We measure years with the Gregorian calendar, which in turn is a modification of the Julian calendar. The Julian calendar, more even-keeled than previous Roman calendars, standardized the solar year to 365 days with one leap day every four years. The 16th-century Gregorian calendar fine-tuned the frequency of leap days, recognizing that the length of a year is slightly shorter than 365-1/4 days (a fact already well-known to astronomers during the realm of Julius Caesar), and also realigned the calendar to the vernal equinox.
When we say it is 2013, we are counting from AD 1. The initial Julian calendar naturally did not measure with respect to Jesus, who would not be born for several decades. Instead, the Romans anchored years on the start of the rule of their consuls and emperors. The concept of Anno Domini, invented by Dionysius Exiguus in the 6th cenury, simply shifted the origin of the Julian calendar to a supposed birth year for Jesus.
The origin point of a calendar is called its epoch. Many other calendars also pin their epochs to religious events: the Jewish calendar, on a presumed beginning of the world; the Muslim calendar, on an event in the life of Muhammed; the Buddhist calendar, on the enlightenment of Siddhartha.
The origin point of a calendar is called its epoch.
Epoch wasn’t always defined in terms of religious events. Cultures more ancient than the Roman Empire, such as the Chinese and Egyptian, tied their calendar epochs to the starts of their dynasties. Whenever a civilization maintains records for centuries and is important enough for neighboring cultures to note the passage of its rulers, this dynastic method marks time with great accuracy. Early astronomers and historians marked events in the context of political leadership. The astronomer Ptolemy, writing his Almagest during the 2nd century, pinpointed all his calculations to a single hour during the first year of the realm of the Babylonian king Nabonassar [747 BC].
Given that political leadership continues to be significant today, why do religious events now dominate the definitions of epoch for our mainstream calendars? After all, the English monarchy has endured for over a millennia, and the British Empire/Commonwealth spans the globe. Perhaps it’s as simple as noting with Stewart Brand that religions tend to be the most enduring human institutions. Nations and empires rise and fall, while religions continue.
When creating a time capsule for future humans or developing a method to communicate with a potential extraterrestrial intelligence, the problem of defining epoch becomes more difficult. The epochs for UNIX (January 1, 1970) and other operating systems are arbitrary and may not endure. In contrast, the first Westinghouse Time Capsule, intended to be opened in the year 6939, attempts to define its own epoch with both religious-based calendars and by astronomical methods. The date of its burial is provided not only in various calendars, but also by providing the positions of the planets in the sky, of the north star Polaris, and information about solar and lunar eclipses. Another example: the LAGEOS satellite, in geosynchronous orbit, contains a plaque of the Earth, to help people in the distant future backdate the launch of the satellite with continental drift.
Are there other good methods to establish epoch across vast time — in addition to religious-based calendars, astronomical data, and geological information?