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Ozone makes twilight wedge blue

Normally, the twilight wedge appears in a rather grey colour. But sometimes, in most cases above an inversion layer, the twilight wedge appears tuquoise-coloured as during this morning twilight on March 2, 2011. The night before had been so clear that the zodiacal light was visible for the naked eye. Towards the sun, above the first light of dawn, the crescent of the moon and Venus made a nice contrast to the blue sky. On the opposite side, there was a definite twilight wedge showing a rarely clear blue colour. This colour is caused by light absorption in the ozone layer.
Above the twilight wedge there was also the venus belt visible. (Photos 1234)

Author: Claudia Hinz, Brannenburg, Germany