Julian Year (a)

AstronomicalCurrent unit

Exactly 365.25 days (31,557,600 seconds). The astronomical standard; a light-year is defined using it.

1 Julian year = 31,557,600 seconds (exactly).

About the Julian year

Astronomers wanted a year that would never change, so they borrowed the average length of the old Julian calendar year: exactly 365.25 days of 86,400 SI seconds, or 31,557,600 seconds. The symbol is a, or aj when it must be told apart from other years. It is not an SI unit, but the IAU lists it among the non-SI units accepted for use in astronomy.

The mean tropical year drifts slightly from one year to the next, so it makes a poor unit, whereas the Julian year is constant because it rests on the SI second. It still approximates both the sidereal and tropical years to within about 0.008 days. The current standard epoch, J2000.0, is exactly 12:00 TT on 1 January 2000, and other Julian epochs are counted in Julian years from it. The light-year is defined as the distance light covers in one Julian year.

Sources: Wikipedia: Julian year (astronomy); Wikipedia: Light-second; NIST SP 811 Appendix B.8: factors for units outside the SI

Julian Year conversion chart

1 Julian year expressed in every other time unit:

More Astronomical units

• tropical year (time)
• sidereal year (time)
• synodic month (time)
• sidereal day (time)
• aeon (time)
• astronomical unit (length)
• light-second (length)
• light-minute (length)
• light-hour (length)
• light-year (length)
• parsec (length)
• megaparsec (length)
• Earth mass (weight & mass)
• solar mass (weight & mass)
• milliarcsecond (angle)
• zodiac sign (angle)

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