04 March 2009

Was that the biggest Full Moon of the year in January?

I am sometimes asked questions about astronomy themes that people hear in the popular media. In January friends sent me links to articles about the "biggest full moon of the year" and they made special efforts to get out and see this particular full moon. What was all the fuss about? Was it worth it?

First this thought: I am as captivated as anyone with the Moon. It is our closest neighbor and it has so many things to offer if you are ready to put in the time to study it. Most amateur astronomers put their telescopes away when the Moon is full, but in fact there is much to see using a simple telescope or binoculars such as the beautiful, large craters Tycho and Copernicus, or the large seas such as the Sea of Tranquility (also known as Mare Tranquillitatis, where Apollo 11 landed).

Back to the "biggest moon" discussion: As the Moon makes its 29 1/2 day journey around the Earth, the path it follows is an ellipse. The average distance from the Earth is about 240,000 miles, but the actual distance of the Moon from the Earth at any given time in its orbit can range 12,000 miles closer or 12,000 miles more distant from the Earth. These two extremes are called perigee and apogee respectively, and when the Moon is at perigee it appears about 15% larger (from our Earth-bound point of view) than it does at apogee.

When the popular press reported the "biggest Full Moon of 2009" they were accurate, but the fact is that every lunar cycle (every 29 1/2 days) the Moon goes through at least one cycle of perigee and apogee and therefore it is, at some point on its monthly journey, as big as the biggest Full Moon was in January -- but the phase will be different each month, from near Full ("gibbous") to First Quarter to Crescent to New Moon and so on around the cycle. The next perigee is on April 1st when the Moon will be in the phase of First Quarter. So I strongly urge everyone to get out on April 1st to see the "biggest First Quarter Moon of 2009." You won't be let down!

1 comment:

Sidewalk Universe said...

Nice write up Paul - very well said. It really is a joy to get familiar with Luna's cycles, phases and features. It is a monthly and daily inexpensive entertainment break - I really get a kick seeing it at different times of the day - and anticipate it's showing in the sky. The Moon never ceases to amaze me!