I think you're partially right about the sunlight. However, I believe that Celestia took over the celestial dynamics that already exist. I think that the problem was with their sun not producing enough energy.
>"..Over the ages the star grew colder and the ponies of Equesrtia adapted as they could to the dimming world they inhabited. Then, in the great Celestial transformation, a distant sun was commandeered and moved to their system to replace their dying sun. Amd all ponies did rejoice and sing their pony songs of tribute to Celestia and happiness to the return of warmth to their world.."