SuperMoon

A number of you have probably heard about the SuperMoon coming next week on March 19.  A friend of mine recently mentioned it, and I'm sure students will bring it up in class next week, so I thought it would make a good blog post (one which will likely disappoint many of you reading this).

There's really nothing astronomically special about this month's full Moon.

In order to understand why, however, we have to discuss a few things about the Moon's orbit around the Earth.  You'd think the orbit of the Moon around the Earth is simple, but it's actually quite complex (much more so than I'll discuss here!).

Way back in 1609, German mathamatician/astronomer Johannes Kepler showed that the planets all orbited the Sun in ellipses, not perfect circles.  The closest approach of a planet is called perihelion and the furthest distance is called aphelian (from "helios", the Greek word for Sun).  The Moon similarly orbits the Earth in an ellipse, but closest and furthest approaches are called perigee and apogee (from "gē", the Greek word for Earth).

Each month, as the Moon orbits the Earth, it has a point at which it's at perigee and a point at which it's at apogee.  At perigee, since it's closer, the full Moon actually looks about 12% larger than it does at apogee.  The distances of perigee and apogee also change slightly with each orbit of the Moon.  Here's a list showing values for 2011 (click to enlarge).

Note that March 19 is a lunar perigee date and it's the closest approach of the year at 356,577 km.  Is it unusually close?  Not really, on December 12, 2008, the distance was 356,567 km - a hair closer.  On November 14, 2016, it will be closer yet at 356,511 km.  So, while this perigee in March is closer than average, it's nothing unusual and close approaches of a similar nature occur every few years.

As an aside, one web site I visited said it was the closest approach in 18 years.  I got my data above from the Lunar Perigee and Apogee Calculator which indicated a closer approach in 2008 as I indicated.  When I checked the data for 18 years ago (1993), I saw March 8 of that year had a perigee of 356,529 km.  A bit closer, but only by a few kilometers.  Either way, my argument holds that this is not a significant event.

As another aside, I've seen the term SuperMoon attributed to an astrology named Richard Nolle who defines it on his website as "... a perigee-syzygy, a new or full moon (syzygy) which occurs when the Moon is at 90% or greater of its mean closest approach to Earth (perigee)."  A lot of woo sites on the Internet are claiming terrible catastrophes with this SuperMoon but these claims simply don't hold up to scrutiny.

If you want really extreme perigees, you missed the last one if you're younger than 81 years old.  It occurred on January 15, 1930 when the Moon was 356,397 km from the Earth.  Forget about the next extreme perigee, it will occur on January 1, 2257 when the Moon is a mere 356,371 km away.

What is a little more interesting about this perigee in March is that it occurs at the time of the full Moon (technically a  perigee-syzygy where syzygy refers to an astronomical alignment of 3 or more celestial objects - in this case the Sun, Earth, and Moon).  Perigee and apogee are not linked to the Moon's phases because of differences in their cycles.  The amount of time from perigee to perigee is 27.55455 days.  The amount of time from full Moon to full Moon is 29.53059 days.  That's a two-day difference.

Since:

    14 x 29.53059 days = 413.43 days

and

   15 x  27.55455 days = 413.32 days

These cycles approximately coincide every 413 days.  So, since we have a perigee full Moon on March 19, 2011, we can expect a perigee full Moon again  1 year and 48 days later on May 6, 2012.  Guess what the Lunar Perigee and Apogee Calculator says? That's right, another perigee full Moon on May 6, 2012 (science works!).  That perigee's not as close at 356,953 km, but it's only about a 0.1% difference!

So, perigee full Moons aren't all that unusual either, they occur every year.

There is one final significance to this month's full Moon, however, and that is that it's occurring near the vernal (spring) equinox.  That's the point when the Earth is neither titlted toward nor away from the Sun and the Sun is directly over the Earth's equator.  On the equinox (literally "equal night" from Latin), the day and night are each about 12 hours in length everywhere on Earth.

Astronomically, the equinox will occur on March 20 at 23:21 UTC. The Full Moon will be March 19 at 18:10 UTC.  The perigee will be on March 19 at 19:10 UTC.  So while perigee and the exact time of the full Moon are only an hour apart, they miss the equinox by slightly over a full day.  Is it unusual for a full Moon to fall on the equinox?  Not really, it happened on the last autumnal equinox of September 22, 2010.  Between 1951 and 2049, there have been three full Moons on vernal equinoxes (1962, 1981, and 2000) and five full Moons one day offset from the vernal equinox (2008, 2011, 2019, 2030, and 2038).  Exact matches, unlike this year, occur every 16.7 years in this century span of time.  Not a big deal astronomically.

By the way, all of this is why Easter is so late this year. Since Easter generally (the formula used is more complicated) falls on the first Sunday after the full Moon after the vernal equinox, it can only fall between March 22 and April 25. This year, because there's a full Moon right before the equinox, we have to wait another lunar month for the full Moon on Sunday, April 17 (my birthday, by the way) and then another full week for Easter on Sunday, April 24 (about as late as it gets - it fell on April 25 in 1943 and won't again until 2038).  If the image at right puzzles you, there are two types of people in the world, those who like peeps, a traditional Easter treat in my house, and those who don't.

What about the big earthquake in Japan?  Was this perhaps caused by how close we are to a SuperMoon?  Sorry, the facts don't support it.  The Moon at the time wasn't even first quarter phase yet (not until March 12 at 23:45 UTC).  This means it's at right angles to the Earth and Sun and at the point of least gravitational influence.

Let's look at the distance from the surface of the sea to the Moon at the time of the earthquake vs. the time of perigee on the 19th.  This data can be obtained from the U.S. Naval Observatory Data Services Topocentric Configuration of Major Solar System Bodies link.  I used 38º 19' N and 142º 22' E as the coordinates of the quake.

When the earthquake occurred on March 11 at 5:46 UTC, the Moon was 390,644 km away.  On March 19 at 19:10 UTC, the point of perigee, the Moon will be 355,167 km away.  That's actually a pretty big difference of about 9%.  Compare that to the only 0.1% difference between the perigee Moon on March 19, 2011 and the perigee Moon on May 6, 2012 - in other words, the earthquake was completely unrelated to the closeness of the Moon.

Here's a big caveat, however.  Just because it's not that big a deal astronomically, and will not result in all kinds of terrible catastrophes, doesn't mean it's not significant.  Our ancient ancestors would have treated this as a special event, a correspondence between the solar and lunar cycles.  Go outside next Friday, look up at the large full Moon as the Northern Hemisphere of the Earth enters the first day of spring, and feel the connection with the natural cycles which connect all living and nonliving things in the eternal passage of time.  I'll be out there.