How we compute the panchangam

How positions are computed

There are two traditions of panchangam computation. The vakya tradition uses tables and mean motions handed down in verse — elegant, portable, and by now several degrees adrift from the sky. The astronomical tradition computes where the sun and moon actually are at a given instant. We use the astronomical method throughout. The formulae are from Jean Meeus's Astronomical Algorithms: the moon's longitude from the sixty-term lunar series of chapter 47 (derived from the ELP-2000/82 lunar theory), the sun's longitude from the shorter solar formula of chapter 25, sunrise and sunset from the NOAA solar-position method built on chapters 25 and 28, and moonrise and moonset by searching the moon's altitude directly against the horizon convention of chapter 15. Each is evaluated for the exact moment being asked about, not interpolated from a daily table. These are compact series rather than a full observatory ephemeris, so an end time here can differ slightly from a source that uses a fuller model.

Ayanamsa

Indian astronomy reckons positions against the fixed stars (nirayana), while modern ephemerides give them against the equinox (sayana). The difference between the two is the ayanamsa, and it grows by roughly fifty arc-seconds a year. We use the Lahiri (Chitrapaksha) ayanamsa, the value adopted by the Indian Calendar Reform Committee in 1955 and used by the Rashtriya Panchang and by most published Indian panchangams. We model it as a steady drift from 23.85° at the start of the year 2000, which is how the companion app computes it too. If your panchangam uses Raman or Krishnamurti ayanamsa, expect small differences in the moment a nakshatra changes — usually minutes, occasionally enough to move a boundary across sunrise.

The five limbs

Tithi is the elongation of the moon from the sun divided into thirty parts of twelve degrees each. Nakshatra is the moon's sidereal longitude divided into twenty-seven parts. Yoga is the sum of the sun's and moon's longitudes divided into twenty-seven parts. Karana is half a tithi. Vaara is the weekday, reckoned from sunrise. We compute each of these directly from the positions, and we find the exact instant each one ends by root-finding on the underlying angle rather than by linear interpolation — which is why our end times are given to the minute rather than the quarter-hour.

Which tithi belongs to which day

A tithi rarely fills a calendar day. The traditional rule, which printed panchangams follow, is that a day is named for the tithi running at sunrise. This site currently uses a simpler reading: it takes the tithi in force at 06:00 local clock time and names the day for that — on the daily panchangam, and when it works out the dates of festivals and vratams. In India the two readings usually agree, because sunrise falls close to six. They differ when a tithi ends between 06:00 and the local sunrise, which happens a few times a year in Indian cities and more often where sunrise is late, such as London or Toronto in winter; on those days the label shown can be a tithi that ended before sunrise. Sunrise, sunset and every kalam are still computed from the true local sunrise — only the naming of the day uses the 06:00 reading. Festivals and vratams tied to another time of day (midday, evening twilight, midnight) are not yet worked out by those rules either; see 'Conventions and regional differences' below. If either reading changes, the change will be listed on the corrections page.

Sunrise, and why it decides so much

Sunrise here means the moment the upper limb of the sun's disc appears on a level horizon, including the standard allowance for atmospheric refraction — a zenith distance of 90°50′. This is the convention used by published panchangams and by national almanacs. It matters far more than it sounds: sunrise starts the panchangam day and is one end of the interval from which rahu kalam, yamagandam, gulika kalam and abhijit muhurta are all derived. We compute it iteratively, re-evaluating the sun's declination at the estimated moment of the event rather than at noon, which removes the error a single noon-time calculation would carry.

Why your city changes everything

Sunrise in Hyderabad and sunrise in Dubai are about forty minutes apart on the same date; between Kolkata and Ahmedabad the gap is nearly an hour. Since the kalams are eighths of the daylight period, they move with sunrise. Every page on this site is computed for a specific latitude, longitude and time zone. We detect your time zone when the page loads and use the nearest city we cover; you can set it exactly from the selector in the header, and the choice is saved on your device. For cities that observe daylight saving — the ones our overseas readers live in — we resolve the offset for the particular date rather than assuming a fixed one.

Festival dates

Festival and vratam dates are computed, not copied. Each festival is stored as a rule — a lunar month, a fortnight and a tithi, or for Makara Sankranti the sun's entry into Makara — and the date shown is the first day, in India Standard Time, whose 06:00 reading meets that rule. Three limits are worth knowing. The rules and the dates they produce have not yet been checked against published panchangams by a qualified reviewer. The time-of-day rules many festivals follow are not yet applied. And adhika (leap) months are not yet modelled, which in some years can move a festival by a month. So a date here can be a day earlier or later than the one your family keeps, and customs also differ legitimately — Vinayaka Chaviti, Deepavali and Sankranti are observed on different days by different communities in the same year. For anything that matters, confirm the date with your family's panchangam or purohit. When a date is found to be wrong, the fix is listed on the corrections page.

Conventions and regional differences

Most disagreements between two panchangams are not errors on either side but different conventions. These are the ones that most often decide which day you see.

This section describes these conventions in general terms. It has not yet been reviewed by a qualified panchangam scholar, and it will be revised when it is.

What we deliberately do not publish

No kundali, no horoscope matching, no rashifal, no predictions of any kind. A panchangam describes the sky; it does not forecast your life. Keeping that line clear is a positioning decision as much as an editorial one, and it is why you will not find a 'lucky colour' anywhere on this site.

Corrections

We publish a corrections policy because a site that computes thousands of numbers a day will eventually get one wrong. If you find an error — a timing, a date, a transliteration, a translation — write to info@redlapdigital.com with the page and what you believe the correct value is; festival, vratam, reference and guide pages have a 'Report it' link at the foot that fills in the page for you. We check every report, correct verified errors in the next build, and will tell you what we found either way. Corrections of substance are listed, with their dates, on the corrections page.

See the corrections log

How often this is rebuilt

The site is meant to be regenerated every morning, India time, so that the static pages describe the current day. A rebuild can fail; when it does, the pages keep the last successful build's date, which is printed at the foot of every page ('Pages computed on …'). When the daily panchangam page loads, it checks the date in your browser and, if the build is behind, says so on the page and links to the current day's panchangam where one exists. Pages for past and future dates are generated across a rolling window; dates outside it are still reachable, they are simply not offered to search engines for indexing.

Sources

• Jean Meeus, Astronomical Algorithms, 2nd edition (Willmann-Bell, 1998) — chapters 7 (Julian Day), 12 (sidereal time), 15 (rising and setting), 22 (obliquity), 25 (solar position), 28 (equation of time) and 47 (lunar position).
• Report of the Calendar Reform Committee, Government of India (1955) — the source of the Lahiri (Chitrapaksha) ayanamsa.