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Close and far moons in 2017

We list the 13 lunar perigees (close moons) and 13 lunar apogees (far moons) for the year 2017, plus share a secret on the intriguing cycle of close and far moons.

Full moons at apogee (left) and perigee (right) in 2011.  Composite image by EarthSky community member C.B. Devgun in India.  Thanks, C.B.!

Full moons at apogee (left) and perigee (right) in 2011. Composite image by EarthSky community member C.B. Devgun in India.

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 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 – 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 May 26, 2017 (221,958 miles or 357,207 km) and the farthest apogee happens on December 19, 2017 (252,651 miles or 406,603 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 2017

Perigee Apogee
January 10 January 22

February 6 February 18
March 3 March 18
March 30 April 15
April 27 May 12
May 26 June 8
June 23 July 6
July 21 August 2
August 18 August 30
September 13 September 27
October 9 October 25
November 6 November 21
December 4 December 19

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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 2021:

Lunar perigees and apogees in 2021

Perigee Apogee
January 9 January 21

February 3 February 18
March 2 March 18
March 30 April 14
April 27 May 11
May 26 June 8
June 23 July 5
July 21 August 2
August 17 August 30
September 11 September 26
October 8 October 24
November 5 November 21
December 4 December 18

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 2017, to the year 2019:

Lunar apogees and perigees in 2019

Apogee Perigee
January 9 January 21

February 5 February 19
March 4 March 19
April 1 April 16
April 28 May 13
May 26 June 7
June 23 July 5
July 20 August 2
August 17 August 30
September 13 September 28
October 10 October 26
November 7 November 23
December 5 December 18

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

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

View larger. | Image via Wikipedia.

View larger. | Image via Wikipedia.

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 2016

2017 EarthSky Lunar Calendar pre-sale…is happening NOW!

Bruce McClure

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