Horologist
Someone who studies the planet Mars is called an areologist.
A person who studies the planets and stars is an astrologist.
A person who studies rainbows is called a "rainbowologist."
A person who studies snowflakes is called a cryologist or a snowflake scientist.
I agree with the above comments. But if you want to get really fancy, you could go with Selenographer (a person who charts the surface of the Moon) or Selenologer (a person who studies the Moon).
What is the name of a person that studies organisms
An entomologist is a person who studies insects.
A person who studies sand is known as a psammologist.
A person who studies sound is called an acoustician.
What is the name of a person that studies derivation of
A person who studies audiology would be called an audiologist.
I believe that they are called chronologist because chrono means time and ology means the study of
A person who studies time.
mr. djw studies dinos
Etymologist
zoologist
A dietician.