Supermoon Nonsense

There seems to be a growing excitement about the “Supermoon” that is due to occur on 19 March 2011, when the Moon will be at its closest to Earth in this orbit, and closer than it has been at any time since 1992.

Moon – not Super

The Moon orbits the Earth in an elliptical orbit, i.e. it is not perfectly circular, and so in each orbit there is a closest approach, called “perigee” and a furthest approach, called “apogee”.

At this month’s perigee the Moon will be 356,577km away from Earth, and will indeed be at its closest in almost 20 years [This is WRONG: see Update 2 below!). But how close is it compared with other perigees?

Let’s start by comparing it to the Moon’s average distance from the Earth, which is ~385,000km. This perigee will be ~8% closer to the Earth than average. OK, that’s a bit closer, but not significantly so.

What about comparing it to the Moon’s average perigee distance, which is ~364,000km. So this “Supermoon” will be ~2% closer to the Earth than it is most months at perigee. Wow!

So what will this mean to you? Nothing at all. The Moon will be a few percent bigger in the sky, but your eye won’t really be able to tell the difference. It will also be a few percent brighter, but your eye will compensate for this too, so altogether this “Supermoon” will look exactly the same as it always does when it’s full.

As to all of those soothsayers claiming that there will be earthquakes and tidal waves. There very well might be, but they’ll be nothing at all to do with the Moon.

UPDATE: I predict that lots of people will report having seen a huge Moon on or around 19 March

UPDATE 2: Thanks to “justcurious” for the comment that inspired this calculation, and to Steve Bell at the UK Hydrographic Office for providing the calculations below:

The Moon orbits the Earth once every 27.321 days (called the sidereal period), but as the Earth is orbiting the Sun at the same time, the Moon’s phases appear to repeat every 29.530 days (called the synodic period, which is the time we use to derive the month).

So the first part of the answer is: the Moon is full every 29.530 days.

The Moon’s orbit is elliptical (a squashed circle) and so you would expect a perigee once every 27.321 days. However the elliptical path around which the Moon orbits the Earth precesses (that is it is not fixed with the perigee occuring at the same part of each orbit; the place where perigee occurs moves, or pressesses) with a period of 8.8504 years, so that perigee doesn’t occur once every 27.321 days but rather once every 27.554 days (called the anomalistic period).

To calculate the frequency of perigee full Moons you need to use the equation:

1/P(perigee&full) = 1/P(perigee) – 1/P(full)

where P(perigee) is the anomalistic period = 27.554 days, and
where P(full) is the synodic period = 29.530 days

and when you put those figures in you find that a full Moon will occur at perigee once every 411.776 days (i.e. P(perigee&full)=411.776, or just less than once per year.

All the articles that cite this as the closest full Moon in 18.6 years are wrong; there was a full Moon at perigee 411.784 days ago, on Feb 28th 2010 when the full Moon occurred at 1700UT and perigee occurred just 19 hours before at 2200UT on Feb 27th 2010.

The next so-called Supermoon will occur on May 6th 2012, when the full Moon will occur at 0400UT, with perigee at the same time.