This opens a much bigger question that I have had: how should we actually define uttara-ayana. It seems that it was originally the winter solstice, where the days begin to get longer, but since it has become associated with Makara Saṅkrānti it has been riding a sidereal year, of which it is not really a sidereal calculation but a tropical calculation. If we truly are observing the Sun returning or the Sura increasing, then there is no relation to the signs, just actual ayanas (solstices).
Namah Shivaya
Freedom T. Cole