Sunrise and Sunset at Solstices

Monday, January 3rd, 2011

It is curious that the days of shortest and longest periods of sunlight (which just about everyone knows are due to the tilt of the earth's spin axis relative to the plane of its orbit)  are not everywhere the same as those of latest and earliest sunrise. This is because the length of the full solar day is not actually constant and so the time of solar noon oscillates around the time that would correspond to noon on a steadily progressing clock.

This is often attributed to the eccentricity of the earth's orbit, and that does play a minor role. But actually, the effect is mainly due to another effect of the earth's tilt - a secondary effect on the difference between the length of a full solar day (taken by an apparent revolution of the sun from noon to noon) and a constant sidereal day (taken for a full revolution relative to the distant stars).