Have you ever looked up into the sky and seen a full moon so big, bright and clear that it looked like it was standing right next to you? Most of us have probably witnessed many of these ‘supermoons,’ in our life, as the moon comes the nearest to earth that it will all year. While the scientific name is a perigee-syzygy of the Earth-Moon-Sun system, the much more easily recognized term ‘supermoon’ was coined by the astrologer Richard Noelle about thirty years ago, who defined the occurrence as “a new or full moon which occurs with the moon at or near (within 90% of) its closest approach to Earth in a given orbit.”
The reason that the moon is closer to us at some times of the year and not others is that the moon’s orbit isn’t a circle: it’s an ellipse. At its closest distance, also known as perigee, the moon can be as little as 357,000 kilometers away from us, but at the farthest point in its orbit (known as apogee), it can be as far away as around 406,000 kilometers away. This range of approximately 50,000 kilometers is what causes supermoons to appear so unnaturally large by comparison.
However, despite its common usage, the term ‘supermoon’ is actually not recognized by most astronomers, due to the fact that Noelle, who first used it, was not an astronomer, and many of his theories were proved to be incorrect. Astronomers prefer to use the term ‘perigee moon’ to describe it at its closest to earth, and this term is much more scientifically correct, as perigee moons are calculated based on better definitions and measurements. True perigee moons only occur once every 14 full moon cycle, which is a little less than once a year, while supermoons based on Noelle’s flawed description may occur as many as 6 times in one year based on his specifications.
When the moon is very close to earth, whether you call it a supermoon or a perigee moon, it has a small effect on ocean tides. The tides are most strongly affected by the moon when it is either new or full, known scientifically as a ‘syzygy’ moon, and when it is a perigee moon, it can cause up to an 18% difference in the strength of tides worldwide, resulting in what are known as the perigean spring tides. However, this extra force usually only results in a couple of centimeters’ difference, which is not anything to be alarmed about. Some people also believe that supermoons are linked with natural disasters such as earthquakes, but no scientific evidence has been found that proves these theories.
The most recent perigee-syzygy moon occurred on June 21st 2013, which means the next one will be due approximately Thursday, August 7th 2014, as they appear about every 412 days. So get ready for a big, beautiful moon late next summer!