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.

The Most Powerful Thing on Earth

On Monday, the recent earthquake in Japan was recalculated to be a 9.0 on the moment magnitude scale. This is not the Richter scale, which is based off of the horizontal shaking amplitude of an earthquake. Instead, the moment magnitude scale is based off of the sheer amount of energy released. As it turns out, this scales to about 3/2 the horizontal amplitude, and as such is calibrated to match closely to the Richter scale.

What does the recalculation from 8.9 to 9.0 mean? Well, the Richter scale itself is a logarithmic scale on base 10; a step of 1 relates to 10x the displacement. However, the moment magnitude scale, while logarithmic as well, is calibrated so that a step of 1 means about 32 times the energy was released.  A magnitude 2 points higher had a whopping 1,000 times the energy. As such, this lowly adjustment of .1 on the scale actually denotes around 40 percent more energy.

This got me thinking, what does this actually mean? Well according to the USGS, the earthquake released 39 zettajoules. What's a zettajoule? You've heard of the prefix mega-? giga-? tera-? peta-? exa-? This is bigger than all of those, zetta- is reserved for 10²¹.

Well, that's a big number, but it's probably meaningless, right? Here's why that's insane.

First off, the world's largest nuclear weapon ever detonated, the Tsar Bomba, had a 50 megaton yield. That means it had the equivalent force of detonating 50 megatons of TNT. How much energy is that? One megaton of TNT would produce 4.184 petajoules when detonated, so this comes out to 209.2 petajoules. It'd take 186,000 of them to match the energy of the earthquake. Fat Man, the bomb dropped on Nagasaki, was a paltry 20 kilotons in comparison, 2,500 times smaller. Even the 1883 eruption of Krakatoa, which was heard 3,000 miles away, had an estimated force the equivalent of only 200 megatons of TNT.

In comparison to our current capabilities, according to World Nuclear News there were 2,558 terawatt-hours of energy produced by the world's nuclear reactors in 2009. That comes out to 9.2 exajoules. That means every nuclear reactor in the world would take four and a quarter millennia to match the earthquake's output.

In fact though, nuclear reactors provide only a small portion of the world's energy: we consumed 484 exajoules overall in 2008. At that rate, if we could have harnessed all the energy from Japan earthquake, we could last on it for 82 years. And yet the earthquake only took 5 minutes to release it.

The even more amazing part: this number is nothing compared to the Sun. The Sun outputs 384.6 yottajoules every second. What's a yottajoule? It's 1,000 zettajoules. So in a single second the Sun outputs nearly 10,000 times the energy produced by the earthquake. Though, to be fair, the Earth only receives a small portion of that output, 174 petajoules a second. And at that rate, it'd still take 2 months to match the earthquake's raw power.

The moral of the story? Never doubt the power of mother nature. To think this is just one of six to break 9.0 on the moment magnitude scale in the past 100 years, not including the 8.8 in Chile last year and the 8.5 in Sumatra in 2005.  After all, this planet is both breathtakingly awesome and entirely terrifying in the forces it can unleash.