Today [21 Dec] is the shortest day of the year

This Sunday, 21 December, the northern hemisphere will experience the shortest day of its year, marked at 22:03 GMT by an astronomical phenomenon known as the winter solstice - the moment the North Pole is tilted furthest from the sun as the Earth continues on its orbit.

The solstice doesn't always occur on 21 December. Sometimes it nudges into the early hours of 22 December, which will happen again next year. The hour of day also varies. Last year's arrived at 17:11. Next year's will at 04:38.

It would seem logical that after the shortest day has elapsed the mornings would start getting lighter earlier, but this isn't what happens - the mornings continue darkening until early in the new year.

In the southern hemisphere, it's exactly the opposite story. In Sydney, Australia, for example, mornings will start getting darker from the middle of December, while the evenings will continue to get lighter until early January.

So what is behind this peculiarity, which appears to fly in the face of received wisdom about the solstice - surely the shortest day should experience the latest sunrise and earliest sunset?

Well, the primary reason behind it all is that a day - a solar day to be precise - is not always exactly 24 hours.

"In fact, it is 24 hours only four times a year, and never in December," explains astronomer Stephen Hurley, who runs a popular science blog called The Science Geek. "It is at its shortest around 23 hours 59 minutes and 30 seconds, in early September, and at its longest around 24 hours 30 seconds in December."

There are two reasons why the length of the solar day varies, the first being the fact that the axis of the Earth's rotation is tilted - 23.5 degrees from vertical - and second, the Earth's speed varies because it moves in an elliptical orbit around the sun, accelerating when it is closer to the star's gravitational pull and decelerating when it is further away.

The sun therefore in effect lags behind the clock for part of the year, then speeds ahead of it for another.

"As you can imagine, it would be complete chaos if our clocks and watches had to cope with days of different lengths," continues Hurley. "So we use 24 hours, the average over the whole year, for all timekeeping purposes.

"So, as the solar days in December are on average 24 hours and 30 seconds, while our clocks and watches are still assuming that each day is exactly 24 hours, this causes the day to shift about 30 seconds later each day."

This cumulative shifting explains why the evenings draw in towards their earliest sunset a couple of weeks before the shortest day, and why the mornings continue to get darker until a couple of weeks after.

Couttesy: BBC NEWS