Because of the movement in earth’s precession, the starting point of the tropical zodiac changes slowly (with respect to fixed stars). Tropical (sayana) zodiac is analogous to measuring the positions of trees and buildings sitting in a slowly moving bus. Sidereal (nirayana) zodiac, on the other hand, considers a fixed zodiac. It considers the motion of the tropical zodiac (bus in our analogy) and makes an adjustment accordingly. We use the sidereal zodiac in Vedic astrology. The difference between the tropical zodiac and the sidereal zodiac is of great importance. What modern ephemeris gives us is the tropical positions of planets. To convert these positions, we have to subtract the difference between the two zodiacs. This difference varies with time. As earth’s precession changes every year, the difference between the tropical zodiac and the sidereal zodiac changes. This difference is called “ayanamsa” (sidereal difference). There are many opinions on the correct value of ayanamsa, because nobody knows exactly which star is supposed to be the starting point of the real zodiac. We will use Chitrapaksha/Lahiri ayanamsa in this book, which is accepted by most Vedic astrologers of India.

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