Here's why:

* Gravity: The Sun's massive size and gravity hold the Earth in its orbit.

* Centripetal Force: The Earth's orbital motion creates a centripetal force that keeps it moving in a circular path around the Sun.

* Observation: We see the Sun appear to move across the sky due to the Earth's rotation, but it's the Earth that is actually moving.

This concept was a major shift in our understanding of the universe and was first proposed by Nicolaus Copernicus in the 16th century.