Today is the winter solstice, the shortest day and longest night of the year, but today is not the latest sunrise and earliest sunset — the latest sunrise will come on December 30 and the earliest sunset happened on December 12.
This bizarre phenomenon is because of our mixed-up timekeeping. Our clocks are actually telling a bit of a fib because they assume that every day is exactly 24 hours long, which isn’t strictly true — the length of a day varies throughout the year.
If you tell the time using a sundial, a day is measured from one noon to the next. This is called a solar day and, strangely, it can vary throughout the year by about 15 minutes. This is all thanks to the tilt of the Earth on its axis, and because of the slightly oval-shaped orbit of the Earth round the Sun. This makes the solar day longer than 24 hours at the winter and summer solstices. It also makes the solar day shorter than 24 hours at the spring and autumn equinoxes.
In the days when sundials were used for timekeeping, this curiously flexible length of day wasn’t a problem. However, keeping solar time with clocks would have meant changing the time every day, which was wholly impractical. So to make life easier, everyone used the average length of 24 hours throughout the year, what is known as Greenwich Mean Time.
As a result, sundial time is out of kilter with GMT. This means that solar noon on December 21 in London, for example, will be at 11.59am, but on December 12, the date of the earliest sunset, solar noon was at 11.54am. On December 30, the date of the latest sunrise, solar noon in London will be 12.03pm.
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These dates also depend on the latitude of a location. Places that are closer to the Equator have their earliest sunset in November. Locations at higher latitudes, on the other hand, have their earliest sunset closer to the date of the winter solstice.
