What Is a Supermoon? A Supermoon Definition

(Last Updated On: January 17, 2019)
What Is a Supermoon? A Supermoon Definition

Generally speaking, a supermoon more or less looks like a regular full moon. Nonetheless, experienced skywatchers will typically point to one notable difference. Supermoons, on average, can be 30% brighter and 14% larger than a run-of-the-mill full moon. So what is a supermoon exactly and why do they matter?

What Is a Supermoon?

Basically, a supermoon is a full or new moon that coincides with the moon’s closest point to earth in an orbit. In other words, it is a full moon that is as near to earth as the moon will get that month. This is why when a full moon is a supermoon, it will tend to appear bigger and brighter than the average full moon. Before the term supermoon was coined, scientist simply referred to these types of full moons as perigean moons. The term perigean simply translates to “falling close to the earth.”

Technically, supermoons have always been happening. However, they only became defined as a notable astrological phenomenon relatively recently. Ironically, it was not an physicist or astronomer, but an astrologer named Richard Noelle, who somewhat arbitrarily decided to coin the term back in 1979.

Noelle stated that in order to qualify as a supermoon, the full or new moon must be at least within the range 90% of its closest orbit to Earth. However, most people just speak of supermoons in terms of full moons, since new moons appear far less notable to the naked eye.

Why Are Some Supermoons More Super Than Others?

Supermoons got a lot of media attention in 2016 because of a rare event, namely three full supermoons occuring in a row, the latter of which was the closest a full supermoon had come to earth since 1948. Since the orbit of the moon in relation to earth is not fixed, the number of times that a supermoon might occur in a given year is random. However, having three happen in a row is certainly remarkable given that, by definition, the odds of a full moon being a supermoon are about 1 in 10.

Complicating the matter is the fact that the moon’s orbit changes every month in relation to Earth. Tides, gravity, and other phenomenon all pull on the moon, and the moon’s path is not just affected by Earth but a bunch of other planets as well. The result is a complex and ever-changing range of gravitational effects that are constantly being exerted on the moon, meaning that the moon’s path is never constant. As such, the moon’s orbit is not a perfect circle, so the closest point that it comes from month to month is always different.

This essentially means that some super moons are more “super” than others. The last time a supermoon occurred that came closer than it did in 2016 was over a hundred years before in 1912. The next supermoon of this caliber won’t occur until November 2034. That said, it should be well worth the wait, since the full moon is expected to come even closer to Earth than it did in the 1912 event.

So Exactly How Close Is Close?

By Noelle’s definition, how close does a supermoon actually get? On an average day, the moon is 238,000 miles from earth. During the last big supermoon in 2016, the full moon was within 221,524 miles. That means that on this date, the full moon was about 16,476 miles closer than the moon is on average. In other words, supermoons are definitely an event worthy of pulling out the telescope for.

Separating the Fact from Fiction

As previously mentioned, the term supermoon has its roots in astrology, and as such, there tends to be a lot of hearsay and magical thinking surrounding these kind of events. Noelle himself warned of an increased risk of what he called “geophysical stress” around these periods.

Although some have speculated that there is correlation between recent disasters like massive earthquakes and tsunamis and the occurrence of supermoons, scientists continue to refute these speculations. They admit that the moon and sun do have an increased effect on the earth’s tides around the time of a full moon, but the difference in force is very slight and cannot account for any major geological events.

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