Calendars and time
Besselian Epoch
Direct answer
Besselian Epoch: A Besselian Epoch is an older astronomical epoch convention based on a Besselian year tied to the Sun mean longitude.
A Besselian Epoch is the historical epoch scale used by many pre-J2000 star catalogs.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Julian Epoch, Epoch, Calendar, Julian Date
A Besselian Epoch is the historical epoch scale used by many pre-J2000 star catalogs.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Besselian Epoch
The system honors Friedrich Bessel and was standard in astrometry before Julian epochs and modern reference frames took over.
Why Besselian Epoch matters
Catalog positions marked B1950.0 cannot be treated as if their label were a Julian epoch or a simple Gregorian date.
Besselian Epoch in practice: an example
B1950.0 and J1950.0 identify slightly different instants and conventions.
Where readings of Besselian Epoch go wrong
A common error is to change the letter B to J without a full coordinate and epoch transformation.
Sources and editorial method
Research references used for this entry include:
- 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
Transform the legacy epoch
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Common questions
What is a Besselian Epoch?
A Besselian Epoch is an older astronomical epoch convention based on a Besselian year tied to the Sun mean longitude.
What do people get wrong about a Besselian Epoch?
A common error is to change the letter B to J without a full coordinate and epoch transformation.
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