Julian year (astronomical unit of time)
A Julian year is an astronomical unit of duration equal to exactly 365.25 days of 86,400 seconds each, used to express long timescales such as orbital periods and proper motions.
The Julian year is a standardized unit of time used in astronomy to express durations on human- to geological scales. It is defined purely as a length of time rather than as a calendar date: one Julian year equals exactly 365.25 days of 86,400 SI seconds each. That definition yields a fixed interval of 31,557,600 seconds and allows astronomers to quote rates and durations consistently across epochs.
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Definition and characteristics
By convention the Julian year uses the day of 86,400 seconds, so 365.25 such days gives the unit often abbreviated as "a" (from the Latin annum). The choice of 365.25 days comes from the average year length in the historical Julian calendar, which introduced a leap day every four years. Because the Julian year is a duration, it does not designate any particular calendar date and should not be equated with a year in the Julian calendar or any civil calendar.
The Julian year is convenient for many astronomical calculations because it is an exact, unambiguous interval. A related derived unit is the Julian century, equal to 100 Julian years and commonly used in ephemerides and long-term series expansions.
History and usage
The term arises from the Julian calendar's arithmetic mean year and from historical practice in Western astronomy. Modern usage treats the Julian year as a non‑SI unit accepted for astronomical work: it appears in expressions for proper motion, orbital elements, rates of precession and nutation, and in older literature when converting between time and distance (for example, a light‑year is sometimes defined using the Julian year).
Because it is exact, the Julian year simplifies comparison of rates given at different epochs. However, for calendar-related purposes or for high-precision timekeeping (e.g., satellite navigation), astronomers use more precise time scales such as the International Atomic Time (TAI) or Barycentric Dynamical Time (TDB).
Key points
- Length: 365.25 days × 86,400 seconds = 31,557,600 seconds.
- Symbol: a (annum); sometimes written "Julian a" to avoid confusion.
- Used for durations, not for calendar dates; distinct from the Julian calendar year.
- Related unit: Julian century = 100 Julian years.
- Not the same as the Julian day, which counts continuous days for dating.
Further reading
Related articles
Author
AlegsaOnline.com Julian year (astronomical unit of time) Leandro Alegsa
URL: https://en.alegsaonline.com/art/51497
Sources
- books.google.com : The explanatory supplement to the Astronomical Almanac
- iau.org : "Measuring the Universe"
- iau.org : "Recommendations Concerning Units" · web.archive.org
- books.google.com : The manual of scientific style