Why is it in geology = geologist astronomy = astronomer not astronomist?

Question

I need a valid explanation on why is it in geology = geologist not geologer or astronomy = astronomer not astronomist

My teacher wants to ask me this question, only ME. I am entitled to -5 in my quiz if I am not able to answer this.

Can you please HELP me?!!

Answer 1

A field that ends in "onomy" is usually practiced by an "onomer" and a field that ends in "ology" is usually practiced by an "ologist."

...an exception may be an agronomist...see the source link.

Answer 2

that the way they call it!