Originally posted on Dark Sky Diary by Steve Owens @darkskyman
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.
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. 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
you wrote all that just to explain that the moon won't be bigger? wow.
The Moon will look unusually big indeed, at least to an amateur astronomer's eye – because it did so, and quite spectacularly, last month when full moon and perigee almost coincided. I had not known that perigee was near, but when I saw the Moon high in the sky – no moon illusion at play! – I was literally blown over by its angular size and the amount of detail visible naked-eye.
The repeat of that wonderful sky show this month should be used as an educational opportunity, to teach about elliptical orbits (and how Kepler discovered them, though using Mars, four centuries ago). And that you can actually see that the Moon's distance from Earth changes considerably, without any technical help, which is quite remarkable.