To "orbit" something means to travel in a circle or oval around it. Planets orbit the sun, and the moon orbits the Earth.

Technically, the word "around" is not needed. A more grammatically correct sentence would be:

As the Earth orbits the sun, the angle at which sunlight hits the Earth changes.

But other phrases use "around" to describe the same motion, like:

• travel around the sun
• move around the sun

So adding "around" to "orbit" might make it easier for someone to understand if they're not familiar with the meaning of the word "orbit".