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Full Moon at Perigee


From the author of   Sizing Up The Universe

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. The greatest separation is called apogee. On average, the moon's distance is 385,000 kilometers. At perigee, it is a bit closer, 356,700 kilometers, whereas at apogee, it is somewhat farther away: 406,300 kilometers.

The full moon on March 19, 2011, coincided almost perfectly with the time of the moon's perigee. Hence, this full moon appeared larger than any other full moon in several years. I took a picture of it (left below). I also took a picture of an almost full moon back on Dec 19, 2010---just a day and a few hours before the famous "solstace lunar eclipse" (see http://www.princeton.edu/~rvdb/images/NJP/lunarEclipse.html). I used the same equipment to take these two pictures. Hence, the relative sizes of the moon in these two pictures correspond to the relative apparent sizes as seen by a visual observer.

Measuring the height of each moon in the picture and dividing, I get that the diameter of the moon on March 19 is 6.0% larger than the December 19 moon (making it 12.4% percent larger in area). My planetarium program tells me that on December 19, the (center of the) moon was 375,820 kilometers from my home in NJ. The same program tells me that on March 19 it was 354,192 kilometers away. The ratio of these two distances is also 6.0%.

Click on image for full-size version (it's big!).

23:46 EDT, Mar 19, 2011
Canon XSi (450D) on 10" RC
4 panel mosaic. Each panel is 3 images of 1/320 seconds and ISO 400.
20:40 EST, Dec 19, 2010
Canon XSi (450D) on 10" RC
2 panel mosaic. Each panel is one image of 1/80 seconds and ISO 100.

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