1. Normally a scientist who specializes in the study of celestial bodies, including their sizes, features, compositions, motions, relative positions, etc.
2. Someone who has a knowledge of the laws of the heavenly orbs, or the principles by which their motions are regulated, with their various phenomena.

Someone who studies planets, moons, and planetary systems, in particular those of the solar system and the processes that form them.

It includes the studies of objects ranging in size from micrometeoroids to gas giants, with the objectives of determining their compositions, dynamics, formations, interrelations, and histories.