Julian year (symbol:a ) - unit of time. One Julian year is equal to 365.25 Julian days of 86,400 SI seconds each, that is, exactly 31,557,600 seconds. This is the average length of a year in the Julian calendar used in Europe in antiquity and the Middle Ages [1] . The Julian years are used in astronomy to express periods of revolution around the Sun of any objects of the Solar System (asteroids, comets, etc.), in geochronology to measure large periods of time, in nuclear physics to express the half-lives of radionuclides.
In the Julian calendar, the length of the year is assumed to be 365ΒΌ days = 365.25 days . Approximation 365ΒΌ is achieved by introducing one leap year for every 4 years. Since the tropical year is somewhat shorter (as of January 1, 2000, it was 365.2421897 days), this gives an error in 1 day in about 128 years. The length of the tropical year is in fact slowly changing, therefore both the Julian year and the average year of the Gregorian calendar (365.2425 mean solar days, an error of 1 day of about 3,300 years) represent only an approximation to the period between two successive spring equinoxes (tropical year determining the change of seasons on Earth).
One hundred Julian years make up the Julian century, equal to exactly 36,525 Julian days (where the Julian day is exactly 86,400 SI seconds). This unit is used as one of the basic units of time in astrometry to express the speed of secular changes of orbital elements, rotation axis positions, etc., in formulas connecting different time scales. For example, the precession angular velocity of the earth's axis is (by an epoch of 2000.0) 5029.0966 angular seconds per Julian century. The Julian Millennium is sometimes used (365,250 Julian days).
Previously, the Julian year was defined as 365.25 average solar days. In connection with the replacement of the astronomical definition of the SI second as 1/86400 of the part of the mean solar day with a more accurate physical definition (atomic second), the definition of the Julian year also changed. The Julian year is now determined, as stated above, through atomic time.
The definition of a light year is now based on the Julian year. By the definition of the International Astronomical Union (IAC), a light year is equal to the distance that light travels in a vacuum, without experiencing the influence of gravitational fields, in one Julian year. This definition is recommended for use in popular science literature. Previously, the light year was based on the duration of the tropical year in the era of 1900.0; a change in the definition resulted in a 0.002% lengthening of the light year, which is of no practical importance due to the much worse accuracy in determining any distances outside the solar system.
The use of the Julian year in science to express large periods of time is based on the fact that it is a precisely defined and unchanging unit, in contrast to a tropical year; on the other hand, the Julian year differs only by thousandths of a percent from the tropical year; therefore, it is very rarely necessary to take into account this difference.
See also
- Bessel year
- Tropical year
- Star year
Notes
- β The Julian calendar was introduced by Julius Caesar in 45 BC. er It was widely used in Europe until 1582 , when Catholic countries switched to the Gregorian calendar. Subsequently, this was done by other countries. Today, the Gregorian calendar is used in almost all spheres of life. However, some countries (for example, Russia and Greece) used the Julian calendar in the 20th century, and the Russian Orthodox Church and some other Orthodox churches still use it.