Observing the Moon: Part 2, Just What is a Super-Moon Anyway?

By Peter Forrester | March 20, 2019

Well, here we go. Another supermoon is coming up tonight / tomorrow (depending on where you are), right after the Spring Equinox which is tonight about 6 pm in US Eastern time. This one is called the Super Worm Equinox Moon, or similar phrases. What exactly do all these terms mean?

Well, first of all, “supermoon” means a Moon that is at its closest approach to Earth, and at full moon (or new moon) phase at the same time, making it appear brighter and slightly larger than it normally would. Every month it has its “perigee”, the closest approach of that orbit, and the opposite “apogee”, but some perigees are closer than others. The moon at perigee normally appears about 14% bigger than at apogee, so we’re not talking a huge difference here.

The term supermoon was first coined by an astrologer in 1979, and has various definitions in different places. The original definition stated that the full or new moon occurs with the Moon within 90% of its closest approach to Earth, but the reason for choosing 90% was never explained, and the claim that a supermoon causes catastrophic effects and stress on Earth has been thoroughly disproven by scientists. It should also be noted that most people using the term “supermoon” are referring to a Full Moon (and very rarely is used for a New Moon).

The Worm refers to the month of March, and apparently this originates with Native Americans who called their full moon in March by this name, owing to earthworms coming out of the thawing soil at this time of year.

Equinox, well that’s happening today. We only have two equinoxes per year, and they always occur around March 20th, and again 6 months later in September. For the Northern Hemisphere, the March equinox is called the Vernal or Spring Equinox, and denotes the astronomical beginning of Spring, while it starts Fall in the Southern Hemisphere. The September equinox is called the Autumnal or Fall Equinox.

But what is an Equinox? According to Wikipedia, equinox occurs at “the instant of time when the plane (extended indefinitely in all directions) of Earth’s equator passes through the center of the Sun”. Or in other words, when the center of the Sun is directly above the equator.

The two equinoxes are when the day and night are the same length (not exactly at the same moment as the Equinox, but pretty close). The equinoxes are also the only times of year that both the northern and southern hemisphere receive the same amount of light from the Sun.

This March Supermoon is the last full moon supermoon of this year, after having them in January and February as well (however, we have three supermoon New moons in August and September). The February one was the closest and brightest, but this one is unique. It is very rare for the supermoon to happen within a day of the equinox. It won’t happen again until 2030.

Owing to the momentous occasion of a Super Worm Equinox Moon happening today / tomorrow, I have deferred a description of the Moon’s seas and craters until Part 3 of this series.

Happy moon observing! Because you’re only going to see the brightest stars right now, skies permitting. If it’s too cloudy, there’s a link on the first reference, the National Geographic article below where you can watch it online.

Have a great day, and Happy Spring (or Fall, if you’re reading this from south of the Equator)!

Previous in series: Observing the Moon, Part 1: Appearance and Phases


Fazekas, Andrew. (2019, March 19). See the first supermoon on the spring equinox in 19 years. Retrieved from https://www.nationalgeographic.com/science/2019/03/see-first-super-worm-moon-on-spring-equinox-in-19-years/.

McClure, Bruce, and Deborah Byrd. (2019, January 1). How many supermoons in 2019? Retrieved from https://earthsky.org/human-world/what-is-a-supermoon.

Wikipedia. (2019, March 20). Equinox. Retrieved from https://en.wikipedia.org/wiki/Equinox.

Wikipedia. (2019, March 14). Supermoon. Retrieved from https://en.wikipedia.org/wiki/Supermoon.

Wikipedia. (2019, February 22). Syzygy (astronomy). Retrieved from https://en.wikipedia.org/wiki/Syzygy_(astronomy).

Author: Peter Forrester

I have been interested in astronomy and stargazing for many years, and now delight to offer some of my learning to others through my weekly blog posts.

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