The moon’s orbit about the Earth is not a perfect circle—it is slightly eccentric. As a result, during part of its orbit it is a little closer to us than at other times.
The closest approach is called perigee, while the greatest separation is called apogee.
On average, the moon’s distance from Earth is 239,228 miles (385,000 kilometers).
At perigee it is a bit closer at 221,643 miles (356,700 kilometers), whereas at apogee it is somewhat farther away at 252,463 miles (406,300 kilometers).
Saturday’s full moon has been called a supermoon, because the moon was closer to us than it had been at any time in the last 18 years, making it appear unusually large in the night sky.
After the event many pictures were posted of this super full moon. But many of my visual observer friends tell me it didn’t look much different from any other full moon. So, how much different was it?
It just so happens that I took a picture of the almost full moon on December 19, 2010—just a day and a few hours before the famous “solstice lunar eclipse” (pictures).
Fortunately, I used the exact same camera and telescope to take a picture of the March 19 supermoon.
Hence, a side-by-side comparison, such as the one shown above, of the two pictures gives a good idea of the relative apparent sizes of these two full moons.
My planetarium program tells me that on December 19 the center of the moon was 233,523 miles (375,820 kilometers) from my home in New Jersey. The same program tells me that on March 19 it was 220,084 miles (354,192 kilometers) away.
Measuring the height of each moon in the picture and dividing, I get that the diameter of the moon on March 19 was 6 percent larger than the December 19 moon, making it 12.4 percent larger in area.
Robert J. Vanderbei is chair of the Operations Research and Financial Engineering department at Princeton University and co-author of the National Geographic book Sizing Up the Universe. Vanderbei has been an astrophotographer since 1999, and he regularly posts new images on his astro gallery website.