Answer:
After 24 hours the sun will be directly over Greg's head again.
Explanation:
Every day, the sun is above our heads. This moment always happens at the same time, at noon, for that reason at that time it seems that the environment gets warmer.
If Grege observed that at noon the sun was directly above his head, he will not be able to notice it until the next day, at noon. That is, Greg will have to wait 24 hours for the sun to set directly above his head.