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The Moon at perigee

Fri, 18 Aug 2017 at09:19 EDT(21 hours away)
13:19 UTC

Dominic Ford, Editor
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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 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's glossary article for the term month.

As the perigee of 18 August 2017 will occur close to the time of new moon, the moon will appear as no more than a thin crescent.

On this occasion the Moon will pass within a distance of 366,000 km of the Earth, and appear with an angular diameter of 32.62 arcsec. 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 06h52m50s +19°21' Gemini 32'37"

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 18 August 2017
Sunrise 06:25
Sunset 20:00
Twilight ends
Twilight begins

26-day old moon
Age of Moon
26 days

All times shown in EDT.
Rise Culm. Set
Mercury 07:45 13:59 20:14
Venus 03:34 10:50 18:07
Moon 03:06 10:16 17:25
Mars 05:47 12:46 19:44
Jupiter 10:52 16:33 22:15
Saturn 15:53 20:40 01:31


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.

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15 Aug 2017, 21:16 EDTMoon at Last Quarter
21 Aug 2017, 14:31 EDTNew Moon
29 Aug 2017, 04:14 EDTMoon at First Quarter
06 Sep 2017, 03:04 EDTFull Moon

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