Look! Up in the Sky! It's Supermoon!

You might want to take a look outside tonight. The Moon will be strutting its stuff as today it's no moon, it's a Supermoon! Obviously something called a "supermoon" must be supernatural in some form or another. I mean it's lining up with the peak of an annual meteor shower (Eta Aquariids) and Cinco de Mayo. Coincidences like this simply don't happen!  Other than today of course, when they do.

If you want to know what it means for your horoscopes, I'd suggest going to speak with a licensed astrologer. You might also need to stock up on some homeopathic medicines while you're at it, as you never know where the day will take you.

Ok, back to the point at hand. So "supermoon" is actually a colloquial term and is not accepted astronomer jargon. Astronomers would tell you tonight is the perigee-syzygy of the Moon in the Earth-Sun-Moon system. Yeah, that's not very helpful is it? I'll explain.

The Moon orbits the Earth in an elliptical orbit. On an ellipse, the perigee and apogee are the closest and farthest points from the center respectively. So the first part of a supermoon is that the Moon is at its closest point to Earth in its orbit. For the Moon specifically, the orbit is around 405,000km away at its apogee and around 360,000 km away at its perigee. It's basically 50,000 km closer at perigee than at apogee! The orbital period of the Moon, the time it takes to make one full revolution from perigee back to perigee, is called the anomalistic month and averages out to around 27.5 days.

Now syzygy should be a more familiar term, however we tend to use other names for it. To clear things up, no, I do not mean that episode of The X-Files where the two high school girls go crazy with mind powers and kill Ryan Reynolds. Syzygy is the term for when the positions of three astronomical bodies make a straight line. A true syzygy with the Earth, Moon, and Sun would be in times of solar and lunar eclipses, as either the Earth is directly between the Sun and Moon or the Moon is directly between the Sun and Earth. The term is also used more generically however to denote new moons and full moons. These aren't times when the three bodies are aligned, but they do represent a specific alignment of the Sun and Moon in relationship to the Earth. A lunation, or synodic month, is the time it takes the Moon to complete one lunar phase cycle; it averages around 29.5 days.

So really a perigee-syzygy in this sense could mean we're experiencing a new moon or a full moon while the Moon is at its perigee. Of course, why would we care about a new moon, we can't see it! So "supermoon" is really just a full moon at its closest point in orbit. Due to it being closer, it will actually be noticeably larger than average (the reported number is 12%). If the Moon were at it's apogee instead, it'd only be three-quarters as large and bright as you can see it today (it turns out 50,000 km can make quite a difference).

[caption id="attachment_1117" align="aligncenter" width="600" caption="The periods of the anachromal and syndochal months are above, with the lower graph representing the period of their interference."][/caption]

Now, as I mentioned, the Moon completes a revolution in about 27.5 days, but the lunar phase cycle takes 29.5 days. This causes a beat period between the two as they go in and out of phase together (shown above). This entire cycle is called the full moon cycle and occurs once about every 14 lunations (just under 14 months).  In fact, the last supermoon was in March 2011. So don't worry too much if you miss this one, the next one should be around July 2013. And if you miss the Eta Aquariids, they'll be back next year the same time as always.

The point of the story? It's just a big moon, spiraling around us at a kilometer a second, as we go spiraling around the Sun at 30 kilometers a second. Same as always, no big deal. But go out and enjoy it, because it'll be cool. If you're lucky, you'll see some shooting stars as well. And if you're really lucky, you won't party too hard for Cinco de Mayo and actually be able to remember your evening under the supermoon.