The Earth tilts on an axis as it orbits around the sun. In the Northern Hemisphere, Dec. 21 is the day when we are tilted the furthest away from the sun — thus the sun appears lowest in the sky (about 19 degrees above the horizon at noon in a city like London). That means the sun spends the least amount of time above the horizon that day. As a result of that it becomes a shortest day of the year.

There is a special significance attached to the celebration of Makar sankranti, the day when according to the Hindu astronomy the sun enters the rashi (zodiac) of Makara (Capricorn).On Makar Sankranti day, 14th January, the beginning of northern journey of the sun (Uttarayan) starts. Makar Sankranti marks the end of a long winter with the return of the sun to the Northern Hemisphere and hence the name Uttarayan. Of course, you must have heard of the fun of Makar Sankranti – kite flying the whole day!!