January 14, 2011

The sun turns

December 22, or the winter solstice, is the shortest day of the year. After that, the days get longer. So without thinking about it much, I grew up imagining two things: one, that in the days before December 22, the sun rises slightly later and sets slightly earlier every day, thus each day turns out to be shorter than the previous. And two, that after December 22, the sun rises slightly earlier and sets slightly later every day, thus each day turns out to be longer than the previous.

Well, things don't quite match that neat picture.

In the days before December 22, the sun actually does rise slightly later every day. But it also sets slightly later every day, though the daily increments are smaller than with the sunrise. The result: the days get progressively shorter.

In the days after December 22, the sun continues to rise slightly later every day. It also sets later every day, though now the daily increments are larger than with the sunrise. The result: the days start to lengthen, just as expected.

And this trend continues till … today, January 14. It is only after today that the sun starts rising slightly earlier every day.

Questions: Is this why January 14 is a marked day on Indian calendars (Pongal, or Sankranth)? And is this date a function of the earth's latitude? Or of its tilt on its axis? Or something else? In other words, can you explain this phenomenon to me in terms I can understand?


PostscriptThere should be a similar, but mirror-imaged, phenomenon around the summer solstice, June 22. Is July 14/15 a similarly marked date anywhere?


Anonymous said...

Perhaps the axis does not stay at a fixed angle? Wiggles a bit like a spinning top?

Anonymous said...

Look up Uttarayana and Dakshinayana if interested in silly blind beliefs of hindoos.

Raman S said...

Dcubed, I never commented here before, but I looked at pvs comment and wondered, can ur various trolls unable to ever let go of their inner anger and hostility twds the world??

Abt ur post, yes, it has to do with the tilt of the Earths' axis. Here's one quick indication that, at least for me, helps to explain the whole phoenomena.

Number of days between solstice and Pongal = 23.

No. of days between solstice and equinox = (approx) 90.

23/90 - abt the same ratio as earth's tilt (23.5 degrees) divided by max possible tilt (90 degrees).

ie., if earth's tilt was maybe 45 degrees, pongal would come about Febuary 4.

Abt ur postscript, the same logic should apply. Sun shd turn south somewhere around Jul 15. Ill make a note to follow it this yr.

Transmogrifier said...

Your observation is correct. It is a function of latitude in that the day after the winter solstice until which the sunrise time will keep advancing before starting to recede varies by latitude.Check out the sun calculator here to get the sun-rise sunset times for various locations around the world. However the shortest length of the day does occur on the same day in the northern hemisphere ~ Dec 21, the winter solstice.

Similar phenomenon occurs with the sunset time after the summer solstice in the northern hemisphere.

The Makar Sankrant date is decided by sidereal astrology to be the date on which Sun enters the zodiac sign of Capricorn (Makar rashi). That is the reason why Makar Sankrant is celebrated on January 14/15 in India. It is the only date in Indian calendar (which is based on Jyotish sidereal astrology) which is based on a sun related event. The actual boundaries of the zodiacal constellations (as defined by IAU) don't follow the neat 30 deg boxes used by sidereal astrology. The ecliptic crosses the boundary of Capricorn at ~ 20h 14m right ascension. So the actual Makar sankraman occurs somewhere around January 22.

Part of the reason for celebrating Makar Sankraman is also because in ancient times the winter solstice occurred in the zodiac sign of Capricorn (Makar rashi). So the Sun entering Makar rashi was the sign of the start of the lengthening on the day. Due to the precession of equinoxes, it now occurs in Sagittarius (Dhanu rashi) and the astrologers have not caught up with modern astronomy yet.