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The Supermoon Is Bogus

Real quick: Saturday's so-called "supermoon" won't cause earthquakes, tsunamis, hurricanes, madness, or any significant uptick in theft/murder/mischief. In fact, the whole concept of the "supermoon" is bogus. Here's why:

The Moon's Orbit. According to breathless reports in the Sun Sentinel and elsewhere, the "supermoon" has the moon "making its closest approach to the Earth in 18 years." That's true, but only barely. The moon came almost as close on January 10, 2005, and on January 30, 2010. Tomorrow, it will be only about 60 miles closer than it was on those days.

The moon's orbit of Earth, like Earth's orbit of the sun, is not a perfect circle. It's elliptical. Once per lunar month, it wanders a goodly distance from Earth (about 250,000 miles at its farthest, or "apogee"), and then wanders back (to a distance of about 220,000 miles at its closest, or "perigee"). A so-called "supermoon" takes place any time the moon arrives at its perigee at the same time as a new or a full moon. That means it's got two chances every 29 - 30 days.

Now, celestial bodies are given to wobble, and perigees and apogees change a bit from month to month. As it happens, we now have a full moon correlating with a perigee which brings the moon about 3,700 miles closer than average. Which sounds like a lot, until you consider: Every two weeks, the moon's distance changes ten times as dramatically. From every monthly apogee to every monthly perigee, the difference is a whopping 30,000 miles. Every two weeks.

The Sun's Light: The fact that a perigee concurs with a full moon is interesting, but not meaningful, for the moon's distance and its phase have little to do with each other. A full moon occurs whenever the sun, moon, and Earth line up, so the sun is illuminating only the side of the moon that faces us. (The moon has what's called a "synchronous rotation," which means the same side is always pointed Earthwards. When the moon is at a right angle to the line between the sun and Earth, sunlight hits exactly half its visible face; when the moon is between the sun and Earth, only its back face is illuminated.)

Earth's tides are affected by the distance of the moon, but not by its phase. Whether the side of the moon facing us is completely dark or completely illuminated, its gravitational pull remains constant. (There is a slight variation in the effects of that gravity during a full moon, since the moon and sun are tugging in opposite directions, but the effect is almost negligible -- a slight raising of tides, which occasionally poses a flood risk in very low-lying areas.)

 The Notion's Provenance: As the astronomer Phil Plait recently pointed out, the whole concept of the "supermoon" comes from an astrologer named Richard Nolle. You can read his rambling about the "supermoon's" significance at his website, but it's probably not a great investment of time. The "supermoon" is not an event; it's coincidence. From your window, you will see the same moon as you always do.

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Brandon K. Thorp

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