Question

What could be the reason for the change in the shadow of the stick?

Answer 1

The angle between the earth and sun change as it rises in the east and sets in the west.

Therefore it has to rotate at 180 degrees and so the angle keeps on changing all through the day.

This means that during the day the angle between them will be from 0 to 90 degrees and hence the length of the shadow will decrease and once it goes from 90 to 180 degrees, the length of the shadow.