Astronomical year numbering
Encyclopedia
Astronomical year numbering is based on AD
/CE
year numbering, but follows normal decimal
integer
numbering more strictly. Thus, it has a year 0
, the years before that are designated with negative numbers and the years after that are designated with positive numbers. Astronomers use the Julian calendar
for years before 1582, including this year 0, and the Gregorian calendar
for years after 1582 as exemplified by Jacques Cassini
(1740), Simon Newcomb
(1898) and Fred Espenak
(2007).
The prefix AD and the suffixes CE, BC or BCE (Common Era, Before Christ or Before Common Era) are dropped. The year 1 BC/BCE is numbered 0, the year 2 BC is numbered −1, and in general the year n BC/BCE is numbered "−(n − 1)" (a negative number equal to 1 − n). The numbers of AD/CE years are not changed and are written with either no sign or a positive sign; thus in general n AD/CE is simply n or +n. For normal calculation a number zero
is often needed, here most notably when calculating the number of years in a period that spans the epoch
; the end years need only be subtracted from each other.
The system is so named due to its use in astronomy
. Few other disciplines outside history
deal with the time before year 1, exceptions being dendrochronology
, archaeology
and geology
, the latter two of which use 'years before the present
'. Although the absolute numerical values of astronomical and historical years only differ by one before year 1, this difference is critical when calculating astronomical events like eclipses or planetary conjunctions to determine when historical events which mention them occurred.
(1627), Johannes Kepler
used a prototype of year zero which he labeled Christi (Christ) between years labeled Ante Christum (Before Christ) and Post Christum (After Christ) on the mean motion tables for the Sun, Moon, Saturn, Jupiter, Mars, Venus and Mercury. Then in 1702 the French astronomer Philippe de la Hire
used a year he labeled at the end of years labeled ante Christum (BC), and immediately before years labeled post Christum (AD) on the mean motion pages in his Tabulæ Astronomicæ, thus adding the designation 0 to Kepler's Christi. Finally, in 1740 the French astronomer Jacques Cassini
, who is traditionally credited with the invention of year zero, completed the transition in his Tables astronomiques, simply labeling this year 0, which he placed at the end of Julian years labeled avant Jesus-Christ (before Jesus Christ or BC), and immediately before Julian years labeled après Jesus-Christ (after Jesus Christ or AD).
Cassini gave the following reasons for using a year 0:
Fred Espanak of NASA
lists 50 phases of the moon within year 0, showing that it is a full year, not an instant in time.
Jean Meeus gives the following explanation:
The XML Schema language, sometimes used in connection with representing data for storage in computers, contains built-in primitive datatypes, date and dateTime, which do not allow a year zero, and designate years BC as negative numbers. Years contain at least four digits. Thus -0001 in that language is equivalent to 1 BC. However, the defining recommendation
indicates a change to a system similar to ISO 8601
and astronomical year numbering is likely in the future.
Anno Domini
and Before Christ are designations used to label or number years used with the Julian and Gregorian calendars....
/CE
Common Era
Common Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...
year numbering, but follows normal decimal
Decimal
The decimal numeral system has ten as its base. It is the numerical base most widely used by modern civilizations....
integer
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...
numbering more strictly. Thus, it has a year 0
Year zero
"Year zero" does not exist in the widely used Gregorian calendar or in its predecessor, the Julian calendar. Under those systems, the year 1 BC is followed by AD 1...
, the years before that are designated with negative numbers and the years after that are designated with positive numbers. Astronomers use the Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...
for years before 1582, including this year 0, and the Gregorian calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...
for years after 1582 as exemplified by Jacques Cassini
Jacques Cassini
Jacques Cassini was a French astronomer, son of the famous Italian astronomer Giovanni Domenico Cassini.Cassini was born at the Paris Observatory. Admitted at the age of seventeen to membership of the French Academy of Sciences, he was elected in 1696 a fellow of the Royal Society of London, and...
(1740), Simon Newcomb
Simon Newcomb
Simon Newcomb was a Canadian-American astronomer and mathematician. Though he had little conventional schooling, he made important contributions to timekeeping as well as writing on economics and statistics and authoring a science fiction novel.-Early life:Simon Newcomb was born in the town of...
