The conversion convention
For the traditional sidereal calculation, Swiss Ephemeris describes sidereal longitude as tropical longitude minus ayanamsha. The reference must be selected, and its value evaluated for the chart’s date. Other projection modes need their own treatment.
Worked example · wrap around zero
Use an illustrative longitude of 12° and an invented offset of 24°. Subtraction gives −12°. Adding 360° puts the answer back on the circle: 348°, or Pisces 18°. Negative intermediate values are normal; they are not a negative zodiac sign.
For a second check, 359° minus the same offset is 335°, or Pisces 5°. Adding 24° to either final result and reducing modulo 360° recovers the original longitude. These examples test arithmetic, not the accuracy of a real ephemeris.
Make the result reproducible
When recording a result, include the named ayanamsha, its numerical value, chart date and time, and the calculation settings. “About 24°” is useful for a diagram but insufficient for a boundary-sensitive personal reading.
Use the glossary entry for Lahiri ayanamsha when that is the selected convention. Do not rename another reference “Lahiri” merely because the numbers are close. Retain the exact displayed choice when sharing or exporting a calculation.
An offset changes the input to an interpretation
Try a comparison with the same birth data and note which sign or mansion assignments change. Keep separate notes for unchanged placements and boundary crossings. A change in classification calls for the matching interpretive rule; the offset itself is not a character description.
When a result is near a boundary, compare the effect of reference choice with any uncertainty in the recorded birth time. Report both sources of uncertainty rather than selecting whichever result feels more agreeable.
Word origin · etymology
Sanskrit transliteration: ayanāṃśa.
Ayana means a course or movement, and aṃśa a part or portion; the compound names the angular offset between zodiac references.
Monier-Williams Sanskrit-English Dictionary: ayana (University of Hyderabad) ↗Cologne Digital Sanskrit Dictionaries: Monier-Williams lexical reference ↗
How we know · epistemology
Coordinate offset convention
The offset is reproducible once its reference and date model are stated. Disagreement between ayanamshas usually reflects different chosen origins, rather than an error in the measured position of a planet.
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