Methodology

How we calculate the Panchang

Last reviewed August 2026 · naamjaap.life

A Panchang is only as trustworthy as the astronomy behind it. Many almanacs online simply copy figures from one another, so an early error is repeated everywhere. We take a different approach: the Naam Jaap Panchang computes every element from first principles, using the same class of astronomical models that professional ephemerides use. This page explains exactly how — the models, the ayanamsa, the sources, and the honest limits of the method — so you can decide for yourself how much to rely on it.

The short version

For any date and city, we compute the true positions of the Sun and Moon in the sky, convert them to the Indian sidereal zodiac using the Lahiri ayanamsa, and read off the five limbs of the Panchang — tithi, nakshatra, yoga, karana and vaara (weekday) — along with the Moon's phase, the lunar month, the samvat years, and the day's muhurats. Everything is calculated for your city's own sunrise, because that is where the traditional day begins.

The astronomical model

Our calculation engine works entirely from established astronomical theory rather than stored tables of results. In outline, for a given moment it:

• Converts the calendar date to Julian Date and applies a ΔT correction (the slowly changing difference between Terrestrial Time and Universal Time), using the Espenak & Meeus (2006) polynomials, so that positions are referred to the correct instant.
• Computes the Sun's true longitude from a truncated VSOP87 series for the Earth's heliocentric motion, then applies aberration — accurate to roughly one arc-second.
• Computes the Moon's true longitude using the full lunar expansion from Jean Meeus's Astronomical Algorithms (Chapter 47), including the principal periodic terms and the additive corrections.
• Applies the IAU 1980 nutation model (63 terms) so both longitudes account for the small nodding of the Earth's axis.
• Subtracts the Lahiri ayanamsa to move from the tropical (sayana) to the sidereal (nirayana) zodiac used in Indian astronomy.

The five limbs (Panchang)

Once the sidereal Sun and Moon longitudes are known, the five limbs follow directly:

• Tithi — the lunar day, from the angular distance between Moon and Sun. Each tithi spans 12°; the one prevailing at sunrise is taken as the tithi of the day, following the standard convention. What is a tithi?
• Nakshatra — the lunar mansion, from the Moon's sidereal longitude divided into 27 segments of 13°20′. Nakshatras explained
• Yoga — from the sum of the Sun and Moon longitudes, divided into 27 parts.
• Karana — half a tithi; there are eleven karanas in the cycle.
• Vaara — the weekday, which in the Hindu reckoning runs from one sunrise to the next.

Why timings are computed for your city

The traditional day begins at local sunrise, not at midnight and not at a single national reference point. Because sunrise depends on latitude and longitude, so does the tithi that rules the day, and so do time-bound elements like Rahu Kaal, the Choghadiya and the Abhijit Muhurat. We calculate sunrise and sunset from each supported city's own coordinates, and evaluate the day's Panchang from that sunrise to the next. This is why the timings you see for, say, Varanasi and Mumbai are not identical — and that difference is a feature, not an error.

Months, paksha and samvat

We report the lunar month under both the widely used systems — amanta (month ending at the new moon, common in western and southern India) and purnimanta (ending at the full moon, common in the north) — so you can read the calendar in whichever tradition you follow. The paksha (waxing Shukla or waning Krishna fortnight) comes from the Moon's phase, and we display the Shaka and Vikram Samvat years with their named samvatsara.

The ayanamsa we use

Indian astronomy is sidereal: positions are measured against the fixed stars, not the moving equinox. The offset between the two frames is the ayanamsa, and the choice of ayanamsa is the single biggest reason two Panchangs can disagree. We use the Lahiri (Chitrapaksha) ayanamsa — the standard adopted by the Indian Astronomical Ephemeris published by the Government of India, and the same value used by drik (observation-based) almanacs. Our implementation is calibrated to agree closely with the Swiss Ephemeris.

Our reference sources

The engine follows well-documented, peer-reviewed astronomy:

• Jean Meeus, Astronomical Algorithms (2nd edition) — solar and lunar position, sunrise/sunset.
• VSOP87 planetary theory (Bretagnon & Francou, 1988) — the Earth's heliocentric longitude.
• IAU 1980 theory of nutation (Seidelmann, 1982).
• Espenak & Meeus (2006) — ΔT polynomial expressions.
• Indian Astronomical Ephemeris — the Lahiri ayanamsa reference.

Honest limits

We want to be candid about what a computed Panchang can and cannot do:

• Festivals can legitimately fall on different days. Whether an observance lands on one date or the next often depends on which tithi is present at sunrise and on regional or sampradaya-specific rules. When calendars differ, it is usually because of these conventions rather than a calculation error. Where a date is genuinely contested, we say so.
• Muhurat is guidance, not a guarantee. Auspicious timings are calculated aids for planning. For weddings, major ceremonies and other important decisions, please also consult a qualified local priest or astrologer who knows your family's tradition.
• Sunrise assumes a level horizon. Local terrain, altitude and atmospheric conditions can shift the actual observed sunrise by a minute or two.
• We are not a substitute for tradition. This tool exists to make an accurate reference conveniently available — not to overrule the practice of your family or community.

Corrections

Accuracy matters to us, and we would always rather be careful than fast. If you believe a figure is wrong — a tithi timing, a festival date, a muhurat — please tell us. Write to thelotuspeacock@gmail.com or use our Contact page, and we will check it against our references and correct it if needed. A single careful reader has more than once made the Panchang better for everyone.