Sidereal

every reader

What the tradition reads

The sidereal zodiac measures a planet's sign by its position against the actual, visible background stars rather than against the equinox, in contrast to the tropical zodiac, which fixes the first degree of Aries to the moment of the spring equinox wherever the equinox happens to fall among the stars in any given era. Vedic astrology uses the sidereal zodiac as its standard, converting a planet's ordinary tropical longitude into a sidereal one by subtracting the ayanamsa, the accumulated gap the slow precession of the equinoxes has opened between the two systems since they last coincided, with the Lahiri ayanamsa serving as the standard figure adopted by the Indian government's own almanac. A minority of Western astrologers also practise sidereal astrology, generally using the Fagan Bradley ayanamsa rather than Lahiri's, holding that a chart's signs should track the constellations the signs were originally named for rather than the seasonal marker the tropical zodiac instead preserves. Because sidereal and tropical longitudes for the same birth moment can differ by more than twenty degrees at present, a chart read sideraly can place the Sun, Moon or a planet in an entirely different sign from the same moment read tropically.

The story

Sidereal comes from the Latin sidus, a star or constellation, so called for its fidelity to the stars themselves rather than to the seasons. Tropical instead traces, by way of Latin tropicus and the English tropic, to the Greek tropikos, itself from tropē, a turn or a solstice, naming the zodiac that turns instead on the Sun's own seasonal turning points at the solstices and equinoxes. The two zodiacs coincided closely around two thousand years ago, and Western astrology has kept its tropical convention essentially unchanged since, defining Aries as beginning at the equinox regardless of where the actual constellation Aries has since drifted to, while Vedic astrology instead kept faith with the stars themselves, adjusting its sidereal zero point as precession, first recognised and assessed, so far as the surviving record shows, by the Hellenistic astronomer Hipparchus in the second century BCE, has slowly carried the equinox backward against the fixed constellations across the intervening centuries.

Written from

Corpus key glossary:sidereal.