The earth's axial rotation leads to seasonal changes as the amount of sunrays in the hemispheres varies due to it. The earth is tilted 23.5 degrees on its axis which impacts days and nights in each hemisphere. The term Makar Sankranti came into existence based on Makar Mandal. Makar Sankranti means the sun reaching the southern end or northern start point of the ecliptic. Since ancient times, when the sun entered the Makar Mandal and reached this Dakshinayanant or Uttarayanarambh point from the southernmost end of the ecliptic, the day became the shortest (December 21 or 22). Now the sun enters the Makar Mandal in mid-January, the reason is that due to the solstice movement, the Dakshinayanant (or Uttarayanarambh) point has now moved towards the west to the Sagittarius.
Makar Sankranti marks the transition of the sun into the Tropic of Capricorn, indicating the northward journey. This change happens after the winter solstice when the days start getting longer in the northern hemisphere, ringing in summer. As the sun moves towards the Tropic of Cancer, solar energy increases in the northern hemisphere, which affects the climate, agricultural cycle and biological rhythms. This also encourages rejuvenation and vitality in people of the northern hemisphere, as the sunray is known to be a stable source of vitamin D. During this period, people traditionally bask in the sun or spend more time in the sun which helps the body produce more vitamin D that strengthens bone health and immunity.
https://www.etvbharat.com/en/!lifestyle/astronomer-explains-science-behind-maha-kumbh-and-sankranti-enn25011403185
