There’s a lot of talk of “Supermoons” these days, but what is a Supermoon, and what causes it? How are they different from a regular full Moon?
What Is A Supermoon?
Supermoons are caused by the shape of the Moon’s orbit, which is not a perfect circle, but an ellipse, or oval, shape. The Moon orbits the Earth once each month, and each month, it reaches a point farthest from the Earth, called apogee, and a point closest to the Earth, called perigee.
According to how most people define a supermoon, it occurs when the Moon is at least 90% of the way to its perigee position at the same time it is in its “full” or “new” phase. An extreme Supermoon is when a full or new Moon happens at the same time the Moon is at perigee.
When we have a supermoon, the Moon can appear as much as 14% larger and 30% brighter than a normal full Moon.
The March 20, 2019, full Worm Moon will be a Supermoon, our final Supermoon of 2019!
Why New And Full Phases?
The reason these two Moon phases are singled out is because each of them means that the Sun, Earth, and Moon are in alignment. When the Moon is full, it sits exactly on the opposite side of the Earth from the Sun. When the Moon is new, it sits between the Earth and the Sun. In both cases, the gravitational pull from these two bodies — the Moon and the Sun — combine to create higher-than-normal tides, called “spring tides,” on Earth. When the Moon is also at perigee at this time, the effect is magnified into what is called a “proxigean spring tide.”
Of course, a new Moon at perigee isn’t very exciting to look at because in this phase, the Moon does not reflect the Sun’s light, so it is invisible to the naked eye. So, understandably, full Supermoons get all the attention.
There are actually about four or five supermoon events each year, only about half of which are full Supermoons. Extreme Supermoons are rarer and occur at varying intervals ranging from as little as a year to 20 years or more.
Not All Supermoons Are Created Equal!
Just as the Moon’s orbit isn’t a perfect circle, it also varies slightly from month to month and year to year. Its perigee during one month may be slightly farther from the Earth than its perigee the next month. The Moon’s average distance from the Earth is 235,000 miles, and its average furthest distance is 248,000 miles.
The Internet is usually abuzz when there’s a supermoon, with many people speculating that extreme Supermoons can cause natural disasters such as earthquakes, volcanoes, and tsunamis. Some even blame earthquakes on supermoon events. But most astronomers dismiss this line of thinking, though, arguing that the 2,000-mile difference is minimal in the grand scheme of things — less than 1% of the Moon’s total distance from the Earth — and unlikely to cause much disruption on Earth, beyond the usual proxigean spring tide.
Regardless of what you believe about a Supermoon’s impact on Earth, one thing is certain: if the sky is clear, the view of the rising Moon will always be amazing, so get outside and enjoy it!