How is your zodiac sign determined by astronomy or and astrology?

Astrology is the study of the motions and relativepositions of of the planets, sun and moon, interpreted in terms of human characteristics and activities

Astronomy is the scientific study of the individuel celestial bodies (excluding the earth) and of the unverse as a whole

The thing that these 2 studies have in common is that in both of the names are begging with "astro" (in greek asrton - star) and they both are studies of stars and planets.