The Sun appears to move across the sky because the Earth is rotating on its axis. Here's why:

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

* Perspective: As the Earth rotates, we, as observers on the surface, see the Sun seemingly moving across the sky. It's like being on a carousel and watching the scenery around you change.

* The Sun's True Motion: The Sun is actually stationary (relatively speaking), and it's the Earth's rotation that creates the illusion of movement.

Therefore, the Sun is not actually moving across the sky, but it appears to do so due to our planet's rotation.