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EarthSky // Astronomy Essentials, Science Wire, Space Release Date: Sep 27, 2015

Intriguing cycle of close and far moons

This is why the moon is so ‘super’ tonight. This post explains lunar perigee and includes dates of all closest and farthest moons for each month of 2015.

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. Nowadays, a full moon at perigee is often called a supermoon. Composite image by EarthSky community member C.B. Devgun in India. Thanks, C.B.!

In September 2015, the moon sweeps to perigee – the moon’s closest point to Earth for the month – on September 28 at 1:46 UTC. That is September 27 at 8:46 p.m. CDT. This month’s moon at its perigee lies 356,877 kilometers (221,753 miles) from Earth. It’s the moon’s closest point to Earth for all of 2015.

The year’s closest perigee is sometimes called the moon’s proxigee.

And there’s more. About one hour after the September 27-28 lunar perigee, the moon will reach the crest of its full phase. That’ll happen at 2:51 UTC. The very close coincidence of perigee and full moon will showcase not only the closest supermoon of 2015, but also a lunar eclipse on night of September 27-28.

Plus, it’s the Harvest Moon for us in the Northern Hemisphere!

We list the dates for this year’s 13 lunar apogees (farthest points) and 13 lunar perigees (nearest points), below:

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 is not a perfect circle. But it is very nearly circular, as the above diagram shows. Diagram by Brian Koberlein.

Lunar apogees and perigees in 2015

Apogee Perigee
January 9 January 21
February 6 February 19
March 5 March 19
April 1 April 17
April 29 May 15
May 26 June 10
June 23 July 5
July 21 August 2
August 18 August 30
September 14 September 28
October 11 October 26
November 7 November 23
December 5 December 21

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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 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

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 2015, to the year 2017:

Lunar apogees and perigees in 2017

Apogee Perigee
January 22 January 10
February 18 February 6
March 18 March 3
April 15 March 30
May 12 April 27
June 8 May 26
July 6 June 23
August 2 July 21
August 30 August 18
September 27 September 13
October 25 October 9
November 21 November 6
December 19 December 4

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

It’s hard to believe that this rather straight-forward four-year apogee/perigee cycle is so little known among professional astronomers and lay people alike. Lunar apogees and lunar perigees align on the same, or nearly the same calendar dates every four years, because 53 returns to perigee (or apogee) is 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.

Close and far moons in 2016

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