Monday, March 21, 2011

Supermoon and the Saros cycle

By now, you cannot possibly have escaped the furious news blitz over the "Supermoon". In case you've been living under an Everest-sized rock, here's the story: the moon, last night had what's known as a "perigee syzygy".

In other words, the moon doesn't have a perfectly circular orbit. It varies from between about 222,000 to 252,000 miles away from Earth (center-to-center); the far distances (apogee) varies little, while the near distance (perigee) varies from 222,000 to 230,000 over a few months. The absolute maximum range is about 221,439 miles to 252,724 miles.

Occasionally, the moon gets to perigee at the same time as the full moon; due to the way the earth's and moon's orbits interact, the closest full moons usually occur between November and March. This is colloquially known as a "supermoon" and some astrologers, in typical astrologer lack-of-brains style, think it's actually significant.

Even worse, some people think yesterday's supermoon could be capable of causing earthquakes. Never mind that a) it's only 2% closer than the supermoons that occur 4-6 times per year and b) the Japan quake was a week before the supermoon, when it was still at first quarter. Phil Plait has a complete debunking.

But there is some neat stuff about supermoons. For example, the extreme ones don't occur completely randomly. Take a look at the current list of the most extreme ones (not sure about the criteria which were used) from Wikipedia:

November 10, 1954
November 20, 1972
January 8, 1974
February 26, 1975
December 2, 1990
January 19, 1992
March 8, 1993
January 10, 2005
December 12, 2008
January 30, 2010
March 19, 2011
November 14, 2016
January 2, 2018
January 21, 2023
November 25, 2034
January 13, 2036

Now look at what happens when I rearrange the dates a bit:

November 10, 1954
November 20, 1972
December 2, 1990
December 12, 2008

January 8, 1974
January 19, 1992
January 30, 2010

February 26, 1975
March 8, 1993
March 19, 2011

January 10, 2005
January 21, 2023

November 14, 2016
November 25, 2034

January 2, 2018
January 13, 2036

Every single one matches up with another or several, a little more than eighteen years apart. In fact, 18 years, 11 days, and 8 hours. That's a number that will immediately ring a bell for astronomers.

It's the Saros cycle - a pattern on which solar and lunar eclipses repeat.

Why, then, do these seemingly unrelated lunar events have the same periodicity? It's because of what the Saros cycle is. It's a combination of three lunar cycles - synodic, draconic, and anomalistic. After one Saros cycle, the moon has the same phase, the same node (location above below the plane on which the see the sun) and the same distance from Earth. This sets up identical eclipses - and also provides the conditions for nearly identical supermoons.

The Saros cycle, due to its combination of the three most significant orbital cycles of the moon, shows up in all sorts of weird places. Jean Meeus, in one section his brilliant book Mathematical Astronomy Morsels, analyzed the closest and farthest distances of the moon over the period 1500-2500. The closest approach and farthest apogee during that millennium occur in the mid 2200s - 1 Jan 2257 and 7 Jan 2266. That's 9 years 6 days - half a Saros!

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