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
Couttesy: BBC NEWS
