March 19, 2011: A Perigeean Full Moon

The moon traces an elliptical orbit around the Earth; the moon's furthest distance from Earth (called apogee) is about 407,000 km, while its closest approach (called perigee) is about 357,000 km away. Now full moons can occur at any points along this orbit, but what makes the moon of March 19, 2011 special is that it became full at almost exactly the same hour as it reached orbital perigee. In other words this full moon occurred about as close to earth as is possible. That turns out to be a pretty rare coincidence and hasn't happened for almost 20 years. So the March 19 full moon appeared to us as an unusually Big One.

To compare with my March 19 moon image, I happen to have one I photographed on January 18 – using the same camera and lens; and the exposures were essentially identical. The animated GIF shown below is derived by superposing these two full-moon images (each at the same scale factor). The animation shows the moon's normal seasonal variation in apparent size––a consequence of the elliptical orbit.

Along with showing the size variation, this animation also illustrates the lunar libration that has occurred between January and March.  "Libration" is the term given to that bit of periodic (back-and-forth) rocking motion that the moon does. Yes, the moon rocks!  Between Jan. 18 to Mar. 19 you can see that the upper part of the moon has tilted slightly toward the viewer on Earth. A much more impressive animation by French "lunatique", Laurent Laveder, shows the moon's to-and-fro librations over a full year (12 consecutive full moons during 2005 and 2006), and that can be seen by clicking here. Because of the moon's libration, a total of about 59% of its surface is visible from Earth.


For the record: my January full moon was only 95% full and waxing (heading toward 100%). The March "Big Moon" was about 99% full and waning (beyond total fullness). This accounts for the fact that the slightly shadowed edges in these two moon images occur on opposite sides of the moon (i.e., bottom edge is shadowed in January vs. the top edge in March).

The images were made with an Olympus E-3, 50-200mm f/2.8 Zuiko lens with a 1.4X telextender (the net focal length of 283mm has a 35mm-equivalence of 566mm). ISO 100, 1/[email protected]/11 and 1/[email protected]/8 for January and March, respectively. Return to my Gallery