(1898) and Fred Espenak
Fred Espenak
Fred Espenak is an American astrophysicist. He works at the Goddard Space Flight Center. He is best known for his work on eclipse predictions....
(2007).
The prefix AD and the suffixes CE, BC or BCE (Common Era, Before Christ or Before Common Era) are dropped. The year 1 BC/BCE is numbered 0, the year 2 BC is numbered −1, and in general the year n BC/BCE is numbered "−(n − 1)" (a negative number equal to 1 − n). The numbers of AD/CE years are not changed and are written with either no sign or a positive sign; thus in general n AD/CE is simply n or +n. For normal calculation a number zero
0 (number)
0 is both a numberand the numerical digit used to represent that number in numerals.It fulfills a central role in mathematics as the additive identity of the integers, real numbers, and many other algebraic structures. As a digit, 0 is used as a placeholder in place value systems...
is often needed, here most notably when calculating the number of years in a period that spans the epoch
Epoch (astronomy)
In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as celestial coordinates, or elliptical orbital elements of a celestial body, where these are subject to perturbations and vary with time...
; the end years need only be subtracted from each other.
The system is so named due to its use in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
. Few other disciplines outside history
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...
deal with the time before year 1, exceptions being dendrochronology
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...
, archaeology
Archaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
and geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, the latter two of which use 'years before the present
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...
'. Although the absolute numerical values of astronomical and historical years only differ by one before year 1, this difference is critical when calculating astronomical events like eclipses or planetary conjunctions to determine when historical events which mention them occurred.
Year zero usage
In his Rudolphine TablesRudolphine Tables
The Rudolphine Tables consist of a star catalogue and planetary tables published by Johannes Kepler in 1627 using data from Tycho Brahe's observations.-Previous tables:...
(1627), Johannes Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...
used a prototype of year zero which he labeled Christi (Christ) between years labeled Ante Christum (Before Christ) and Post Christum (After Christ) on the mean motion tables for the Sun, Moon, Saturn, Jupiter, Mars, Venus and Mercury. Then in 1702 the French astronomer Philippe de la Hire
Philippe de La Hire
Philippe de La Hire was a French mathematician and astronomer. According to Bernard le Bovier de Fontenelle he was an "academy unto himself"....
used a year he labeled at the end of years labeled ante Christum (BC), and immediately before years labeled post Christum (AD) on the mean motion pages in his Tabulæ Astronomicæ, thus adding the designation 0 to Kepler's Christi. Finally, in 1740 the French astronomer Jacques Cassini
Jacques Cassini
Jacques Cassini was a French astronomer, son of the famous Italian astronomer Giovanni Domenico Cassini.Cassini was born at the Paris Observatory. Admitted at the age of seventeen to membership of the French Academy of Sciences, he was elected in 1696 a fellow of the Royal Society of London, and...
, who is traditionally credited with the invention of year zero, completed the transition in his Tables astronomiques, simply labeling this year 0, which he placed at the end of Julian years labeled avant Jesus-Christ (before Jesus Christ or BC), and immediately before Julian years labeled après Jesus-Christ (after Jesus Christ or AD).
Cassini gave the following reasons for using a year 0:
Fred Espanak of NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
lists 50 phases of the moon within year 0, showing that it is a full year, not an instant in time.
Jean Meeus gives the following explanation:
Signed years without year 0
Although he used the usual French terms "avant J.-C." (before Jesus Christ) and "après J.-C." (after Jesus Christ) to label years elsewhere in his book, the Byzantine historian Venance Grumel used negative years (identified by a minus sign, −) to label BC years and unsigned positive years to label AD years in a table, possibly to save space, without a year 0 between them.The XML Schema language, sometimes used in connection with representing data for storage in computers, contains built-in primitive datatypes, date and dateTime, which do not allow a year zero, and designate years BC as negative numbers. Years contain at least four digits. Thus -0001 in that language is equivalent to 1 BC. However, the defining recommendation
W3C recommendation
A W3C Recommendation is the final stage of a ratification process of the World Wide Web Consortium working group concerning a technical standard. This designation signifies that a document has been subjected to a public and W3C-member organization's review. It aims to standardise the Web technology...
indicates a change to a system similar to ISO 8601
ISO 8601
ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date and time-related data. It was issued by the International Organization for Standardization and was first published in 1988...
and astronomical year numbering is likely in the future.