A unit of time, a version of the year defined in terms of the SI second = 31 556 925.445 seconds.¹ Symbol, a. It is typically used with an SI multiplier prefix, either k, M or G, so "ka", “Ma" and "Ga".

which is the number of seconds in the tropical year 2000. In choosing this number, harkened back to the pre-atomic clock definition of the second, which in 1956 was defined as “the fraction 1/31 556 925.9747 of the tropical year for 1900.” Of course, the number of seconds in a tropical year is constantly changing.

This unit is used for very long periods of time, and users fall into three catagories: nuclear chemists studying the half-lives of radioactive isotopes, geologists studying the history of the earth, and astronomers.

Annus is the Latin word for year.The Latin phrase, per annum, is often used Conventionally, when a Greek or Lttin word is adopted as th ename of a unit, th enominative case for example, erg not er

1. Norman E. Holden, Mauro L. Bonardi, Paul De Bière, Paul R. Renne and Igor M. Villa.
IUPAC-IUGS common definition and convention on the use of the year as a derived unit of time (IUPAC Recommendations 2011)
Pure and Applied Chemistry, vol. 83, no. 5, pages 1159-1162 (2011).

a for year

Astronomers had used the symbol a for the year for some time, usually meaning the Julian year. see source 1 below.

An increasing number of geological books and journals are using a standard system of units in which Ma (mega-ans) is used for millions of years and Ga (giga-ans) for billions of years. In other usages, m.y. is used for millions of years and b.y. for billions of years.

Frank Press and Raymond Siever.
Earth. 4th ed.
New York: W. H. Freeman, 1986.
Page 25, footnote.

The Quaternary spans the last 1.5 Ma.¹

1. The abbreviations Ma (Mega-annum) refers to units of yr × 10⁶ measured from the present (1950 A.D. by international agreement) pastward. It means the same as the cumbersome “millions of years before present” and is a fixed chronology analogous to the calendars tied to historical events. The abbreviation my (million years) is used to express simple duration in units of yr × 10⁶ in any given past interval.

Treatise on invertebrate paleontology. Part A.
Raymond C. Moore, director.
W. A. Berggren and J. A. Van Couvering.
Introduction: Concept of the Quaternary.
Geological Society of America, 1979.
Page A506.

Table 6 Units accepted for use with SI minute hour day, all defined in seconds, clock time. The year is not mentioned nor the a symbol is mentioned

These are not derived units, a classification that in SI is reserved for new combinations of base units defined solely by multiplication or division of base units. Since SI is a coherent system, defining a new unit by multiplying a base unit by a constant (e.g., = 60 seconds) is not permitted.

In 2004 Renne and Ville letter

Dear Editor,

We write to encourage GSA journals to conform to the Sisteme International (SI) regarding units of time. The SI unit of time, the second (s), is impractical for earth scientists, astronomers and nuclear physicists alike. In such cases, the SI tolerates other units, and for geological applications the annum (a) is used, where 1 a = 3.16 × 10⁷ s (Holden, 2001). As with other units, thousands, millions, and billions of these are appropriately designated ka, Ma, and Ga, respectively. So far, so good—these derived units are in widespread use in earth science literature. The departure lies in the use of different units (e.g., m.y.) for time differences such that the interval between 90 Ma and 100 Ma, for example, would be designated as 10 m.y. in, e.g., Geology. Following correct SI usage (Nelson, 2002) units must follow algebraic rules such as the distributive law: 100 Ma – 90 Ma = (100 – 90)Ma = 10 Ma, and so on. Similarly, rates and decay constants should be expressed in (ka)⁻¹, (Ma)⁻¹ or (Ga)⁻¹. Analogies are useful: we would all agree that the interval between 100 m and 200 m depths in a borehole is 100 m, or that a magma at 1000 °C is 100 °C hotter than one at 900 °C. Why should we treat time units any differently? We urge GSA to abandon the policy of expressing time differences in k.y., m.y., or g.y., and thereby achieve compliance with the SI standard.

Paul R. Renne and Igor M. Villa,
co-chairs IUGS Working Group on Decay Constants in Geochronology.

References Cited
Holden, N.E., 2001, Table of the Isotopes, in CRC Handbook of Chemistry and Physics: CRC Press, Boca Raton, section 11, p. 50–197.
Nelson, R.A., 2002, Guide for metric practice: Physics Today 55, p. BG15–BG16.

GSA Today, vol 14, no. 10, October 2004, page 62.
Copyright © 2004, The Geological Society of America, Inc. (GSA). Reprinted by permission.

The reference to Holden (2001) is in error. That edition of the CRC Handbook actually says (page 11-50):

[Column No.] 4; [Column Title] Half-life; [Description] Half-life in decimal notation. µs = microseconds; ms = milliseconds; s = seconds; m = minutes; h = hours; d = days; and y = years.

The word annum is not mentioned, much less defined. This is puzzling, as for more than 10 years Holden had routinely used the a symbol for year, for example:

The recommended values listed are given in units of second (s), day (d), and year (a).

