The Times, November 29^th 2005

Winter Solstice

It is often remarked that the evenings get lighter before the winter solstice (about December 21) while the mornings get darker into the new year. This arises because the Sun does not transit (cross the meridian, be due south when it is half way between rising and setting) at exactly 12h as shown by our clocks.

Before the 25th the Sun transits before 12h and this makes sunset early. Over the next few days it transits closer to 12h and this offsets the still slightly shortening length of the day and sunsets start to become a few minutes later. After the 25th the Sun transits after 12h and this makes sunrise later by our clocks. At first this offsets the slightly lengthening day. The combined effect is for the earliest sunset to be about the December 12-13 and the latest sunrise about January 2.

Four times each year the Sun transits at 12h and December 25 is one such day. In early February transit is as late as 12h 14m while in early November it transits at 11h 44m. The Earth’s distance from the Sun varies throughout the year and the Sun does not follow the equator so the Sun does not move eastwards at a uniform rate each day. When a Greenwich sundial shows 12h noon it will not usually be 12h GMT. For practical reasons we need every day, hour and minute throughout the year to be of equal length.

Astronomers invented mean (or average) time where a fictitious sun moves uniformly along the equator giving equal intervals of time. When related to the Greenwich meridian this became Greenwich Mean Time (GMT) now equivalent to Universal Time (UT). In practice it is observations of stars that are used to define UT because the Sun’s position is difficult to measure accurately. The discovery that the Earth’s rate of rotation and thus the length of a day is subject to short-term variations required a more uniform time scale still, now provided by atomic clocks. Leap seconds are added to UT from time to time to keep it more nearly in step.