017, 02:26 EDT||– Moon at Last Quarter|
|20 Sep 2017, 01:31 EDT||– New Moon|
|27 Sep 2017, 22:55 EDT||– Moon at First Quarter|
|05 Oct 2017, 14:41 EDT||– Full Moon|