"While the exact day on which the winter or summer solstice occurs remains steady (within one day error), there is a slight change in the way the Earth's rotation axis is aligned to the sun. Hence, over a period of a few hundred years, this drift means that even though the sun begins its Uttarayan on December 21, it is not in the Makara rashi as it was about 1,500 years ago. So, 1,500 years ago, during the time of Aryabhata, the Uttarayan and Makar Sankranti coincided. Now Makar Sankranti comes on January 14, but Uttarayan happens on Dhanu Sankranti!"