Question

why is sunlight not bright beyond 66 degree latitude?

Answer 1

1. As we move poleward the sun rays become slanting or oblique. They spread over large area of earth's surface and hence amount of energy received per unit area is less.
2. Oblique rays has to cover a long distance and pass through a thicker portion of the atmosphere whereas the same is not the case with vertical rays.
Therefore, sun appears brighter near equator than poleward.