Methods

Four ways to read
the same sky.

Nothing here is a preference toggle. Each profile is immutable and versioned, reproduces a named authority, and publishes how well it does.

Profiles

Profile	Reproduces	Sunrise is	Ayanāṃśa	Checked against
pac-rp@2026.1	Rashtriya Panchang. The default.	Upper limb, 31′ + 16′	lahiri-iae1985	PAC daily page: 53 end-times; one station sunrise
classical@2026.1	The traditional Hindu sunrise	Disc centre, no refraction	lahiri-iae1985	Swiss Ephemeris, the same sunrise model
iskcon-gcal@2026.1	Vaiṣṇava rulings	Upper limb, 31′ + 16′ (as pac-rp, for now)	lahiri-iae1985	Not measured yet (GCal 11)
compat-drikpanchang@2026.1	Comparison only. Never the default.	Upper limb, 34′	lahiri-iae1985	Not measured yet (private spot-checks)

What “sunrise” means

Kolkata, 15 December 2025. Four published conventions put the same sunrise 4 min 00 s apart, which is enough to change the tithi that names a day.

06:08:58Upper limb, 31′ + 16′Rashtriya Panchang 2016

06:08:43Upper limb, 34′Common almanac practice

06:10:19Disc centre, 30′Calendar Reform Committee 1955

06:12:43Disc centre, noneTraditional “Hindu sunrise”

Solid disc: the Sun as the convention sees it at the horizon. Dashed: where it is, before the air bends the light.

The object

What ?profile= accepts, and what every response echoes.

{
  "id": "iskcon-gcal",
  "version": "2026.1",
  "title": "Vaiṣṇava (GCal)",
  "purpose": "The astronomical basis for Vaiṣṇava rulings reconciled to the Gaurabda Calendar.",
  "ayanamsa": "lahiri-iae1985",
  "solarAyanamsa": "fixed-23-15",
  "rise": {
    "sun": {
      "limb": "upper",
      "refractionArcmin": 31,
      "semidiameter": 16,
      "dip": false
    },
    "moon": {
      "limb": "centre",
      "refractionArcmin": 31,
      "semidiameter": 16,
      "dip": false
    }
  },
  "kala": {
    "arunodaya": "sunrise-minus-4-ghatika",
    "pratah": "day-muhurta-1-3",
    "sangava": "day-muhurta-4-6",
    "madhyahna": "midday-pm-1-ghatika",
    "aparahna": "day-muhurta-10-12",
    "sayahna": "day-muhurta-13-15",
    "pradosha": "night-muhurta-1-2",
    "nishita": "midnight-pm-1-ghatika"
  },
  "monthSystem": "amanta",
  "dayBoundary": "sunrise",
  "notes": [
    "Only aruṇodaya (4 ghaṭikās before sunrise) is set from the source so far; the rise model and month naming are those of pac-rp until reconciled against GCal output.",
    "Moonrise and moonset: the sources read do not state the Rashtriya Panchang convention; the centre of the disc with 31′ refraction is assumed."
  ]
}

Ayanāṃśa is a named function, never a number

One arc-second moves a saṅkrānti 23.6 s, a nakṣatra 1.8 s, a yoga 3.4 s. Tithi and karaṇa do not depend on it.

Variant	Definition	Against default, 11 November 2026	Used for
lahiri-iae1985	23°15′00.658″ at 21 March 1956, IAU 1976 precession. Today 24°13′56″.	default	PAC and Rashtriya Panchang parity
lahiri-icrc	23°15′00.000″ at the same epoch, Newcomb	−1.1″	Fixtures printed before 1985
fixed-23-15	A constant 23°15′	−59′04″	RP “religious” solar months: Vaisakhi, Vishu, Pongal
true-citra	Spica held at 180°	−57.8″	Tamil and Kerala traditions that use it
lahiri-vp285	Zero at the mean equinox of 285 CE	+23″	Documenting the 285 CE definition

Conformance · tier A

Against the Positional Astronomy Centre

53 of 53 end-times that PAC printed are reproduced inside the budget of sixty seconds for print; the widest difference is 30 s. Measured 10 November 2026. Below, the 8 widest.

