Lesson 1A: Sidereal Zodiac vs Tropical Zodiac
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Sidereal Zodiac vs Tropical Zodiac
| Vedic astrology uses the sidereal or 'star based' zodiac. This system locates the zodiac with reference to the fixed or known star constellations. The sidereal zodiac differs from the Tropical or 'sun based' zodiac employed in western astrology. The tropical zodiac is not calculated with reference to the fixed or known star groups. Rather the tropical zodiac locates its starting point (zero degrees in Aries) from the point of the Vernal (spring) equinox. This point is found where the ecliptic (sun's apparent path around the earth) intersects or crosses the equator. This intersection occurs every year on the day of the vernal (spring) equinox in the northern hemisphere. |
Westward Drifting Zodiac Due to a phenomenon known as the precession of the equinoxes, the point of the vernal equinox is drifting westward with reference to the constellations at a rate of approximately 50 arc seconds per year. So the tropical zodiac which uses the vernal equinox point as its starting point (zero degrees in Aries) is also drifting westward with reference to the heavens. It has been estimated that the 2 zodiacs coincided on the day of the vernal equinox in 285 AD. Since that time, the tropical (sun based) zodiac has drifted westward from the sidereal (star based) zodiac to the point where at the time of writing they are now approximately 23 degrees, 51 arc minutes apart (almost 1 sign). This difference between the 2 zodiacs is known as the ayanamsa. |
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