What Is A Supermoon?
Supermoons are caused by the shape of the Moon’s orbit, which is not a perfect circle. It orbit is an ellipse, or oval, shape. Each month, it reaches a point farthest from the Earth, called apogee, and a point closest to the Earth, called perigee.
A “supermoon” occurs when the Moon is at least ninety percent of the way to its perigee position at the same time that it is in its “full” or “new” phase. When either of which occurs when the Moon happens to be at perigee, the Moon is considered to be an “extreme perigean supermoon.”
Why New And Full Phases?
The reason these two 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.”
Full Moons get all of the attention around supermoons because they reflect the Sun’s light. Supermoons during a new phase on the other hand are invisible to the naked eye.
Fun fact: On Saturday, January 21, 2023, the New Moon was at its closest distance to Earth in nearly 1,000 years (992 to be exact).
What’s In A Name?
The term “supermoon” became popular in March 2011 when the Moon’s perigee brought it to 221,565 miles of Earth—within 127 miles of the absolute closest that it can come (the absolute closest the Moon can come is 221,438 miles from Earth—an exceedingly rare occurrence).
In actuality, the designation should only be applied to “extreme” perigees, in which the full Moon approaches a distance of 221,472 miles or less. Between the years A.D. 1500 to 2500, this condition is met only 14 times, or on average once about every 71 years. The last time the full Moon came this close to Earth was January 15, 1930 (221,454 miles) and the next time will be on December 6, 2052 (221,469 miles).
But if you follow the 90% rule noted above, you can have as many as three—and on some occasions even four or five—supermoons in a single year!
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.
Many people speculating that extreme supermoons can cause natural disasters such as earthquakes, volcanoes, and tsunamis. But most astronomers dismiss this line of thinking, though, arguing that the 2,000-mile difference (less than 1% of the Moon’s total distance from Earth) is minimal in the grand scheme of things and unlikely to cause much disruption on Earth, beyond the usual perigean spring tide.
Regardless of what you believe about a Supermoon’s impact on Earth, or what you call it, 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!
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