This Saturday night, the largest full moon of the year will appear. This week’s full Moon, which takes place on May 5, will be a “SuperMoon,” and should appear quite a bit larger and brighter than a normal full Moon.
Last year, the March SuperMoon got a lot of media attention because it was the largest full Moon in nearly 20 years. This week’s SuperMoon will not be quite as extreme as last year’s, but will come close.
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% perigee. May’s full Moon will occur within an hour of the Moon’s 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.
The Moon won’t be as big and bright as this week’s Moon for another two years – August 10, 2014, to be exact – so be sure to get outside this weekend and enjoy it!