Close and far moons in 2018

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).

Lunar perigees and apogees in 2018

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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

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

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

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