Planetary Positions for Another Time
About These Positions
Current planetary positions show the transits (gochara) of each planet through the signs and nakshatras.
Astrologers combine them with a person's birth chart and running Vimshottari dasha to make predictions.
Ayanamsa: sidereal vs tropical
Vedic astrology uses the sidereal zodiac, measured against the fixed stars. Western astrology uses the
tropical zodiac, measured from the vernal equinox, which slowly moves westward against the stars. The
difference between the two is the ayanamsa. MoonAstro uses the Lahiri (Chitrapaksha) ayanamsa, the official
standard in India — about 23.85° in the year 2000 and increasing by roughly 50 arc-seconds a year.
To find a planet's sidereal longitude, its tropical position is first calculated astronomically and the ayanamsa is then
subtracted. This is why Indian and Western ephemerides differ, even though the planets' rates of motion are the same.
How positions are calculated
One simple approach starts from the known positions of the planets at a fixed moment (an epoch) and adds the distance each
has travelled since, based on its orbital period. Jupiter, for example, takes about 4,332 days (11.86 years) to complete a
360° circuit, so after every 4,332 days it returns to the same longitude. Modern calculations refine this with
corrections for each planet's elliptical orbit.