Ayanamsa Calculations

Ayanamsa (also spelled Ayanamsha) represents the precession of the equinoxes - the difference between tropical and sidereal zodiac positions. This is crucial for Vedic astrology calculations.

What is Ayanamsa?

In Vedic astrology, planetary positions are calculated using the sidereal zodiac, which is based on fixed stars. The tropical zodiac (used in Western astrology) is based on the seasons. Due to the precession of Earth's axis, these two systems drift apart by about 50 seconds of arc per year.

The ayanamsa value represents the angular offset between these two systems at any given time.

Basic Usage

Use the type-safe get_ayanamsa() function with a date and system name:

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa, AyanamsaSystem
from typing import cast

date = datetime(2026, 1, 11, 12, 0, 0)

# Calculate Lahiri ayanamsa
lahiri = get_ayanamsa(date, cast(AyanamsaSystem, "lahiri"))
print(f"Lahiri: {lahiri:.6f}°")
# Output: Lahiri: 24.260000°

# Or use without cast if your IDE supports it:
raman = get_ayanamsa(date, "raman")  # type: AyanamsaSystem
kp_new = get_ayanamsa(date, "krishnamurti_new")
fagan = get_ayanamsa(date, "fagan_bradley")

print(f"Raman: {raman:.6f}°")
print(f"KP New: {kp_new:.6f}°")
print(f"Fagan-Bradley: {fagan:.6f}°")

Type-Safety: The AyanamsaSystem type ensures autocomplete support in your IDE and catches invalid system names at development time.

Available Ayanamsa Systems

NDAstro engine supports 17 different ayanamsa calculation methods:

Popular Systems

Lahiri Ayanamsa

The most widely used ayanamsa in Indian astrology, officially adopted by the Indian government. Uses the modern Swiss Ephemeris SIDM_LAHIRI constants with NASA JPL DE440T — the most accurate available implementation.

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa

date = datetime(2026, 1, 11, 12, 0, 0)
ayanamsa = get_ayanamsa(date, "lahiri")
print(f"Lahiri: {ayanamsa:.6f}°")

Traditional Lahiri (lahiri_traditional)

A compatibility mode that replicates the ayanamsa used by JHora, DrikPanchang, and AstroSage. These platforms share two systematic deviations from the modern "lahiri" mode:

1. IAU-1940 constants (SE SIDM_LAHIRI_1940): They use the original Lahiri epoch constants published in the 1940s (epoch = J1900, ayan₀ = 22.4458°, rate = 50.2798"/yr) rather than the refined modern values.
2. Double Delta-T application: Their Swiss Ephemeris wrapper pre-converts birth UT to TT manually, then passes TT to swe.calc_ut() which adds Delta-T a second time, advancing the effective Moon computation time by ~69 seconds (in 2025).

When used with DasaContext, dasa start dates match JHora/DrikPanchang/AstroSage within approximately 4 hours for a 6-year Mahadasa. Proportionally larger offsets apply to longer dasas (up to ~13 hours for a 20-year Venus Mahadasa).

This mode exists for cross-platform compatibility only. Use "lahiri" for astronomically accurate results based on NASA JPL DE440T.

from datetime import datetime
import pytz
from ndastro_engine.dasa import DasaContext, get_dasa_timeline

# Reproduces JHora 'Traditional Lahiri' dasa dates
context = DasaContext(
    birth_datetime=datetime(2026, 1, 1, 0, 0, 0, tzinfo=pytz.timezone("Asia/Kolkata").localize(
        datetime(2026, 1, 1)).tzinfo),
    lat=12.9833,
    lon=77.5833,
    ayanamsa_system="lahiri_traditional",
)
timeline = get_dasa_timeline(context)

Note: The lahiri_traditional ayanamsa value returned by get_ayanamsa() uses the IAU-1940 constants only (no time shift). The double-Delta-T Moon shift is applied automatically inside get_dasa_birth_info() and get_dasa_timeline().

Krishnamurti (KP) Systems

Two variants used in the Krishnamurti Paddhati system.

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa

date = datetime(2026, 1, 11, 12, 0, 0)
kp_new = get_ayanamsa(date, "krishnamurti_new")
kp_old = get_ayanamsa(date, "krishnamurti_old")

print(f"KP New: {kp_new:.6f}°")
print(f"KP Old: {kp_old:.6f}°")

Raman Ayanamsa

Used by B.V. Raman and his followers.

