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The Moon at perigee

Thu, 25 May 2017 at21:22 EDT(241 days ago)
01:22 UTC

Dominic Ford, Editor
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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 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 25 May 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 357,000 km of the Earth, and appear with an angular diameter of 33.44 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 04h28m40s +16°47' Taurus 33'26"

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 25 May 2017
Sunrise 05:48
Sunset 20:23
Twilight ends
22:16
Twilight begins
03:55

29-day old moon
Age of Moon
29 days

All times shown in EDT.
Rise Culm. Set
Mercury 04:48 11:31 18:14
Venus 03:46 10:08 16:30
Moon 06:07 13:02 19:58
Mars 06:57 14:25 21:54
Jupiter 15:55 21:45 03:40
Saturn 21:48 02:39 07:26

Warning

Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.

Source

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.

Related news

25 May 2017, 15:46 EDT  –  New Moon
01 Jun 2017, 08:43 EDT  –  Moon at First Quarter
09 Jun 2017, 09:11 EDT  –  Full Moon
17 Jun 2017, 07:34 EDT  –  Moon at Last Quarter

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39.04°N
77.49°W
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