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In short
A Julian Day is a moment expressed as a single continuous count of days and fractions of a day, with no months, years or time zones. Astronomical formulas work in it. This app turns every birth instant into a Julian Day first, from the time in UTC, and every position it computes takes that one number as input.
What the term means
A Julian Day is a date written as one number: days counted continuously, with the time of day as a decimal fraction. It has no months, leap years or time zones, so the time between any two moments is a subtraction. The formulas that give a graha's longitude are written in it. One fixed value matters here: Julian Day 2451545.0 is noon UT on 1 January 2000, the reference point the formulas measure from.
Where the term comes from
The term is from modern astronomy and does not occur in the classical texts in this project's library. BPHS has its own continuous day count for a related job. In Ch. 27 Sl. 13 the lord of the year of birth is found from the Ahargan, which the translator's note explains as the number of days passed since the beginning of Creation, taking a count from Burgess's translation of the Surya Siddhanta; dividing a day count by seven then gives the weekday.
How this app computes it
- The local birth time is first turned into UTC, using the birth place's time zone for that date. The UTC year, month, day and time then go through the standard conversion in Meeus's Astronomical Algorithms, including the correction for the Gregorian calendar.
- A birth at 11:14 IST on 19 January 1985 is 05:44 UT, which is Julian Day 2446084.7389. Midnight UT on 13 October 2026 is Julian Day 2461326.5.
- Most formulas then use Julian centuries from 2000: the 1985 birth is -0.1495 centuries. The Sun, Moon, planets, sidereal time, the tilt of the ecliptic and the ayanamsa are all computed from that one number.
- A mistake in the time zone moves the Julian Day, and with it every position. That is why the engine prefers the place's named time zone to a fixed offset.
Pages that use this term
- Methodology - one birth taken from clock time to its Julian Day and on to the chart.
- Kundali - the casting steps, the Julian Day second among them.
- Calculators - the hub that describes the shared first step.
- Ayanamsa calculator - a value computed from a date's Julian Day.
- Why apps disagree on your nakshatra - time-zone errors that shift the moment.
Related terms
- Ephemeris - the positions computed from it.
- Vara - the weekday, another day count.
- Panchang - the almanac computed for a date.
- Birth-time rectification - when the time itself is uncertain.