Norman E. Holden,
Total half-lives for selected nuclides.
Pure & Applied Chemistry, vol. 62, no. 5, pages 941-958 (1990).
Page 944.

Is it possible the CRC editor changed Holden's notation? Whatever the cause


Updates to editorial style: Annum (a) is used to denote year as a unit of measure; use a, ka, Ma, or Ga rather than year (or yr) kyr, Myr (or m.y.), or Gyr.

AGU Editorial Style Guide for Authors. online, update of 21 Sept 2007.


The authors write to urge the use of SI, yet dodge a major goal of the system's founders, namely the eliminating of the foot-inch-line, pound-ounce-dram style of mensuration through the use of prefixes representing decimal multiples and submultiples. To conform with SI, half-life constants would have to be stated in the SI unit of time, seconds, not seconds-days-years. Thus hydrogen-7's half-life would be described as 23 yoctoseconds and tellurium-128's as 69,400,000 yottaseconds.


concern with year led to committee



(c) Convention and abbreviations.—The age of a stratigraphic unit or the time of a geologic event, as commonly determined by numerical dating or by reference to a calibrated time-scale, may be expressed in years before the present. The unit of time is the modern year as presently recognized worldwide. Recommended (but not mandatory) abbreviations for such ages are SI (International System of Units) multipliers coupled with “a” for annum: ka, Ma, and Ga for kilo-annum (10³ years), Mega-annum (10⁶ years), and Giga-annum (10⁹ years), respectively. Use of these terms after the age value follows the convention established in the field of C-14 dating. The “present” refers to 1950 AD, and such qualifiers as “ago” or “before the present” are omitted after the value because measurement of the duration from the present to the past is implicit in the designation. In contrast, the duration of a remote interval of geologic time, as a number of years, should not be expressed by the same symbols. Abbreviations for numbers of years, without reference to the present, are informal (e.g., y or yr for years; my, m.y., or m.yr. for millions of years; and so forth, as preference dictates). For example, boundaries of the Late Cretaceous Epoch currently are calibrated at 65 Ma and 99 Ma, but the interval of time represented by this epoch is 34 m.y.

North American Commission on Stratigraphic Nomenclature.
North American Stratigraphic Code.
AAPG Bulletin, v. 89, no. 11 (November 2005), pages 1547–1591.
Page 1564.

Half lives and mean lives are commonly given in years (unit a).

E. R. Cohen, T. Cvitas, J. G. Frey, B. Holmström, K. Kuchitsu, R. Marquardt, I. Mills, F. Pavese, M. Quack, J. Stohner, H. L. Strauss, M. Takami, and A. J. Thor.
Quantities, Units and Symbols in Physical Chemistry.
IUPAC Green Book, 3rd Edition, 2nd Printing.
Cambridge: IUPAC & RSC Publishing, 2008.
page 24, note 19.



When the proposal for the unit was published in 2009, some geologists objected, for two reasons

the use of two different symbols made The nuclear chemists scoffed: the geologists a and y were measures of the same quantity, and to have two was contrary to the principles of SI. The journal Science said it would stick with the old usage.

editorial opinion

Having date and interval Distinguished ingin the notation serves a useful purpose. Yr shoudl be used instead of y, because Gy is already an SI symbol, for the gray.  

Marie-Pierre Aubry, John A. Van Couvering, Nicholas Christie-Blick, Ed Landing, Brian R. Pratt, Donald E. Owen and Ismael Ferrusquía-Villafranca.
Terminology of geological time: Establishment of a community standard.
Stratigraphy, vol. 6, no. 2, pages 100-105 (2009).

A. This article is available online at www.ldeo/, where the authors comment:

“Consistent with a convention that has been used widely since the 1970s, this paper recommends standardization of the distinction between geohistorical dates (with the symbols ‘a’ for annus, ‘ka’, ‘Ma’ and ‘Ga’ for years and thousands, millions and billions of years before present) and spans of geological time (expressed in ‘yr’, ‘kyr’, ‘Myr’ and ‘Gyr’). The manuscript was rushed into print in the fall of 2009 in an unsuccessful effort to head off the formal abandonment of this convention by the International Union of Geological Sciences (IUGS) and the International Union of Pure and Applied Chemistry (IUPAC) in favor of adopting the symbols ‘a’, ‘ka’, ‘Ma’ and ‘Ga’ for use in both senses.”




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.

George A. Wilkins.
The IAU Style Manual (1989)
The Preparation of Astronomical Papers and Reports.

Available online at
Page 24.

Editorial opinion

If the nuclear chemists wish to confrom strictly  to SI, half-lives should be reported in seconds, that is ks, Ms. and not seconds, days and years. It is easy to imagine the effect these unfamiliar units would have on visualizing ages. On the other hand, perhaps would reveal some interesting a patterns.

on a definition of the present, namely 1950, and , ut it is not clear what is gained by introducing another present and a different length of the year, whn the workers in radiocardon dating had already standardized on the tropical year , and NIST supplies reference.

unfortunate homophone.


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