Some background information on how Chithirai 1 came to be selected as Tamil New Year's day by the ancient Tamils is found here: Why Chiththirai? Excerpts: Chiththirai 1 or Thai 1? Both days have significance in astronomy as per the solar calendar. They are reckoned by calculating the entry of the Sun into the mathematical points or beginnings of the zodiacal signs of Aries and Capricorn respectively, according to the Fixed Zodiac system of all traditional almanacs. Chiththirai New Year, marks the entry of the Sun into Aries (Mesha Rasi). The first point of Aries is recognised as the beginning point of the zodiac in the ancient astronomy of the civilisations of both East and West. The entry of the Sun into the Aries marks the beginning of spring. The day celebrated as Thaipongal marks the entry of the Sun into the first point of Capricorn (Makara Rasi). The first degree of Capricorn is the 271st degree of the zodiac and entry of the Sun into it marks the transit of the Sun from the southern hemisphere to the northern hemisphere of the earth ( uththirayanam - உத்திராயணம்). Ramani