The sun itself doesn't actually move across the sky. It's the Earth's rotation that makes it appear to move. Here's why:

* Earth's Rotation: Our planet spins on its axis, completing one rotation approximately every 24 hours. This rotation is what causes day and night.

* Perspective: As the Earth spins, we on its surface see the sun rise in the east and set in the west. This is just a matter of perspective. The sun stays in its position in space, but we are moving, creating the illusion of the sun's movement.

Imagine a merry-go-round: If you're standing on the ground watching someone ride a merry-go-round, it looks like they're moving in a circle. But, they're actually staying on the merry-go-round while the merry-go-round itself is rotating. It's the same concept with the Earth and the sun.

Let me know if you have any more questions!