December 26, 2001
The winter solstice has passed, but careful observers will note that the time of sunrise in the northern hemisphere continues to get later each day, and that earliest sunset already occurred around December 8. Are the days really getting longer? Yes, indeed they are, but a combination of the tilt of the Earth’s axis (obliquity) and the eccentricity of its orbit causes the sun to move at a variable rate through the sky during the year. This variation is described by the Equation of Time. From mid-November to early February tilt and eccentricity work together to make the solar day (sundial noon to noon) longer than 24 hours by as much as 30 seconds in late December. The cumulative effect of these extra seconds each day create an offset in the time of local noon (and the times of sunrise and sunset) by as much as +/- 16 minutes, and is most noticeable near the solstice. The end result is later sunrises until about January 5 when the sun climbs high enough into the sky for obliquity to become the dominant factor in the Equation of Time.