What's the name of the person who looks at the constellation?

People who look at constellations are called astrologers.

An astrologer is a person who follows the principles of astrology and draws an astrolabe by using a person's birthplace and birth time to explain a person's personality and destiny.

Astrology uses the relative positions and motions of celestial bodies (especially the positions of planets in the solar system) to explain or predict people's things and behaviors.