Glossary

In manufacturing, IT and government systems, the "Julian date" is the day of the year (1–365, or 1–366 in a leap year), usually prefixed by the year. Common forms are YYYYDDD (e.g. 2026202) and YYDDD (e.g. 26202). It has no direct connection to the astronomical Julian Date.

See also: Day of year (ordinal date) , Astronomical Julian Date (JD)

A continuous count of days and fractions of a day since 12:00 Universal Time on 1 January 4713 BC (proleptic Julian calendar). Used throughout astronomy so that intervals between events are simple subtractions. Example: 2451545.0 is the J2000.0 epoch.

See also: Julian Day Number (JDN) , Modified Julian Date (MJD) , J2000.0

The integer part of the astronomical day count: the whole number assigned to a day that begins at noon Universal Time. The Julian Date equals the Julian Day Number plus the fraction of the day since that noon.

See also: Astronomical Julian Date (JD)

Defined as JD − 2,400,000.5. The subtraction removes leading digits and shifts the day boundary to midnight. MJD 0 is 17 November 1858. Widely used in satellite tracking, geodesy and computing.

See also: Astronomical Julian Date (JD) , Reduced Julian Date (RJD)

JD − 2,400,000. Like MJD but without the half-day shift, so it still begins at noon. RJD 0 is 16 November 1858 at 12:00.

See also: Modified Julian Date (MJD)

Introduced by NASA in 1979 as ⌊JD − 2,440,000.5⌋, giving a compact day count that started at 24 May 1968. Used in some spacecraft telemetry.

See also: Modified Julian Date (MJD)

JD − 2,415,020, with epoch 31 December 1899 at 12:00. Introduced by the International Astronomical Union in 1955 and used in some older ephemerides.

See also: Astronomical Julian Date (JD)

The position of a date within its year, from 1 (1 January) to 365 or 366 (31 December). The ISO 8601 "ordinal date" format writes it as YYYY-DDD. This is the number at the heart of the industrial "Julian date".

See also: Julian date (ordinal / day-of-year)

The solar calendar introduced by Julius Caesar in 45 BC, with a leap year every four years. It drifted from the seasons and was replaced by the Gregorian calendar beginning in 1582. It is unrelated to the industrial day-of-year "Julian date".

See also: Gregorian calendar

The calendar in civil use today, introduced by Pope Gregory XIII in 1582. It refines the leap-year rule (century years are leap years only if divisible by 400), keeping the calendar aligned with the seasons.

See also: Julian calendar

The standard astronomical epoch of 1 January 2000 at 12:00 Terrestrial Time, equal to Julian Date 2451545.0. Used as the reference moment for coordinate systems and orbital elements.

See also: Astronomical Julian Date (JD)

A 7,980-year cycle devised by Joseph Scaliger in 1583, the product of the 28-year solar cycle, 19-year Metonic (lunar) cycle and 15-year indiction. Its start, 4713 BC, is the origin of the astronomical Julian Date. Scaliger named it after his father, Julius Caesar Scaliger.

See also: Astronomical Julian Date (JD)

A calendar extended backward to dates before it was actually adopted. The "proleptic Julian calendar" and "proleptic Gregorian calendar" apply each system to years before 45 BC and 1582 respectively — necessary for defining continuous day counts.

See also: Julian calendar , Gregorian calendar

The time standard at the Greenwich meridian on which the Julian Date is based. A Julian Date "now" is measured in UT, so it is independent of local time zones.

See also: Astronomical Julian Date (JD)