Julian date
Cross-traditionAstronomical factEditorial synthesis
Definition
A continuous count of days used in astronomy instead of calendar dates. It has no months, leap years or time zones, which makes it the natural way to label an instant when doing arithmetic across long spans.
Also called: JD · Julian day number
In more depth
Calendar dates are awkward to compute with: month lengths vary, leap rules changed historically, and the Julian-to-Gregorian transition happened in different years in different places. The Julian date sidesteps all of it by numbering days continuously from a fixed origin far in the past, with the fractional part giving the time of day.[1]
Because it is a bare count, a Julian date must be qualified by the time scale it is expressed in; the same instant has different values in different scales. Astro Gazer's provenance panel uses Julian dates to state the span its Earth-orientation data covers, so you can see whether your birth instant fell inside the tabulated range or beyond it.[1]
References
Numbered citations in the text link to the records below; each record links to the full source registry.
- Sean E. Urban (editor); P. Kenneth Seidelmann (editor). Explanatory Supplement to the Astronomical Almanac. Sean E. Urban and P. Kenneth Seidelmann. University Science Books. 2013. Astronomical fact View source record