Calendars and time
Astronomical Day
Direct answer
Astronomical Day: An Astronomical Day is a context-dependent day interval used in astronomy, historically often beginning at noon to keep one observing night under one date.
An Astronomical Day is a technical day convention that must be defined with its starting instant.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Civil Day, Solar Noon, Calendar, Julian Date
An Astronomical Day is a technical day convention that must be defined with its starting instant.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Astronomical Day
Astronomers once began the observing day at noon; the practice was largely replaced by midnight-based dates in the twentieth century.
Why Astronomical Day matters
The term prevents confusion when reading older observations or Julian dates whose day boundary differs from civil midnight.
Astronomical Day in practice: an example
The Julian Date integer changes at noon UT, so one astronomical night can share an integer day number.
Where readings of Astronomical Day go wrong
It is a mistake to assume all modern observatories use the historical noon-start convention.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Julian Date Converter - Official explanation of Julian dates and conversion between calendar dates and continuous day counts.
- NASA Goddard: Modified Julian Dates - NASA explanation of JD and MJD conventions and their reference epochs.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
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More in Calendars & Timekeeping
- Callippic Cycle
- Draconic Month
- Geocentric Coordinate Time
- Intercalation
- Local Mean Solar Time
- Ordinal Date
- Sidereal Year
- Unix Time
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Common questions
What is an Astronomical Day?
An Astronomical Day is a context-dependent day interval used in astronomy, historically often beginning at noon to keep one observing night under one date.
What do people get wrong about an Astronomical Day?
It is a mistake to assume all modern observatories use the historical noon-start convention.
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