Astronomy is a word that was first used in the Middle ages. According to Macmillan dictionary, its definition is: "the scientific study of the stars, planets, and other objects in the universe.
Each sign of the zodiac is related to a planet from the solar system (in this case, the moon and the sun are in the group of the “planets”):
Aries -> Mars
Taurus -> Venus
Gemini -> Mercury
Cancer -> Moon
Leo -> Sun
Virgo -> Mercury
Libra -> Venus 
Scorpio -> Pluto
Sagittarius -> Jupiter
Capricorn -> Saturn
Aquarius -> Uranus
Pisces ->  Neptune.
Anyway, most people think that the Zodiac signs are related to the zodiac constellations, but that is not true, just their names come from them.