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

You're talking to a child. You're explaining why the temperature is different at different times of year. You say:

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

As (something happens), (something else happens)

Use this kind of grammatical structure to describe two things that happen at the same time. It's most commonly used for two continuing processes that happen together:

As you get older, you learn what your own strengths and limitations are.

"As ___, ___" is good for describing how things work. Here's a technical description of how water boils:

As the temperature of the water rises, some of the water molecules begin to go into a gaseous state.

(something) orbits around (something)

To "orbit" something means to travel in a circular path 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".

at which

The example at top includes the phrase "the angle at which sunlight hits the Earth". This is a reordered version of "hit (something) at an angle":

The light from the sun hits different parts of the Earth at different angles.

When you reorder a sentence, you still have to keep the preposition "at". There are two choices for where to put it:

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

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

The first version is a lot more casual. The second version sounds more intelligent. It's also considered more grammatically correct, so please use it when you're writing or answering a grammar question on a test.

This same structure works with other prepositions:

• for which
• by which
• on which