PAC prints	Printed, IST	Computed	Difference	Budget
Viṣṭi ends 7 Nov 2026	23:06	23:05:29	−30 s	60 s
Hasta ends 6 Nov 2026	04:44	04:43:32	−28 s	60 s
Moon enters Vṛścika 9 Nov 2026	02:48	02:48:27	+27 s	60 s
Indra ends 5 Nov 2026	06:27	06:26:33	−27 s	60 s
Pūrva Phalgunī ends 11 Sep 2026	13:16	13:16:25	+25 s	60 s
Āśleṣā ends 2 Nov 2026	03:47	03:46:34	−25 s	60 s
Moon enters Siṃha 2 Nov 2026	03:47	03:46:34	−25 s	60 s
Kṛṣṇa Trayodaśī ends 7 Nov 2026	10:48	10:48:24	+24 s	60 s

PAC prints to the minute, and one row to a tenth of a minute. Each difference is taken to the printed value exactly as written. Every row and every other measurement is in fixtures/report.json.

Conformance · tier C

Against the Rashtriya Panchang’s festival lists

162 of 181 festival dates the Positional Astronomy Centre printed for 2022 to 2027 are the dates the ruleset gives, worked at the Panchang’s own central station. The target is every one. The 19 that differ are below, none corrected: the rules are the Calendar Reform Committee’s rows as printed, and the lists follow practice those rows do not record.

Observance	PAC prints	Ruleset gives	What is known of why
Akṣaya Tṛtīyā	3 May 2022	4 May 2022	Our reading, not the Centre’s statement. Tṛtīyā ran from 05:19 on the 3rd to 07:33 on the 4th and touched the forenoon of both days, so the row's 'paraviddha' gives the second. On the second day it outlasted sunrise by 2 h 08 m, under three muhūrtas. The list appears to ask for three muhūrtas after sunrise before it takes the second day; the row does not say so.
Śaṅkara Jayantī	6 May 2022	5 May 2022	Our reading, not the Centre’s statement. Pañcamī covered the whole noon window on both days and the row marks neither, so the ruling is a boundary and falls to the first by the declared fallback. The list keeps the day pañcamī is current at sunrise.
Govardhana Pūjā	25 Oct 2022	26 Oct 2022	Our reading, not the Centre’s statement. Pratipadā began at 16:18 on the 25th and covered noon only on the 26th. The row marks no part of the day, so the preface's noon applies and 'purvaviddha' has nothing to choose between. The list keeps the day pratipadā begins, as it does in 2023 and 2026; in 2024, when it began after sunset, the list took the next day.
Yama Dvitīyā	26 Oct 2022	27 Oct 2022	Dvitīyā began at 14:42 on the 26th and covered noon only on the 27th; the row reads 'madhyahna, purvaviddha'. The list itself prints two reckonings that year: Bhai Duj on the 26th, and Bhratri Dvitiya (Bengal) on the 27th. The row's noon gives the date the list prints for Bengal.
Makara Saṅkrānti	15 Jan 2023	14 Jan 2023	Our reading, not the Centre’s statement. The Sun entered Makara at 20:45 on the 14th, after sunset (17:34). The rule of the solar festivals counts the day from midnight, which gives the 14th. The list prints the Makara Snana and Tila Samkranti on the 15th, with Pongal, and 'Makara Samkranti (Bengal)' and Magha Bihu on the 14th. It appears to give the bath the day the Tamil rule gives the month (report, p. 244): the next day when the transit falls after sunset. Pongal is ruled that way here, on the strength of that rule and its 'S. India' tag, and agrees; for the bath the appendix gives only the day from midnight. In 2024, 2025 and 2026, when the transit fell before sunset, the list and the rule agree.
Śrī Pañcamī	26 Jan 2023	25 Jan 2023	Our reading, not the Centre’s statement. Pañcamī began at 12:34 on the 25th, two minutes before the noon window closed, and ended at 10:28 on the 26th: the ruling is a boundary for that margin. The list keeps the day pañcamī holds through the forenoon.
Vasanta Pañcamī	26 Jan 2023	25 Jan 2023	Our reading, not the Centre’s statement. The same tithi and the same two minutes as Śrī Pañcamī 2023.
Akṣaya Tṛtīyā	22 Apr 2023	23 Apr 2023	Our reading, not the Centre’s statement. Tṛtīyā ran from 07:49 on the 22nd to 07:47 on the 23rd: the forenoon of both days, 2 h 14 m after sunrise on the second. As in 2022.
Śaṅkara Jayantī	25 Apr 2023	24 Apr 2023	Our reading, not the Centre’s statement. Pañcamī ran from 08:25 on the 24th to 09:40 on the 25th and covered noon only on the 24th. The list keeps the day it is current at sunrise.
