Astronomical year numbering

Astronomical year numbering is structured around the AD/CE system but adheres more rigorously to standard decimal integer principles. This system includes a year 0, with years preceding it assigned negative values and those following it designated as positive. For years before 1582, including year 0, astronomers utilize the Julian calendar, while the Gregorian calendar is applied for years after 1582. Notable astronomers such as Jacques Cassini (1740), Simon Newcomb (1898), and Fred Espenak (2007) have employed this system.

In this framework, the prefixes AD and the suffixes CE, BC, or BCE (representing Common Era, Before Christ, or Before Common Era) are omitted. The year 1 BC/BCE corresponds to year 0, while the year 2 BC is labeled as −1. Generally, the year n BC/BCE is represented as "−(n − 1)" (a negative number equivalent to 1 minus n). The years in the AD/CE format remain unchanged and can be expressed without a sign or with a positive sign; thus, n AD/CE is simply represented as n or +n. Zero is often necessary for calculations, particularly when determining the span of years across the epoch, where only the end years need to be subtracted from one another.

This system is specifically referred to as astronomical year numbering due to its application in the field of astronomy. Other disciplines outside of historical studies rarely deal with time prior to year 1, though there are exceptions like dendrochronology, archaeology, and geology, which often use 'years before the present.' While the absolute numerical values of astronomical and historical years differ by just one before year 1, this distinction is crucial when calculating astronomical phenomena such as eclipses or planetary alignments to ascertain the timing of historical events that reference them.

Read also

• Julian day, another calendar commonly used by astronomers
• Astronomical chronology
• Holocene calendar
• ISO 8601