Uttarayan and Sankranti......

The season occurs based on tropical sun (without ayanamsha). This is due to earth's tilt of 23.45 degrees. The earth circles around sun with this tilt. When the tilt is facing the Sun we get summer and when the tilt is away from the Sun we get winter. That is the reason when there is a summer in north of equator there will be winter in the south of equator, as it is facing away from the sun. Because of this tilt it seems like the Sun travels north and south of the equator. This motion of the sun going from south to north is called Uttarayana – the sun is moving towards north and when it reaches north it starts moving south and it is called Dakshinayana – the sun is moving towards south. This is a phenomenon. The sun doesn't go up and down, it is the position of earth in orbit around the Sun makes as look like it's moving up and down. This motion of the sun moving towards the north is called Uttarayana. When it is moving towards the south it is called Dakshinayana. This causes rise to seasons. They are dependent on equinoxes and solstices.

There is a common misconception that Makar Sankranti is the Uttarayana. This is because at one point in time Sayana and Nirayana Zodiac were same. Every year equinoxes slides by 50 seconds due to precession of equinoxes, giving birth to Ayanamsha and causing Makar Samkranti to slide further. As a result if you think Makar Samkranti is uttarayana then as it is sliding it will come in June after 9000 years. However Makar Samkranti still holds importance in Hindu rituals as a Samkranti. All Drika Panchanga makers like mypanchang.com, datepanchang, janmabhumi panchang, rastriya panchang and all drik panchanga will use the position of the tropical Sun to determine Uttarayana and Dakshinayana.

Also when Uttarayana starts it is a start of winter. When equinox slides it will increase ayanamsha and makar sankranti will also slide. 1000 years back in 1000 AD makar samkranti was in Dec 31 and now January 14 and after 9000 years when makara sankranti will be in June that is the summer. You can not have uttarayana in June when sun is about to begin its ascent upwards —dakshinayana. This misconception is continued as there is not much difference between actual Uttarayana date of Dec 21 and January 14. But the difference will be visible more as equinoxes slide further.