Astrovarg

How we calculate

How Astrovarg calculates

When you ask for a Panchang or a Kundli, the answer is worked out step by step, from your place to the sky. Here is each step in plain words, with a short note for the curious.

From your place to your Panchang

  1. 1.Your place and its time zone

    You choose a place. Our own location service finds it and gives its coordinates and its official time zone.

    The detail

    Every place carries its IANA time zone name, such as Asia/Kolkata, so a searched place needs no second lookup. We keep the name, not a fixed offset, because offsets move with the date where clocks change for summer.

  2. 2.From clock time to the time of the sky

    Clock time is not the time the planets keep. We turn your local time into the time scales astronomy uses.

    The detail

    Civil time (UTC) becomes Terrestrial Time through the published leap seconds, and Barycentric Dynamical Time is used to read the planetary data. Earth's rotation is uneven, so the difference ΔT comes from measured Earth-rotation values where they exist and from the long-term model of Stephenson, Morrison and Hohenkerk elsewhere.

  3. 3.Where the Sun, Moon and planets are

    Positions come from NASA JPL's DE441, the planetary data that covers about 13,200 BC to AD 17,191. We read it with our own reader.

    The detail

    The reader was written from NAIF's published specifications for these data files, with no outside libraries and no unsafe code. Our earlier reference engine has its own reader, written separately from the same specifications, and the two are compared. The engine then corrects for the time light takes to travel, for the Earth's motion and for the Sun's gravity on light, and turns the positions to the date's own frame with the IAU 2006 precession and IAU 2000A nutation models.

    We compared our reader with SPICE, the official software of NASA's Navigation and Ancillary Information Facility (NAIF), on 1,094,849 positions and velocities (31 October 2026). Every one came out the same, to the last digit.

  4. 4.The sidereal zodiac

    Vedic astrology counts the signs from the stars. We use the Lahiri ayanamsha, the standard of Indian almanacs.

    The detail

    The ayanamsha moves with the slow turning of the Earth's axis. We compute it with the IAU 2006 precession model and add nutation for the true value of the date.

  5. 5.Your city's own sunrise

    The Hindu day starts at sunrise, so the Panchang is reckoned from the sunrise at your place.

    The detail

    Sunrise is the moment the Sun's upper edge meets the horizon: its centre at 50 arcminutes below it, which is 34′ for the bending of light by the air and 16′ for the Sun's own half-width. This fixed standard is the treatment of refraction; no weather model is added.

  6. 6.The Panchang and its times

    Each Panchang element is a share of the sky, and each begins and ends at a moment we search for.

    The detail

    A tithi is each 12° the Moon moves ahead of the Sun, and a karana is half of one. A nakshatra is one of 27 parts of 13°20′, and a yoga adds the Sun and the Moon and divides by the same 13°20′. Each change is found by searching the day and refining the moment to within a millionth of a day. The weekday belongs to the most recent sunrise. Rahu Kaal, Yamagandam and Gulika Kaal are parts of the daylight, one eighth each, picked by the weekday.

  7. 7.Festival dates

    A festival is a rule over the sky, not a fixed date. Each one is placed on the day its own rite holds the tithi.

    The detail

    The tithi is tested at the rite's own time at your place: midday for Rama Navami, the evening (Pradosha) for Diwali, midnight (Nishitha) for Maha Shivaratri. The same festival can fall on different days in different cities.

What we built ourselves

  • The Astrovarg Ephemeris: the reader for the planetary data, the time scales, the reference frames and the apparent positions.
  • The astrology engine: the Panchang, the festivals, the charts and the day's timings, built on the Astrovarg Ephemeris.
  • The location service that finds your place and its time zone.

Data and credits

These sources make the calculations possible. Their notices are shown as their licences ask.

Planetary data
NASA Jet Propulsion Laboratory, DE441 (Park et al., 2021), distributed by NAIF.
Earth rotation and leap seconds
IERS and USNO published data.