ज्योतिषे विश्वासटिप्पन्यः

Intro to sidereal positioning

“There are also sidereal traditions of astrology, both a Hindu tradition and a western tradition, which derives itself from ancient Hellenistic and Babylonian astrology. They use a so-called sidereal zodiac, which consists of 12 equal-sized zodiac signs, too, but it is tied to some fixed reference point, i.e. usually some fixed star. These sidereal zodiac signs only roughly coincide with the sidereal zodiacal constellations, which are of variable size.” [SW]
“Sidereal planetary positions are usually computed from tropical positions using the equation: sidereal_position = tropical_position – ayanamsha(t) , where ayanamsha is the difference between the two (tropical and siderial) zodiacs at a given epoch.” [SW]
अयनांश-सूचिः - प्रारम्भस्थाने अयनांश-चिन्ता, Swiss Ephemeris सारःअत्र, सूचिर्अत्र।
- Info about Lahiri ayanAMsha there is slightly inaccurate, since the calendar reform committee recommended two ayanAMsha-s (one fixed - for official solar and lunar religious months, and another variable one for determining naxatra spans)
The most popular Indian ayanAMshas adjust ecliptic coordinates so as to roughly place Spica/Citra at ecliptic longitude 180° or 0 Libra.

Here, one sets the ayanAmsha to adjust for precession.