How do people know, say, that some specific event happened in China 1200 BC? Or the Mayans did something in AD 150? Back then, didn't they all use a different calendar system?
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1By tracing the different calendars backwards from a known, synchronised date.– SemaphoreApr 27, 2015 at 1:35
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@Semaphore Ok... but how do they know this date was synchronized? Do you have an example?– JackApr 27, 2015 at 1:52
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Web search: chronology.– Tyler DurdenApr 27, 2015 at 2:01
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1 Answer
The term you're looking for is chronology (Wikipedia's article on the subject is rather sub-par).
In general, there are two major ways of synchronizing dates.
Find an event recorded in more than one calendar. For example, if a treaty was signed on "a.d. VIII Kal. Oct, Julius et Caesar consulibus" and "7.14.19.11.14", you can establish an equivalence between the two and convert dates back and forth; a sufficient chain of conversions will let you figure out what the date would be in a modern system. In practice, you'll want multiple dual-dated events, because of errors in the historical record.
Find an astronomical event that was recorded with a date. Total solar eclipses are especially good for this, since they're rare and only visible from a small area. If you've got even a "within a century or two" idea of how the calendar you're working with aligns to the modern dating system, there's usually only one eclipse that's a candidate. Again, you want multiple events to deal with possible error (eg. an annular eclipse that was recorded as total, or a "great comet" that was on a hyperbolic orbit rather than the short-period comet you thought it was).
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Addendum: The astronomers use so-called Julian day (not to be confused with Julian calendar). It is also sometimes adopted as a reference where calendars are common, but not regular (eg. switch from Julian calendar to Gregorian with lack of 10 days in 1582, different leap years in Roman calendar etc.).– VoitcusApr 27, 2015 at 6:09