This explains why the evenings start drawing out before the shortest day, but it continues to get darker in the mornings until the new year. This causes the day to shift about 30 seconds later each day, as shown in the diagram below. So, in December solar days are on average 24 hours and 30 seconds in length, while our clocks and watches are still assuming that each day is exactly 24 hours. For example, a value of 10 means a solar day is 24 hours 10 seconds long, 20 means a solar day is 24 hours 20 seconds long, and -10 means a solar day is only 23 hours 59 minutes 50 seconds long.Īs you can imagine, it would be complete chaos if our clocks and watches had to cope with days of different lengths, so we use 24 hours, the average over the whole year, for all timekeeping purposes. The graph shows the difference between the length of a solar day and its average value of 24 hours throughout the year. It is at its shortest, around 23 hours 59 minutes 38 seconds, in mid September and at its longest, around 24 hours 30 seconds around Christmas Day. The length of a solar day varies throughout the year. The table shows that during December the solar noon drifts later by about 30 seconds each day.Ī solar day is the period of time between solar noon on one day and solar noon on the next day. The final column shows the solar noon, the time of day that the Sun is at its highest in the sky or, to put it another way, the mid-point between sunrise and sunset. This clearly shows that 21 December has the shortest period of daylight, but while the time of sunrise continues to get later and later throughout the whole of December, the time of sunset stops getting earlier around 12 December. In the table above, the daylight column shows the number of hours, minutes and seconds between sunrise and sunset. The table below shows the sunrise and sunset times for London for December at three day intervals. This post aims to explain this interesting phenomenon. The evenings in fact start to draw out a week or so before December 21, so it is already getting lighter in the evenings, although it does not start to get lighter in the mornings until early in the new year. Many people think that it will continue to get dark earlier each day in the afternoon until we reach 21 December, the winter solstice. As I complete this post from my home in Manchester, England, it is 4:30 pm and already dark outside.
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