.The landmass in those ancient times were different from what we find today.
Please read my articles on Rodina, Pangea Super continents of ancient times.
2.Santana dharma was present throughout the world.
3. The Vedas were not compiled in one shot. The Truth ,as revealed to the Rishis were compiled as and when they were revealed.For details please read ‘when were the Vedas compiled’
So what is found in the Rig Veda is the oldest records.
What is mentioned in the Rig Veda about movement of Stars and Planets refer to the period and indicate the area wherethe people lived.
While the later Vedic texts mention Dakshinayna and Uttarayana, they refer to what they observed,that is the movement of the Sun to South or to North.
This is what we practice today.