Weather

Why do supermoons happen in fours?

The March full moon is the second in a string of supermoon is in 2020, NASA Ambassador Tony Rice explains why we tend to get three or four in a row.

Posted Updated

By
Tony Rice
, NASA Ambassador

Look for a big bright supermoon tonight. The Moon will appear a bit larger but you'll probably most notice it is about 30% brighter than full moons at their furthest from Earth, also known as a minimoon. The best time to look is right after sunset when the Moon is closest to the horizon and looks even bigger due to an optical illusion.  If you miss it Monday night, look again on Tuesday.

The Moon reached full phase at 1:47 this afternoon. The Moon reaches perigee, the closest point in its orbit around Earth a few hours after midnight. That makes it a supermoon by any of the popular definitions (more on that below). This is the second of a quartet of supermoons in 2020 (February 9, March 9, April 7 and May 7).

Elizabeth Gardner raised an excellent question during this morning’s 8 am newscast. How unusual is it to have several super moons in a row? Not at all! Supermoons tend to come 3-4 In a row. We’ll have four supermoons in during 2021-2024 as well.  2029-2033 bring five supermoons each year.

This happens because the Earth and Moon are both moving around the Sun. This creates similar but not quite the same length of repeating cycles between the Moon and Sun and Moon and Earth.

The synodic month, or the time between lineups of Sun, Earth and Moon (a.k.a. a full moon) is about 2 days longer than the anomalistic month or the time it takes for the Moon to return to the same perigee point. Every 14 synodic months and 15 anomalistic months, things line up bringing the full moon at its closest.  This last happened in 2018 and will happen again in 2026, but the 10% wiggle room the definitions of whether a given full moon is super or not (see below on) create sliding windows of time that overlap in groups of usually four months a year, but sometimes three or five.
supermoons occur when a full moon happens near perigee, minimoons occur when a full moon happens near apogee (Source: NASA/JPL)

Many definitions, none official

The term supermoon was originally coined by astrologer Richard Nolle in 1979 in a very non-scientific discussion in Dell Horoscope magazine about the Moon’s effects on Earth. There it sat for about 30 years before it became popular. Supermoon is rooted in astrology not astronomy.  There is no official definition or formula to calculate which full moons are super and which are not.  But it is a fun concept so several have created their own definitions.  Sometimes they lineup, sometimes they don't.

Nolle’s definition includes both new and full moons “within 90% of its closest approach to Earth”, though new moons are not as easy to see so you won’t hear as much about them. Nolle. Over the years Nolle has narrowed his definition to include on the closest full moons.

Another definition comes from retired NASA astrophysicist Fred Espenak. He adopted the same 90% rule which he applies just that orbit (Nolle applies the 90% rule to the closest perigee and furthest apogee, without explanation).

Astronomy Sky and Telescope magazine and Norwegian based timeanddate.com offer their own definitions of 223,000 miles (358,884 km) and 360,000 km (223,694 mi) again without explanation for the round number choice.

NASA has no official stand on the topic, but tends to follow Espenak’s lead on when to encouraging people get outside and see the big beautiful moon.

 Credits 

Copyright 2024 by Capitol Broadcasting Company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.