Astronomy · Reference
The Julian Day
In astronomy, the Julian Day is a single, unbroken count of days used to timestamp observations across centuries. Here's what it is, where it came from, and how it relates to MJD, JDN and the calendar.
Right now
The current Julian Date is approximately 2461242.978, the Julian Day Number is 2,461,243, and the Modified Julian Date is about 61242.478. See live values on the home page.
Definition
The Julian Date (JD) is the number of days — including the fraction of a day — that have elapsed since 12:00 Universal Time on 1 January 4713 BC, in the proleptic Julian calendar. That instant is Julian Date 0.0. Because the count begins at noon, midnight civil dates fall on half-integer Julian Dates: for example, Tuesday, July 21, 2026 at 00:00 UTC is JD 2461242.5.
The Julian Day Number (JDN) is the integer part: the whole-number label for the day that runs from one noon to the next. The Julian Date is then the JDN plus the elapsed fraction of that day. So JD and JDN differ only by the time of day.
Why noon, and why 4713 BC?
Starting the day at noon meant a whole night of observing shared a single date — convenient before electronic records. The epoch of 4713 BC comes from the Julian Period, a 7,980-year cycle proposed by Joseph Justus Scaliger in 1583. It is the product of three older cycles used in calendar reckoning:
- the 28-year solar cycle (when weekdays repeat on the same dates in the Julian calendar),
- the 19-year Metonic cycle (lunar phases repeating on the same dates), and
- the 15-year indiction cycle (a Roman taxation period).
28 × 19 × 15 = 7,980 years. Scaliger placed the start of the period at the last time all three cycles began together, which works out to 4713 BC. He named it after his father, Julius Caesar Scaliger. In the 19th century, astronomer John Herschel turned the period into the continuous day count now standard in astronomy.
The day-count family
The raw Julian Date is a large number, so several offset versions are used in practice:
| Name | Formula | Epoch (day 0) | Typical use |
|---|---|---|---|
| Julian Date (JD) | — | 4713 BC-01-01 12:00 | General astronomy |
| Modified JD (MJD) | JD − 2400000.5 | 1858-11-17 00:00 | Satellites, geodesy, computing |
| Reduced JD (RJD) | JD − 2400000 | 1858-11-16 12:00 | Variable-star work |
| Truncated JD (TJD) | ⌊JD − 2440000.5⌋ | 1968-05-24 00:00 | NASA spacecraft telemetry |
| Dublin JD (DJD) | JD − 2415020 | 1899-12-31 12:00 | Older ephemerides |
| CNES JD | JD − 2433282.5 | 1950-01-01 00:00 | French space agency |
The Modified Julian Date is by far the most common of these.
How it's calculated
Converting a calendar date to a Julian Date uses a compact integer algorithm. The version popularised by Jean Meeus, valid for any date and automatically handling the 1582 calendar reform, is what powers the converters on this site. For code you can copy, see the Python, JavaScript and Excel pages.
Key reference values
| Moment | Julian Date |
|---|---|
| Julian Period epoch (4713 BC-01-01 12:00) | 0.0 |
| MJD epoch (1858-11-17 00:00) | 2400000.5 |
| Unix epoch (1970-01-01 00:00) | 2440587.5 |
| J2000.0 (2000-01-01 12:00 TT) | 2451545.0 |