Hindu astrology is based on the simpler and more accurate sidereal system as compared to the tropical system used by western astrologers.
The zodiac is an imaginary band on the celestial sphere that represents the path of the principal planets, the moon, and the sun. This band is divided into 12 equal parts each 30° wide, bearing the name of a constellation for which it was originally named. This division is done with respect to a given frame of reference. Hindu astrology follows the sidereal system that has as the reference the distant “fixed” stars. Current western astrology uses the tropical system that is based on the movement of the overhead position between the tropics.
The sidereal system was used by most ancient civilizations. It has several advantages. It is not affected by the many peculiarities of the earth’s rotation and revolution. Also the system is simpler to use. The only information required is the location of either Taurus or Scorpio. One of these is always visible in the night sky within the Milky Way. Once this is done any point can be easily located. The tropical system requires adjustments to be made for the peculiarities of the earth’s rotation and revolution. Today these adjustments are made by computers but are practically complicated. Also several indicators are required to chart out the complete coordinate system of the zodiac. These are the current mean sidereal time, the observer's terrestrial longitude and latitude, and the epoch the observer wishes to utilize.
In Hindi the signs of the zodiac are called “rashis” (singular is rashi). The twelve rashis along with their western counterparts are as follows: Mesha (Aries), Vrishabha (Taurus), Mithuna (Gemini), Karka (Cancer), Simha (Leo), Kanya (Virgo), Tula (Libra), Vrushchik (Scorpio), Dhanush (Sagittarius), Makar (Capricorn), Kumbha (Aquarius) and Meena (Pisces). The period of the rashi starts about after 3 weeks from the start of the corresponding zodiac sign. For example Mesha rashi starts on April 14, whereas Aries starts on March 21. The Hindu or sidereal dates are more in tune with the Astronomical dates given by the International Astronomical Union.
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