One sky · two starting points
What is ayanāṃśa?
Ayanāṃśa is the angle between the starting point of the tropical zodiac and the starting point of the sidereal zodiac. Both systems measure the same sky from different starting points.
Why there are two starting points
One follows the seasons; the other is referenced to the stars.
The tropical zodiac begins at the March equinox, a seasonal marker. A sidereal zodiac begins from a point defined relative to the stars. Earth’s axis slowly changes direction through the precession of the equinoxes, so the two starting points gradually separate.
Ayanāṃśa measures the angle between them for a particular date. Subtracting that angle from a tropical longitude gives a corresponding sidereal longitude.
A named astronomical choice
Different sidereal methods place their starting point slightly differently.
Indian astronomical and calendar work includes several documented ayanāṃśa conventions, including Citrāpakṣa/Lahiri, Raman, and Krishnamurti. Their values are close, but a small difference can change a name or transition time near the boundary between two nakṣatras, rāśis, or yogas.
Tithi and karaṇa depend on the changing Moon–Sun angle, which remains the same after subtracting an equal ayanāṃśa from both longitudes. Vāra is simply the weekday.
Historical reference: Report of the Calendar Reform Committee (1955).
For readers who enjoy word histories
How the Sanskrit word is built
ayana + aṃśa → ayanāṃśa
Ayana can mean going, a course, or an advance; in this astronomical setting it points toward the changing course associated with precession. Aṃśa means a part or share, including an angular portion. When the final a of ayana meets the first a of aṃśa, they join as the long vowel ā.