Astrology is more a strong cultural belief than an exact science as the West understands science. But in India, it cannot be dismissed as merely the art of fortune-telling. Hindus believe that it is a divine science, a gift of the gods. Astrology is a branch of the Vedas (Vedanga) with the aid of which it is possible to forecast the future and probe all past events by considering the position of the planets at the time of birth. Astrology has three broad divisions: Siddhanta, which deals with the astronomical side of the science, Samhita, which deals with world events and Hora which has to do with the horoscope of human and animate objects. Our sages, it is believed, with their spiritual powers, had direct contact with Brahma and were initiated into the science. Among the sages associated with astrology are-Varahamihira, Kalidasa, Varunchi, Vasishta, Vyasa, Parashara, Venkatesa, Kashyapa, Neelakantha, Jayadeva, Ganapati, Satyacharya, Manitha, Jeeva Sharma, Aryabhatta and Bhaskaracharya.
Grahas
The nine grahas or planets influence the earth … these are Ravi or Surya (Sun), Chandra, Soma (Moon), Mangala, Kuja (Mars), Budha, Soumya (Mercury), Guru, Brahaspati (Jupiter), Sukra, Bhrigu (Venus), Sani, Manda (Saturn), Rahu or Sarpa and Ketu or Sikhi. Rahu and Ketu are known as shadowy planets (chhaya graha) since they do not have physical or celestial bodies like the other planets. The north point where the moon crosses the ecliptic (the path of the sun) is called Rahu and the south point, Ketu. Despite their inferior status among planets, these two chhaya grahas have a powerful influence.
Zodiac
The Zodiac is a broad celestial belt extending to about nine degrees on each side of the ecliptic which is the path of the sun which passes through the centre of the Zodiac. The Zodiac circle of 360 degrees is divided into 12 parts of 30 degrees each. Each one of these parts is known as a sign or a rasi. The Zodiac revolves on its axis once a day from east to west.
Lagna
There are 12 signs of the Zodiac that go round the earth once in 24 hours. Each of these signs or rasis takes two hours to rise- this ascendance is called lagna. For instance if the rasi Simha is on the rise, the two-hour period is called ‘Simha Lagna’. A person’s lagna is determined by the rasi that has risen at the exact time of his birth. In a horoscope the lagna of a person is called the first house.
Nakshatras
Literally. stars. While there are 12 rasis, the Zodiac is further marked by 27 constellations or nakshatras. Like the rasis, the nakshatras are owned by the various planets including Rahu and Ketu. The 27 nakshatras are Ashwini, Bharani, Krittika, Rohini, Mrigasira, Arudra, Punarvasu, Pushyami, Ashlesha, Makha, Poorva Phalguni, Uttara Phalguni, Hasta, Chitra, Swati, Visakha, Anuradha, jyestha, Moola, Poorva Ashada, Uttara Ashada, Sravan, Dhanishta, Satabisha, Poorva Bhadra, Uttara Bhadra, Revati.
Rasis
The Zodiac or Bhachakra is divided into 12 rasis (signs): Mesha (Aries), Vrishabha (Taurus), Mithuna (Gemini), Kataka, Karkataka or Karka (Cancer), Simha (Leo), Kanya (Virgo), Tula (Libra), Vrischika (Scorpio), Dhanus, Dhanur or Dhanu (Sagittarius), Makara (Capricorn), Kumbha (Aquarius) and Meena (Pisces).
Rasi-Graha
The 12 rasis are owned by seven planets, Rahu and Ketu not owning any.
RASI Â Â Â Â Â Â Â Â Â SYMBOLÂ Â Â Â Â Â Â Â GRAHA
Mesha          Ram             Mangala
Vrishabha     Bull              Sukra
Mithuna       Twins            Budha
Kataka          Crab           Chandra
Simha           Lion             Surya
Kanya          Virgin           Budha
Tula             Balance         Sukra
Vrischika    Scorpion       Mangala
Dhanu        Centaur          Guru
Makara       Crocodile        Shani
Kumbha    Water carrier     Shani
Meena         Fish              Guru
Yogas
Just as there are 27 nakshatras (constellations) there are also 27 yogas. These yogas (not to be confused with Yoga meditation exercise) have not been given any significance in predictive astrology except that birth in certain yogas is considered auspicious while in others it is not so. Every nakshatra has a corresponding yoga and each yoga has its own attributes.