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Close and far moons in 2018

In 2018, the moon swings farthest from Earth for the year on January 15, only 2 weeks after coming closest to Earth for the year on January 1.

Here’s a comparison between the December 3, 2017 full moon at perigree (closest to Earth for the month) and 2017’s farthest full moon in June at apogee (farthest from Earth for the month) by Muzamir Mazlan at Telok Kemang Observatory, Port Dickson, Malaysia.

The moon’s distance from Earth varies throughout its monthly orbit because the moon’s orbit isn’t perfectly circular. Every month, the moon’s eccentric orbit carries it to apogee – its most distant point from Earth – and then to perigee – the moon’s closest point to Earth – roughly two weeks later.

In this post, beneath the illustration below, we list the year’s 14 perigees and 13 apogees. Yes, the moon’s apparent size in our sky does change across this cycle of the moon. The variation in the moon’s apparent size – across its monthly orbit – is akin to that of a U.S. quarter versus a U.S. nickel.

Also in this post, we share with you a little-known fact about the intriguing cycle of close and far moons.

This year’s closest perigee comes on January 1, 2018 (221,559 miles or 356,565 km) and the farthest apogee happens on January 15, 2018 (252,565 miles or 406,464 km). That’s a difference of about 30,000 miles (50,000 km). Meanwhile, the moon’s mean distance (semi-major axis) from Earth is 238,855 miles (384,400 km).

The moon's orbit around Earth is not a perfect circle.  But it is very nearly circular, as the above diagram shows.  Diagram by Brian Koberlein.

The moon’s orbit around Earth isn’t a circle, but it’s very nearly circular, as the above diagram shows. Diagram by Brian Koberlein. Used with permission.

Lunar perigees and apogees in 2018

Perigee Apogee
Jan 1 Jan 15
Jan 30 Feb 11
Feb 27 Mar 11
Mar 26 Apr 8
Apr 20 May 6
May 17 June 2
June 14 June 30
July 13 July 27
Aug 10 Aug 23
Sept 8 Sept 20
Oct 5 Oct 17
Oct 31 Nov 14
Nov 26 Dec 12
Dec 24

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Amazingly, in periods of four years, lunar apogees and perigees fall on the same, or nearly the same calendar dates. Let’s look four years ahead, to the year 2022:

Lunar perigees and apogees in 2022

Perigee Apogee
Jan 1 Jan 14
Jan 30 Feb 11
Feb 26 Mar 10
Mar 23 Apr 7
Apr 19 May 11
May 17 June 2
June 14 June 29
July 13 July 26
Aug 10 Aug 22
Sept 7 Sept 19
Oct 4 Oct 17
Oct 29 Nov 14
Nov 26 Dec 12
Dec 24

Also, in cycles of two years, the calendar dates remain the same, or nearly so, except that the lunar apogees and perigees trade places. For instance, let’s look two years beyond 2018, to the year 2020:

Lunar apogees and perigees in 2020

Apogee Perigee
Jan 2 Jan 13
Jan 29 Feb 10
Feb 26 Mar 10
Mar 24 Apr 7
Apr 20 May 6
May 18 June 3
June 15 June 30
July 12 July 25
Aug 9 Aug 21
Sept 6 Sept 18
Oct 3 Oct 16
Oct 30 Nov 14
Nov 27 Dec 12
Dec 24

Want to know more? Here’s for a complete listing of all lunar perigees and apogees for the 21st century (2001 to 2100) and a perigee and apogee calculator.

Here’s a little-known fact of the moon’s apogee/perigee cycle, among both professional astronomers and lay people. That is, the cycle causes lunar apogees and perigees to align on the same, or nearly the same, calendar dates every four years. That’s because 53 returns to perigee (or apogee) are nearly commensurate with four calendar years.

The mean length of the anomalistic month (perigee to perigee, or apogee to apogee) is 27.55455 days, whereas the average Gregorian year equals 365.2425 days. Hence:

27.55455 x 53 = 1460.3912 days

365.2425 x 4 = 1460.97 days

This animation by Peter Lowenstein in Zimbabwe contrasts the size of the May 27, 2017 waxing crescent moon, which was close to Earth, with the June 9, 2017 full moon, which was far from Earth. Read more about this image.

Bottom line: In periods of four years, lunar apogees and perigees fall on the same, or nearly the same calendar dates.

Close and far moons in 2017

Bruce McClure

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