That's a great question! You're absolutely right, all motion is relative. Here's how it applies to sunset:

* The Sun Isn't Actually Moving: The Earth is rotating on its axis. As our planet spins, the Sun appears to move across the sky.

* Our Frame of Reference: The frame of reference for sunset is the Earth's surface. As the Earth rotates, different parts of the planet face the Sun. When the Earth's rotation brings the Sun below the horizon at our location, we experience sunset.

* Relative to the Sun: From the Sun's perspective, the Earth is the one moving. But since we live on Earth, we experience sunset as the Sun setting in the west.

So, in summary, the reference for sunset is the Earth's surface, and it's really the Earth's rotation that causes us to see the Sun appear to set.