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This montage of two images of the Moon illustrates how its apparent size changes during the month. On the left, it is near its closest to the Earth. Half a month later (right), it shows not only the opposite phase, but is also near its furthest distance from Earth. The distance - and hence apparent diameter - varies about 7.5% either way from the average.
These two images were in fact not taken during the same month. The image on the right is from 2005-02-18 and the one on the left from 2007-04-13.
When perigee falls close to Full Moon, this is called by some a "supermoon". Due to its larger size, it is 0.15 mag brighter than the average Full Moon. Casual observers will notice neither the increased size nor the increased brightness.
This phenomenon must not be confused with the optical illusion that a Moon on the horizon appears to the human brain larger than a Moon high in the sky. The opposite is true: When the Moon is in the zenith, the observer is 1.7% closer to the Moon than when it rises on the horizon.
Canon EOS 300D, f = 800 mm, f/12.6, 400 ISO, 5 ms, unsharp mask.
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