Calendars and time

Julian Epoch

Direct answer

Julian Epoch: A Julian Epoch is an astronomical epoch expressed from J2000.0 using Julian years of exactly 365.25 days.

A Julian Epoch labels an epoch on a fixed 365.25-day year scale.

Calendars & TimekeepingThe Great Almanac DictionaryUpdated 2026-09-26

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At a glance

Found in: Calendars & Timekeeping
Related: Julian Century, Besselian Epoch, Calendar, Julian Date

An engraved clock movement with an hourglass and a sundial — Calendar and clock · visual guide **Julian Epoch**
A Julian Epoch labels an epoch on a fixed 365.25-day year scale.
A cycle or timeline schematic for comparing the named timekeeping concept.

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The story behind Julian Epoch

Julian epochs replaced older Besselian epochs in many modern astrometric standards.

Why Julian Epoch matters

Julian Epoch provides a concise time coordinate for star catalogs, orbital elements, and reference-frame data.

Where readings of Julian Epoch go wrong

It is a mistake to interpret the decimal part of a Julian Epoch as an ordinary Gregorian date fraction without conversion.

Sources and editorial method

Research references used for this entry include:

U.S. Naval Observatory: Terrestrial Time - Official definition of TT, its relation to TAI, and the J2000.0 epoch.
U.S. Naval Observatory Circular 179: The IAU Resolutions on Astronomical Reference Systems, Time Scales, and Earth Rotation Models - Technical reference for TAI, TT, TCG, TCB, TDB, UT1, sidereal time, epochs, and Earth rotation.

Source: The Great Almanac Dictionary
Category: Calendars & Timekeeping
Last updated: 2026-09-26

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Julian CenturyA Julian Century is exactly 36,525 days in astronomical formulas. Besselian EpochA Besselian Epoch is the historical epoch scale used by many pre-J2000 star catalogs. CalendarA Calendar organizes days into named and numbered cycles for reckoning time. Julian DateJulian Date is a continuous astronomical count of days and day fractions. Coordinated Universal TimeCoordinated Universal Time is the international reference standard for world time. Sidereal TimeSidereal Time is star-based rotational time used to orient the sky.

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Common questions

What is a Julian Epoch?

A Julian Epoch is an astronomical epoch expressed from J2000.0 using Julian years of exactly 365.25 days.

What do people get wrong about a Julian Epoch?