Julian Date in Astronomy
The system Joseph Scaliger laid out in 1583 is still the universal time index of observational astronomy and space operations. Right now — Thursday, June 18, 2026 — the astronomical Julian Date is about 2461210.25815.
Julian Day Number (JDN)
2461210
Julian Date (JD)
2461210.25815
Modified Julian Date (MJD)
61209.75815
Origin: Scaliger, 1583
Joseph Justus Scaliger, a French classical scholar, was trying to reconcile ancient chronologies — Egyptian, Roman, Greek — and needed a single continuous day count that reached back before every recorded calendar. He chose as his epoch the proleptic Julian calendar date Monday, 1 January 4713 BC at noon Universal Time, picked because it was the most recent coincidence of the 28-year solar cycle, the 19-year Metonic cycle, and the 15-year Roman indiction. He named the count the “Julian Period” after his father, Julius Caesar Scaliger, and astronomers have used it ever since.
The 4713 BC choice was deliberate. By starting the count before any known historical event, Scaliger guaranteed that every date in recorded history has a positive Julian Day Number — no negative days, no “BC” qualifiers in the continuous count. The fuller story is in the history of Julian dates.
JDN, JD, and the noon convention
The Julian Day Number is an integer: day 0 is noon of 1 January 4713 BC, day 1 is noon of 2 January 4713 BC, and so on. The Julian Date extends it with a fractional day for the time of day. The noon boundary (not midnight) is a holdover from Scaliger’s era, when astronomers worked at night and it was convenient for an entire night’s observing to fall inside a single JD. That is why JD 2460000.0 is noon UT on 24 February 2023, not midnight.
This noon epoch is the single most common source of off-by-half-day errors when converting between Julian dates and civil time. At 00:00 UTC the JD is 0.5 less than the JDN for that calendar day; at 12:00 UTC the JD equals the JDN exactly. The distinction matters wherever a 12-hour error would point a telescope at the wrong part of the sky. The how-to-read guide walks through reading a JD without tripping on it.
Modified Julian Date
The Modified Julian Date, introduced by the Smithsonian Astrophysical Observatory in 1957, is MJD = JD − 2,400,000.5. Subtracting 2,400,000 shortens the count to five digits for the modern era; the extra 0.5 shifts the day boundary from noon to midnight, matching civil time. MJD 0.0 is 17 November 1858 at 00:00 UT. Right now the MJD is about 61209.75815.
The IERS publishes Earth-orientation parameters keyed to MJD, GPS time is internally an MJD offset, and most modern spacecraft-telemetry systems store timestamps as MJD-plus-seconds. MJD caught on precisely because it fixes the two practical problems with the classical JD: the large, unwieldy number and the noon boundary that conflicts with civil midnight. See all seven formats for how JDN, JD, and MJD relate to the day-of-year codes.
J2000.0 and the standard epoch
When a star catalog or an orbital element set specifies an epoch, it almost always means J2000.0: Julian Date 2451545.0, equivalent to 1 January 2000 at 12:00 TT. Positions and proper motions in modern catalogs (Hipparcos, Gaia) are expressed at J2000.0 and propagated forward or backward in Julian years of exactly 365.25 days.
The “J” prefix distinguishes Julian-year epochs from the Besselian epochs (B1950.0, B1900.0) of older catalogs. A Julian year is exactly 365.25 days and a Julian century exactly 36,525 days; those fixed definitions simplify epoch propagation and remove any dependence on the irregularities of leap years.
Heliocentric and Barycentric Julian Date
For microsecond-accuracy timing — pulsar timing, exoplanet transits, eclipsing binaries — Earth’s orbital motion introduces a light-travel-time variation of up to about ±500 seconds between observations made six months apart. To remove it, astronomers convert ordinary Julian Date to Heliocentric Julian Date (HJD, referred to the center of the Sun) or Barycentric Julian Date (BJD, referred to the Solar System barycenter).
BJD in the Barycentric Dynamical Time scale (BJD−TDB) is the modern standard for high-precision timing and is what the NASA Exoplanet Archive uses for transit observations. The conversion from JD to BJD is not a simple offset: it depends on the observer’s position on Earth and the direction to the target, so the same instant carries a slightly different BJD for different stars.
Time scales: UTC, TAI, TT, TDB
A Julian Date can be expressed in several time scales, and which one you use matters for precision work. They differ by at most about 70 seconds, which rounds to zero at the resolution of a calendar day — but for sub-second timing the scale is as important as the number itself.
| Time scale | What it is |
|---|---|
| UTC — Coordinated Universal Time | Civil time, kept in step with Earth rotation by leap seconds. Sufficient for nearly all practical purposes. |
| TAI — International Atomic Time | UTC without leap seconds: TAI = UTC + the current leap-second count (37 s as of 2025). A smooth, continuous atomic scale. |
| TT — Terrestrial Time | TAI + 32.184 s. The scale used for ephemerides and for the J2000.0 epoch. |
| TDB — Barycentric Dynamical Time | TT corrected for the relativistic effects of Earth’s orbital motion. The standard scale for Barycentric Julian Date. |
Compute JD and MJD in code →
Reference formulas and snippets for Julian Day Number, JD, and MJD in Python, JavaScript, and SQL.
Two-way Julian converter →
Convert any date to JDN, JD, and MJD, or take a Julian value back to a calendar date and time.