Govardhana Pūjā	13 Nov 2023	14 Nov 2023	Our reading, not the Centre’s statement. Pratipadā began at 14:57 on the 13th and covered noon only on the 14th. As in 2022.
Śaṅkara Jayantī	2 May 2025	1 May 2025	Our reading, not the Centre’s statement. Pañcamī ran from 11:24 on the 1st to 09:15 on the 2nd and covered noon only on the 1st. The list prints the 2nd, the day it was current at sunrise. As in 2022 and 2023.
Karaka Caturthī	10 Oct 2025	9 Oct 2025	Our reading, not the Centre’s statement. Caturthī ran from 22:54 on the 9th to 19:39 on the 10th. The Moon rose at 19:14 on the 9th, before it began, and at 20:07 on the 10th, 28 minutes after it ended: it was current at moonrise on neither day. The row does not say what then, and the day the tithi begins is ruled, as a boundary. The list prints the 10th, the day caturthī held from sunrise until after sunset.
Sūrya Ṣaṣṭhī	28 Oct 2025	27 Oct 2025	Our reading, not the Centre’s statement. Ṣaṣṭhī ran from 06:05 on the 27th, three minutes after sunrise, to 08:00 on the 28th, and covered noon only on the 27th. The row names no part of the day, so the preface's noon applies. The list prints the 28th, the day ṣaṣṭhī was current at sunrise. In 2022, 2023, 2024 and 2026 the list and the rule agree.
Rathayātrā	16 Jul 2026	15 Jul 2026	Our reading, not the Centre’s statement. Dvitīyā ran from 11:51 on the 15th to 08:53 on the 16th and covered noon only on the 15th. As with Mahāṣṭamī 2026, the list keeps the day the tithi is current at sunrise.
Rakṣā Bandhana	28 Aug 2026	27 Aug 2026	Our reading, not the Centre’s statement. Pūrṇimā ran from 09:09 on the 27th to 09:48 on the 28th, and its second half began at 21:32 on the 27th. The row says only 'in the second half of purnima'. Of that half, 8 h 08 m fell in the day that began at sunrise on the 27th, all of it by night, and 4 h 08 m after sunrise on the 28th. The row does not say which day to take; the day that holds more of the half is ruled, as a boundary. The list prints the 28th. In 2022 and 2023, when the half outlasted the next sunrise by 91 and 84 minutes, it printed the first day. It appears to take the second day when pūrṇimā lasts three muhūrtas past sunrise, as with Akṣaya Tṛtīyā; the row does not say so.
Mahāṣṭamī	19 Oct 2026	18 Oct 2026	Our reading, not the Centre’s statement. Aṣṭamī ran from 08:28 on the 18th to 10:52 on the 19th and covered noon only on the 18th, so 'paraviddha' has nothing to choose between. The list keeps the day aṣṭamī is current at sunrise, the day it joins navamī.
Govardhana Pūjā	9 Nov 2026	10 Nov 2026	Our reading, not the Centre’s statement. Pratipadā began at 12:32 on the 9th, 25 minutes after the noon window closed, and covered noon only on the 10th. As in 2022.
Ekādaśī	20 Nov 2026	21 Nov 2026	Our reading, not the Centre’s statement. Ekādaśī ran from 07:16 on the 20th to 06:32 on the 21st and was current at sunrise only on the 21st (06:17), for 15 minutes; the row gives the day it is current at sunrise. The list prints the 20th for Smārtas and the 21st for Vaiṣṇavas and widows, with a Trispṛśā Mahādvādaśī. It appears not to count a tithi that outlasts sunrise by so little.
Makara Saṅkrānti	15 Jan 2027	14 Jan 2027	Our reading, not the Centre’s statement. The Sun enters Makara at 21:10 on the 14th, after sunset (17:34). As in 2023.

The printed dates are in fixtures/tier-c, each list with its address and the hash of the file it was read from. Where the Panchang prints a date for an observance, the ruling’s own page lists it beside ours.

APAC daily page, captured by the week
BEnd-times in the Rashtriya Panchang editions 2022–27. Not built yet.
CRashtriya Panchang festival lists: 162 of 181 dates. Vaiṣṇava dates (GCal 11) not built yet.
DStation sunrise tables: one row of 150
EPrivate spot-checks, aggregate rates only. Not built yet.
FInvariants: contiguous intervals, in the test suite

Profile: iskcon-gcal@2026.1
Manifest: /v1/methods
Engine · ruleset: 0.1.0 · 2026.3-draft
Last conformance run: 10 November 2026 · 53 of 53 inside budget

Places from GeoNames, CC BY 4.0. Ephemeris: JPL DE440 (de440-1900-2100). Time zones: the IANA database, 2026f. Rules: Calendar Reform Committee, 1955. Printed dates: Positional Astronomy Centre, Kolkata.