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At sunset for the observer, sunbeams (blue lines) are nearly parallel, since the Sun is so far away. The Moon will appear illuminated from above.


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The illusion is due to the sun being so far that the difference in angles shown in the drawing would not be noticeable. Imagine two planes where both of them contain the sun and moon, the level plane would be how an observer would see the moon when both the moon and the sun were at the horizon. The tilted plane is how an observer would normally see the moon ...


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I found one study from 2001 {1}: It was also determined that for a fulltime outdoor worker, the additional UVA exposure could approach approximately that of one third of a full winter’s day. For indoor workers with an outside lunch break of noon to 1 pm, the additional UVA exposure was on average 6.9 kJ m$^{-2}$...


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