vineri, 6 iulie 2012

Dusk/dawn asymmetry around the solstice

Despite my amateurish enthusiasm for astronomy, recently I came across a puzzle relating to the solstice that I couldn't immediately fathom. Why is the shortest day not also the day of the latest sunrise? In fact, in the Northern hemisphere, sunrise continues to get later as we pass the winter solstice, before it gradually begins getting earlier again.

Obviously there is some kind of asymmetry going on here. The first thing that came to my mind was the fact that the Earth is nearer the Sun in January (perihelion), though after reflection this does not seem to be important. The real explanation is (I think at least mostly) due to the difference between the sidereal day and the solar day. Thus, because of its rotation around the Sun, the Earth has to rotate slightly more each day before sunrise at a particular location. Dawn gets later after the winter solstice in the Northern Hemisphere because the Earth rotates in the same direction as it orbits the Sun.

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