Astronomy Essentials

Close and far moons of 2021

Side-by-side photos of the full moon, the one on the left considerably larger than the one on the right.
Here’s a comparison between the December 3, 2017, full moon at perigee (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, some two weeks later, to perigee – the moon’s closest point to Earth in its monthly orbit.

In this post, beneath the illustration below, we list the year’s 13 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 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 farthest apogee comes on May 11, 2021 (252,595 miles or 406,512 km), and the closest perigee occurs on December 4, 2021 (221,702 miles or 356,794 km). That’s a difference of roughly 30,000 miles (50,000 km). Meanwhile, the moon’s mean distance (semi-major axis) from Earth is 238,855 miles (384,400 km).

EarthSky lunar calendars are cool! Order now. Going fast!

Perfect circle around Earth, overlapped by slightly eccentric dotted line indicating orbit of moon.
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 2021

Perigee Apogee
Jan 09 Jan 21
Feb 03 Feb 18
Mar 02 Mar 18
Mar 30 Apr 14
Apr 27 May 11
May 26 June 08
June 23 July 05
July 21 Aug 02
Aug 17 Aug 30
Sept 11 Sept 26
Oct 08 Oct 24
Nov 05 Nov 21
Dec 04 Dec 18

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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 2025:

Lunar perigees and apogees in 2025

Perigee Apogee
Jan 08 Jan 21
Feb 02 Feb 18
Mar 01 Mar 17
Mar 30 Apr 13
Apr 27 May 11
May 26 June 07
June 23 July 05
July 20 Aug 01
Aug 14 Aug 29
Sept 10 Sept 26
Oct 08 Oct 23
Nov 05 Nov 20
Dec 04 Dec 17

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 2021, to the year 2023:

Lunar apogees and perigees in 2023

Apogee Perigee
Jan 08 Jan 21
Feb 04 Feb 19
Mar 03 Mar 19
Mar 31 Apr 16
Apr 28 May 11
May 26 June 06
June 22 July 04
July 20 Aug 02
Aug 16 Aug 30
Sept 12 Sept 28
Oct 10 Oct 26
Nov 06 Nov 21
Dec 04 Dec 16

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.

A little-known fact of the moon’s apogee/perigee cycle

Both professional astronomers and lay people are often unaware that the apogee/perigee 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

Small full moon alternating with large crescent moon.
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 perigees and apogees fall on the same, or nearly the same, calendar dates. Look here for more information – and an explanation – about the cycle of close and far moons.

January 1, 2021
Astronomy Essentials

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Bruce McClure

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