Out of 88 known constellations, vedic astrology uses 12 major constellations known as Rashi. These rashis are more significant because they follow ecliptic. Ecliptic is an astronomical imaginary line made by suns motion around the Earth. Even other planets and Rashis and moon follows ecliptic. Hence these rashis are given more importance over rest 76 constellations out of 88.
30 degree *12 Rashi = 360 degree.
Since rashis are subdivided into 2.25 nakshatras, each nakshatra shows different characteristics, making the analysis tending towards perfection.
And hence it possess the characteristics of the same creature or shape upto some extent. In vedic astrology out if 12 rashis one Rashi belongs to us as a birth rashi, generally known as moon sign.
These rashi are calculated mathematically and a sky map is made which is commonly known as Kundli or horoscope. It includes position on Rashi and all the navgrahas i.e. nine vedic planets and rest 3 planets namely Uranus, Neptune and Pluto.