Maharishi Parashara is the source of all the Vedic astrology we know today. Introduction to Vedic astrology was given to his disciple Maitreya in the form of Questions and Answers in the classical text translated by many learned astrologers called Brihat Parashara Hora Shastra, is the book which needs no reference. Vedic Astrology is the limb of the Veda, it’s referred to as the Vedanga (the limb of the Veda).
Parashara Hora Shastra talks about seven planets and two nodes: Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, Rahu, Ketu. It also speaks about the constellations and Zodiac in which these planets are placed at the time of birth.
The ecliptic (Sun’s apparent path around the sky, which is the mean plane of the earth’s orbit about the sun) cuts the celestial equator at 23.5°. The constellation or Nakshatra at this point is Ashwini. This is also called the first point of Aries, all measurement commences from this point.
Zodiacs are divided into 12 parts 30° each which equals 360°. Each segment is called a Zodiac Sign: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.
Zodiacs are also divided into 27 segments of 13°20°20′each which are called Constellations or Nakshatras:
Ashwini,Bharini,Kritika,Rohini,Mrigashira,Aridra,Punarvasu,Pushya,Ashlesha,Magha,Purva Phalguni,Uttra Phalguni,Hastam,Chitra,Swati,Vishakha,Anuradha,Jyestha,Moola,Poorva Ashadha,Uttra Ashadha,Shravana,Dhanistha,Shatabhisha,Purva Bhadrapada,Uttara Bhadrapada and Revati.
Planets orbit around the Sun, there positions can be ascertained with accuracy by keeping constellations or Nakshatra as reference.
Thus, in simple terms the study of Constellations, Planets and Zodiacs is Vedic Astrology….