Namaste
It is a common practice among many in TamilNadu that Uttarayanam starts only on Pongal/ makarashankaranthi day (atleast to my limited knowledge).
According to Drika Panchanga makers like mypanchang.com, datepanchang, janmabhumi panchang, rastriya panchang and Vishuddha Siddhanta Panjika, Uttarayanam started on December 21st 2011
I would like to share the below mentioned information and would like to know which panchagam is widely used in India and outside India to figure out festival dates.
Please read the reasoning below very carefully. Its quite a read if one wants to understand the basics of Hindu calendering system.
Namaste
Hari
Drik Siddhanta and Uttarayana
http://en.wikipedia.org/wiki/Uttarayana
This festival is celebrated on January the 14 or the 15.The season occurs based on tropical sun (without ayanamsha). The earth revolves around sun with a tilt of 23.45 degrees. When the tilt is facing the sun we get summer and when the tilt is away from the sun we get winter. That is the reason when there is summer north of the equator, it will be winter south of the equator. Because of this tilt it appears that the sun travels north and south of the equator. This motion of the sun going from south to north is called Uttarayana – the sun is moving towards north and when it reaches north it starts moving south and it is called Dakshinayana – the sun is moving towards south. This causes seasons which are dependent on equinoxes and solstices.
There is a common misconception that Makara Sankranti is the Uttarayana. This is because at one point in time Sayana and Nirayana zodiac were the same. Every year equinoxes slide by 50 seconds due to precession of equinoxes, giving birth to Ayanamsha and causing Makar Sankranti to slide further. As a result if you think Makar Sankranti is Uttarayana then as it is sliding, it will come in June after 9000 years. However Makar Sankranti still holds importance in Hindu rituals.
All Drika Panchanga makers like mypanchang.com, datepanchang, janmabhumi panchang, rastriya panchang and Vishuddha Siddhanta Panjika use the position of the tropical sun to determine Uttarayana and Dakshinayana.
Also when Uttarayana starts, it is a start of winter. When equinox slides it will increase ayanamsha and Makar Sankranti will also slide. In 1000 AD, Makar Sankranti was on Dec 31 and now it falls on January 14; after 9000 years when Makara Sankranti will be in June. It would seem absurd to have Uttarayana in June when sun is about to begin its ascent upwards —Dakshinayana. This misconception continues as there is not much difference between actual Uttarayana date of Dec 21 and January 14. However, the difference will be significant as equinoxes slide further.
Thapas 