The shortest day is usually 21 December, but the earliest sunset is 12 December, and the latest sunr

The shortest day is usually 21 December, but the earliest sunset is 12 December, and the latest sunrise is 31 December. Why are the earliest sunset and latest sunrise so far apart?

Richard Miller, Addlestone, UK

• The reason is that a “solar day” – i.e. the time it takes for the sun to return to the meridian you happen to be on – is typically not quite 24 hours. It’s sometimes more and sometimes less. 24 hours is actually the length of a “mean solar day” or the average length of the solar day over a year. The length of a solar day can differ from 24 hours by as much as 16 and a half minutes. It varies due to effects caused by the tilt of the earth’s axis and the fact that the orbit of the earth around the sun is not perfectly circular (which causes the speed at which the earth orbits the sun to vary slightly through the year). When the solar day is shorter than 24 hours the position of the sun in the sky at “clock noon” edges gradually westwards day by day, when it’s longer than 24 hours it edges eastwards. Sunset and sunrise are symmetric around “solar noon” (the time the sun crosses your meridian), not clock noon. Between around the beginning of September and Christmas Day the sun is already west of your meridian at clock noon so solar noon is earlier than clock noon (by around 3 minutes on 12th December). As the day is symmetrical around solar noon, less than half the day’s sunlight occurs after clock noon and more than half before. After Christmas Day (until mid-April) the sun is still east of your meridian by clock noon so solar noon is later than clock noon (again by around 3 minutes on the 31st December) and more than half the day’s sunlight occurs after clock noon and less than half before. Close to the shortest day the actual length of daylight time varies very little so the effects of these changes in the difference between solar time and clock time become quite apparent in this asymmetry in sunrise and sunset times. The changing difference between solar time and clock time through the year is known as the “equation of time” and can sometimes be found on old sundials in the form of an “analemma” - a figure of eight etched on the face of the sundial which is used to correct the reading on the dial (solar time) to clock time.

  Frank Roberts, London UK