Be sure to head outside this week to observe the largest full Moon in nearly 20 years. This month’s full Moon, which takes place on March 19, will be a “SuperMoon,” and could appear up to 14% larger and 30% brighter than a normal full Moon.
So, what is a SuperMoon, anyway, and what causes it? 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 reaches a point farthest from the Earth, called apogee, and closest to the Earth, called perigee.
A SuperMoon occurs when the Moon is at least 90% of the way to its perigee position at the same time it is full or new. An extreme SuperMoon is when a full or new Moon happens at the same time the Moon is close to 100% average perigee.
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 larger 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 the new Moon does not reflect the Sun’s light, it is invisible — so full SuperMoons get much more attention than new SuperMoons.
There are actually about four or five SuperMoon events each year, only about half of which are full SuperMoons. Extreme SuperMoons are more rare and occur at varying intervals ranging from as little as a year to 20 years or more.
This week’s extreme SuperMoon will be the fourth we’ve seen since 2005. It will also be the largest and brightest since 1992. You may wonder how that’s possible. 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. This week, the Moon will be 221,567 miles away, just a tiny bit closer than its average closest distance of about 223,500 (the Moon’s average distance from the Earth is 235,000, and its average furthest distance is 248,000 miles).
The Internet is currently abuzz with many people speculating that extreme SuperMoons can cause natural disasters such as earthquakes, volcanoes, and tsunamis. Some are even blaming last week’s earthquake and tsunami activity in Japan and the Pacific Ocean on the upcoming SuperMoon event. 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. These tides are usually stronger when the Moon is new than when it’s full, so the conventional wisdom is that the upcoming event will result only in slightly higher than normal spring tides.
Regardless of what you believe about the SuperMoon’s impact on Earth, one thing is certain: if the sky is clear, we’ll all get to see one amazing view this coming Saturday night. Don’t miss it!