Here's why:

* Earth's Rotation: The Earth rotates on its axis from west to east. This rotation makes it seem like the sun is moving across the sky from east to west.

* Perspective: As the Earth rotates, the sun appears to descend lower in the sky from our perspective. This creates the illusion of the sun "setting" below the horizon.

It's important to note that the sun isn't actually moving downwards. It's our planet's rotation that creates this visual effect.