Wednesday, May 9, 2012

On the Sizes of Things in the Sky, Part II

On the 6th of May 2012, many people noticed and reported on the so-called "abnormal" size and brightness of the moon. It just goes to show how little people notice if its not pointed out. I wrote last month about the sizes of things in the sky, in which I explained why the moon varies in size, anywhere from 9.3% smaller than to 4.8% larger than the sun.

The moon has an elliptical orbit, which means that it can be considerably closer at one point of the orbit than at another point. Over 40 000 km closer, in fact (which is actually less than 6% closer than it's average distance). This point in the moon's orbit is called the perigee.

(On a side note: The perigee has an interesting property - the rate at which the moon is getting closer or further from the Earth is slowest at this point. That means that the perigee is a gradual thing, and the moon stays close to this distance for a couple of months at a time.)

A supermoon is arbitrarily defined to occur when the moon is within 90% of its perigee during a full or new moon. Astrologers refer to the event as a supermoon, and like to believe that it has some special significance on natural disasters and the like. Astronomers, on the other hand, use the term perigee-syzygy, and point out that apart from causing perigean spring tides, there's hardly any evidence suggesting that it has any significance whatsoever.

Just in case the media frenzy seems to have made the supermoon of earlier this month appear to be an incredibly rare occurrence, I've taken the effort to look up all the dates of supermoons in the 21st century (i.e. I clicked the second link in the Google results for "super moon dates"). The dates, from the very astrologer who coined the term, for the last and next five years are as follows.

Table 1: Supermoon dates for full moons from 2009 to 2015.

11 January 2009
31 December 2009
30 January 2010
28 February 2010
18 February 2011
19 March 2011
18 April 2011
6 April 2012
6 May 2012
4 June 2012
25 May 2013
23 June 2013
22 July 2013
12 July 2014
10 August 2014
9 September 2014
29 August 2015
28 September 2015
27 October 2015

There is another type of supermoon - the so-called extreme supermoon, which occurs when the moon is closer than its average closest approach. The table below lists the dates on which this occurs for the first half of the 21st century. You cannot help but notice that 2012 is suspiciously absent from the list.

Table 2: "Extreme supermoon" dates for full moons from 2001 to 2050.

12 December 2008
30 January 2010
16 March 2011
14 November 2016
2 January 2018
25 November 2034
13 January 2036

Although, the actual event of an "extreme" supermoon is not that different from a normal full moon, as demonstrated in the picture below. Note that the March 2011 supermoon was supposedly the biggest since 1993.

The "supermoon" of 19 March 2011 (right), compared to a "typical" full moon of 20 December 2010 (left).
Image by Marco Langbroek, 2011 under a Creative Commons Attribution-Share Alike 3.0 Unported license.

Fascinating. It seems that the whole supermoon frenzy is simply just another case of a journalist getting hold of a piece of information and getting extremely excited about it without bothering to do any research on the topic first.

(On a side note: If you do want to read further on supermoons, please bear in mind that logic is to astrology as the finer points of Russian literature are to the bacteria on a decomposing dead penguin in the Antarctic.)

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