The term “supermoon” is relatively new, having been popularized by social media in the last few years. But the astronomical coincidence underlying the phenomenon has existed for eons. In this instalment… The making of a modern myth.
Constructing a myth
This month, the Earth–Moon distance will vary between 356,000 and 406,000 kilometres — a difference of 14 percent from perigee to apogee. A “supermoon,” known scientifically as a perigeean full moon, occurs whenever the Moon’s orbit brings it closest to Earth. In addition, the rising full moon always appears enormous, especially when it is just on the horizon. However, this is an optical illusion (hyperlien) which diminishes as the Moon climbs higher in the sky. Despite the truth, the effect is absolutely riveting. Indeed, according to “supermoon” promoters, a full Moon at perigee should appear even larger when rising than an “ordinary” full Moon…
The difference is imperceptible
In actual fact, the difference in size is undetectable to the naked eye. The eye-brain system is incapable of making absolute measurements: from memory it can’t compare the size of the full moon in the sky with the Moon last month, or six months ago. However, a camera can permanently record an observation, and compare it with past observations…a fact that is illustrated by the image accompanying this article: on the left, a “supermoon” on August 10, 2014 (distance 357,000 km); on the right, taken with the same setup, the full moon on March 5, 2015, at apogee (406,000 km). The size difference is evident in these side-by-side images, but is imperceptible to the naked eye when we see the Moon in the sky.
Higher tides in sight
However, “supermoons” do have a directly observable effect on tide levels. Pronounced tides occur when the moon is full (and to a lesser extent around new moon). But they are amplified when the Moon is closest. During full moon, and two or three days after, the difference in water level between successive low and high tides is greatest: tides are extremely low and extremely high. When a storm arrives during this period, it can cause flooding along low-lying coastlines, especially when the wind blows toward shore.
A recurring phenomenon
The conditions that cause a “supermoon” recur every year. But the lunar apogee–perigee cycle is completely independent of the Moon’s phases: the moment when the full moon coincides with lunar perigee occurs about a month later from year to year. In 2015, perigeean full moons occur on August 29 (21 hours before perigee), September 28 (less than an hour after perigee) and October 27 (24 hours after perigee).