The Lahiri (Chitrapaksha) ayanamsa
Lahiri, also called Chitrapaksha, is the ayanamsa adopted by the Indian government's calendar committee in 1955 and is the default on this site. It places the sidereal zodiac so that the star Chitra (Spica) lies at 180 degrees, opposite Aries 0.
Origin
Lahiri is named after N. C. Lahiri, who worked on the Indian national calendar. The Calendar Reform Committee adopted a sidereal reference based on this ayanamsa in 1955. The name Chitrapaksha refers to the star Chitra (Spica) in the Hindu tradition of reckoning.
Value and use
This site uses 23.8571 degrees at J2000, with the yearly drift of about 50.3 arcseconds. It is the default in the panchang, birth chart and checks tools. Most Indian almanacs in print use Lahiri.
When to choose it
Choose Lahiri if you want the most common convention in Indian practice, or if your almanac or teacher uses it. Results will match most printed panchangs that state Lahiri.
Verification status
The base value and formula are standard. The site has not yet checked its panchang output against published almanacs for a set of dates. This is planned and will be listed on the method page when done.
Where the value comes from
The base value used here, 23 degrees 51 minutes 26 seconds at J2000, is the figure associated with the Lahiri convention. The convention fixes the sidereal zodiac so that the star Chitra, or Spica, falls at 180 degrees sidereal, exactly opposite the zero point of Mesha. The committee that adopted a sidereal standard for the Indian national calendar in the 1950s used a reference of this kind. Printed panchangs across much of India follow it, and so the value is the one most readers will meet first. The yearly drift is then added to the base value for any date, using the same precession rate as the other ayanamsas on the method page.
Using Lahiri in practice
To check a result against a Lahiri panchang, pick a date and city, compute the Moon's sidereal longitude, and compare its nakshatra. Most printed Lahiri almanacs will agree with the tool to within a few minutes of the end time of a tithi or nakshatra, as long as the sunrise convention is the same. Where they differ, the cause is usually the time zone, the city coordinates or the sunrise altitude rather than the ayanamsa. Keep a note of all four settings when you compare, so that a difference can be traced to a single cause.
Limits of the convention
Like every ayanamsa, Lahiri is a convention and not a physical constant. The base value depends on a chosen reference star and on a particular measurement of its position in an earlier era, and different astronomers would set the reference slightly differently. The convention is useful because it is widely adopted, which gives Indian readers a common frame for dates, festivals and almanacs. It should not be presented as the one astronomically true zodiac, since no such zodiac exists. A careful reader holds the convention lightly, understands what it fixes, and checks any result near a boundary against a second ayanamsa.