Bulging Full Moon rising over the Temple of Poseidon, Cape Sounion, Greece. Imaged by Elias Chasiotis on April 28, 2010. ©Elias Chasiotis, shown with permission.

The rising and setting moon, like the sun, is usually slightly flattened.

Refraction by our curved thin shell of atmosphere rapidly thickening near the surface refracts the moons rays differentially.

Rays from the lower limb travel a curved path, concave towards earth’s centre, so that the limb appears higher in the sky than its true position. The effect is weaker for upper limb rays and thus the moon appears flattened. From a spacecraft the effect is extreme.

Here the flattened moon has an extra bulge at the lower limb. The bulge is actually an area of reduced flattening as the superimposed ellipse and circle reveal.

A temperature inversion is the cause. The lower air layers are warmer and therefore slightly less dense than usual. The less dense air refracts the rays from the lower limb less strongly causing it to appear bulged downwards.

Red light is refracted less than green and blue producing a red rim on the moon's lower rim and a hint of green on the top.

The Temple of Poseidon with its graceful Doric columns was built in c440 BC on the site of a much earlier one. It stands on a headland with the Aegean on three of its sides.

Elias imaged the temple from 2 km away. Canon EOS 450D, Canon EF 70-200 F2.8, 2x converter at F7.1, ISO 200, exposure 0.4 sec.


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