The Sciences

Bad Moon Rising

By Bob BermanMar 1, 1993 12:00 AM
Moon rise stock
(Credit: Sdecoret/Shutterstock)

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We’re about to be moonstruck. Get ready for a rare and powerful lunar event that starts at sunset on March 7, with the rising of an enormous full moon. Thanks to the famous moon illusion, Earth’s nearest neighbor always looks bigger when it hovers over the horizon, perhaps because of its proximity to foreground objects. But this night’s moon will win the size prize. Quite simply, we’ll see the largest moon until the next century.

A celestial chain of coincidences begins when March’s full moon lands on the very day that the moon comes closest to Earth. Such alignments happen infrequently, since the 29.5-day cycle of lunar phases doesn’t march in step with the 27.5-day interval between close approaches.

There’s more: the moon’s orbit changes shape, depending on the relative positions--and gravitational pull--of the sun and Earth. Sometimes it’s more circular; at other times it becomes unusually elongated. This month the orbit distorts to maximum eccentricity.

This stretched-out lunar orbit will push the moon to an anomalously great distance from Earth on March 21, a distance that won’t be surpassed until 1998. But the real showstopper is the strangely close lunar approach, or perigee, two weeks earlier. On the night of March 7, at closest approach between midnight and dawn, the surfaces of our two worlds will pass barely 216,000 miles apart.

And there’s still more. Perigee occurs at 9 A.M. Greenwich mean time, while the full moon arrives at 9:47. So as another bonus, full moon and the most extreme perigee come not just on the same night but less than an hour apart.

Will the close separation affect us? You bet. Tides vary with the cube of distance: if the moon ventured 3 times nearer, its tidal pull would be 27 times stronger. Proximity dwarfs other factors such as mass. That’s why the sun, 400 times more distant than the moon, exerts less than half the lunar effect even though it’s 27 million times heavier.

Since brawny spring tides (which have nothing to do with the season; the word comes from the German springen, to rise up) occur at every full moon, this bull’s-eye combination of full moon and extreme perigee will cook up extraordinary tides. They even have their own Scrabble-friendly term: proxigean. History has shown a disquieting correlation between such astronomical conditions and coastal flooding.

The proxigean spring tides on March 8 will be dramatic. High tide will climb far up the beach while low tide will expose areas normally submerged. If there’s a storm at sea or strong onshore winds that day-- anything to raise the oceans a bit higher still--we can expect impressive coastal damage. Even the air pressure can be important, since oceans behave like the pool of mercury in a barometer. A 1-inch barometric drop will raise the seas 13 inches.

Tidal forces act only on large bodies such as Earth and its oceans. The moon’s distance to your head and to your feet being essentially the same, there’s no tidal effect on your body--just as tea displays no tendency to climb the sides of the cup when the moon passes overhead. Contrary to astrological myth, the moon cannot coax bodily fluids to migrate.

But you might want to migrate your entire body to high ground that day if you have a house on the beach. From there gaze safely as the bloated moon’s invisible fingers exert their strongest grip until the next millennium.

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