January 14, 2005

Longer Days, But Here Comes the Sun

January 14th, Pongal. Makar Sankranti here in Bombay. Consider the length of the days since the winter solstice on December 22, the shortest day of the year. (Well, shortest here in the Northern Hemisphere).

On December 22 '04, the day (sunrise to sunset) lasted 10 hours and 59 minutes in Bombay. It lengthened to 11 hours the next day, and stayed there till December 30, when it stretched to 11 hours and 1 minute. On Jan 3 '05, it got to 11:02; then 11:03 on Jan 6, 11:04 on the 9th. Today, Jan 14, the day will last 11:06 hours.

Nothing odd there, the days are getting longer, is all. As they should.

But now consider sunrise since December 22. That day, the sun rose at 0708. 0709 on the 24th, 0712 by the 31st, 0713 on Jan 4th. Today, Jan 14, the sun rose at 0715, which is the latest it will rise for the year. In a few days, the sun will start rising earlier every day.

What's happening here? The days are getting longer, as they should. But the sun has risen steadily later since the winter solstice. (The days have got longer for the last three weeks because the sunset has got steadily, and correspondingly, later). Today, January 14, is when this trend of later sunrises finally turns around.

A small prize of my choosing to those who can adequately explain why this is happening. And tell me what, if anything, it has to do with January 14th and Pongal.

1 comment:

Tanuj said...

hmm.. interesting.

ok, so this is because the eath's orbit is elliptical AND the axis tilted at 23.5 deg. (The ellipse itself is not entirely symmetrical around the sun along its longer axis but that's another matter.) because of this, the locus of the position of the sun in the northern hemisphere follows a tiled '8', (called the analemma) i.e. the top part of 8 is the summer, and the bottom winter. and if you follow the path of the sun on the 8, you will see that a small part of the 8 is actually lower than the dec 22 point (which lies on the vertical line that splits the 8 into two symmetrical halves). everything lower that the dec 22 point has sunrises later than dec 22, even though days are longer in this period. jan 14 represents the lowest point on the 8.

similarly, as you move up along the 8 you will see that the earliest sunrise actually happens before the summer solstice.

and then there are sunsets.

for a (much!) more detailed explanation and cool animation on this phenomenon, visit www.analemma.com :)