Annus, -ī
Annus (year in Latin) is the unit of time that is exactly 365.25 d (86400 s).
Although there is no universally accepted symbol for the year,
International Organization for Standardization,
International Union of Pure and Applied Chemistry,
International Union of Geological Sciences,
National Institute of Standards and Technology,
and the International Astronomical Union
recognise and recommend the use of the symbol a for a year.
In Astronomical publications, the deprecated abbreviation yr is still frequently used.
However, the symbol a is widely used in other scientific disciplines (Paleonthology, Archaelogy, Geology, Biology...).
"Year" in other languages
año (ES)
année (FR)
anno (IT)
ano (PT)
Jahr (DE)
nián (CN)
varsh (HI)
a instead of yr
• IAU Style Manual
(International Astronomical Union; IAU Commission 5, IAU Transactions XXB, 1989)
The IAU has used the Julian century of 36 525 days in the fundamental formulae for precession, but the more appropriate basic unit for such purposes and for expressing very long periods is the year. The recognised symbol for a year is the letter "a", rather than "yr", which is often used in papers in English; the corresponding symbols for a century (ha and cy) should not be used. Although there are several different kinds of year (as there are several kinds of day), it is best to regard a year as a Julian year of 365.25 days (31.5576 Ms) unless otherwise specified.
• NIST SP811
(National Institute of Standards and Technology; Special Publication 811)
• ISO 80000-3:2019
(Quantities and units - Part 3: Space and time)
• IUPAP Red Book
(International Union of Pure and Applied Physics; Symbols, Units, Nomenclature and Fundamental Constants in Physics)
• IUPAC Green Book
(International Union of Pure and Applied Chemistry; Quantities, Units and Symbols in Physical Chemistry)
• IUGS, Holden et al. 2011, Pure. Appl. Chem. 83, 1159
(International Union of Geological Sciences; IUPAC-IUGS common definition and convention
on the use of the year as a derived unit of time)
• Aubry et al. 2009, Stratigraphy 6, 2
(Terminology of geological time: Establishment of a community standard)
• UCUM (Unified Code for Units of Measure)