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa

date = datetime(2026, 1, 11, 12, 0, 0)
ayanamsa = get_ayanamsa(date, "raman")

Parsing ISO Datetime Strings

When working with datetime strings from external sources, use parse_iso_datetime() to safely parse ISO format strings with automatic UTC default for missing timezone information:

from ndastro_engine.utils import parse_iso_datetime
from ndastro_engine.ayanamsa import get_ayanamsa

# Parse ISO datetime strings
# If no timezone is present, UTC is assumed
date1 = parse_iso_datetime("2026-01-11T12:00:00")
date2 = parse_iso_datetime("2026-01-11T12:00:00Z")
date3 = parse_iso_datetime("2026-01-11T12:00:00+05:30")

# All parsed datetimes are timezone-aware
print(f"Date1 timezone: {date1.tzinfo}")
print(f"Date2 timezone: {date2.tzinfo}")
print(f"Date3 timezone: {date3.tzinfo}")

# Use with ayanamsa calculations
lahiri = get_ayanamsa(date1, "lahiri")
print(f"Lahiri ayanamsa: {lahiri:.6f}°")
print(f"Raman: {ayanamsa:.6f}°")

Fagan-Bradley Ayanamsa

Western sidereal astrology system.

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa

date = datetime(2026, 1, 11, 12, 0, 0)
ayanamsa = get_ayanamsa(date, "fagan_bradley")
print(f"Fagan-Bradley: {ayanamsa:.6f}°")

Traditional Systems

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa

date = datetime(2026, 1, 11, 12, 0, 0)

# Traditional Indian systems
kali = get_ayanamsa(date, "kali")
janma = get_ayanamsa(date, "janma")
yukteshwar = get_ayanamsa(date, "yukteshwar")
suryasiddhanta = get_ayanamsa(date, "suryasiddhanta")
aryabhatta = get_ayanamsa(date, "aryabhatta")

print(f"Kali: {kali:.6f}°")
print(f"Janma: {janma:.6f}°")
print(f"Yukteshwar: {yukteshwar:.6f}°")

Star-Based Systems

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa

date = datetime(2026, 1, 11, 12, 0, 0)

# Star-based calculations
true_citra = get_ayanamsa(date, "true_citra")
true_revati = get_ayanamsa(date, "true_revati")
true_pusya = get_ayanamsa(date, "true_pusya")

print(f"True Citra: {true_citra:.6f}°")
print(f"True Revati: {true_revati:.6f}°")
print(f"True Pusya: {true_pusya:.6f}°")

Other Systems

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa

date = datetime(2026, 1, 11, 12, 0, 0)

# Additional systems
true = get_ayanamsa(date, "true")
madhava = get_ayanamsa(date, "madhava")
vishnu = get_ayanamsa(date, "vishnu")
ushashasi = get_ayanamsa(date, "ushashasi")

Complete List of System Names

System Name	Description
lahiri	Lahiri (Chitrapaksha) — modern SE SIDM_LAHIRI constants, NASA JPL DE440T
lahiri_traditional	Traditional Lahiri (IAU-1940 / SE SIDM_LAHIRI_1940) — matches JHora/DrikPanchang
krishnamurti_new	KP New (0°0'0" at 291 CE)
krishnamurti_old	KP Old (15" less than KP New)
raman	B.V. Raman's ayanamsa
fagan_bradley	Fagan-Bradley (Western sidereal)
kali	Kali (0° at Kali Yuga start)
janma	Janma system
yukteshwar	Sri Yukteshwar's system
suryasiddhanta	Surya Siddhanta
aryabhatta	Aryabhatta system
true_citra	Based on Citra (Spica) star
true_revati	Based on Revati (ζ Piscium) star
true_pusya	Based on Pusya nakshatra
true	True ayanamsa
madhava	Madhava system
vishnu	Vishnu system
ushashasi	Ushashasi system

Understanding the Calculations

All ayanamsa calculations use a quadratic formula based on Julian centuries from J2000.0 epoch:

ayanamsa = c0 + c1 × b6 + c2 × b6²

Where: - c0: Constant term (ayanamsa at reference epoch) - c1: Linear term (degrees per Julian century) - c2: Quadratic term (degrees per square Julian century) - b6: Time parameter in Julian centuries from J2000.0

Comparing Different Systems

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa

date = datetime(2026, 1, 11, 12, 0, 0)

# Compare popular systems
systems = ["lahiri", "krishnamurti_new", "raman", "fagan_bradley"]

for system in systems:
    value = get_ayanamsa(date, system)
    print(f"{system:20s}: {value:8.4f}°")

Historical Values

The ayanamsa value changes over time due to precession:

from datetime import datetime
from ndastro_engine.ayanamsa import get_ayanamsa

# Historical dates
dates = [
    datetime(1900, 1, 1, 12, 0, 0),
    datetime(1950, 1, 1, 12, 0, 0),
    datetime(2000, 1, 1, 12, 0, 0),
    datetime(2026, 1, 1, 12, 0, 0),
    datetime(2050, 1, 1, 12, 0, 0),
]

for date in dates:
    ayanamsa = get_ayanamsa(date, "lahiri")
    print(f"{date.year}: {ayanamsa:.4f}°")

See Also

• API Reference: Ayanamsa
• Quick Start Guide