Sankranti means to go from one place to another place (to change direction). It also means one meets another. The time when the sun changes direction from one constellation (of the zodiac) to another is known as Sankranti. Transition of the Sun from Sagittarius to Capricorn during the winter solstice in the northern hemisphere (Uttarayana) is known as Makar sankranti http://www.hinduism.co.za/makar.htm