Example: Historians can know when the Jewish Temple was destroyed, when certain Pharaohs reigned, when the Gallic wars occurred, etc.
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Because people back then recorded these events in writing. For some things that weren’t explicitly recorded, they can make an estimation based on other records. For example, the earliest recorded reference to some event, even if it doesn’t give the year of that event, at least provides a latest possible limit for that event taking place.
Because scholars and historians of the time recorded events as they happened and included dates. While they may have used different calendar systems, when you collect enough corroborating data across different cultures, each one allows us to find out about the other.
For example, let’s say I’m a scholar and I write about the coronation of King Tut In year 520 of whatever calendar I’m using, and I chronicle his reign and I note that he died 30 years in his reign.
Some visiting Greek scholar was there who also recorded the death of King Tut and he uses his own calendar system which is in the year 213 and notes that it also happened to be on Easter.
Then some cleric in Rome is going around chronicling all the Easter celebrations from around the Roman empire and in fact even invents a new calendar system just for that, and that calendar system is the one we end up using today.
Well we could use these hypothetical connections to recalculate the reign of King Tut with respect to our own calendar.
Lots of ancient civilizations had calendars. Once you’re able to figure out one of those calendar systems, you can match up events in calendars that you know with identical events in calendars that you don’t know.
This relies on pinpointing specific days. This can be done by ancient records of eclipses or other easily determined geological/astrological events. We can use science to pinpoint this exactly for thousands of years in the past. You can also use references like seasonal changes (the beginning of summer is pretty constant from year to year) as well.
For specific dates you have to rely on historical accounts.
Obviously people can lie or just be wrong, so all historical accounts have to be taken with a grain of salt.
BUT, if you were to find say 2-3 different, unrelated historical accounts that have the SAME facts, for example in Ancient Rome if one account was written in Rome, and the other written in Greece hundred of miles away by two different people who didn’t speak, you can be a bit more reliant on that info because it is unlikely that the same misinformation would show up in two different sources. And the more agreeing sources you have, the better. And that’s for trying to know when a specific day/week event happened.
Long term things are even easier, because rulers often kept near track of things going on during their rule. Or if they didn’t, it was not uncommon for years to be measured by how long that ruler had been ruling, for example “in the 5th year of the reign of XYZ” before calendars were widespread and standardized.
So then you can just line up all these records with a day you know/can verify, and work your way backwards.
Archeological evidence also helps, archeological discoveries can be dated to different eras just by the artifacts present, as well as chemically testing those artifacts to find their age. Usually with Radioisotope dating.
Historians also can now turn to astronomers in some cases. Say a historical text mentions an eclipse or a comet. Well eclipses and comets are things that happen on a pretty standard path, the moon is always going around the earth at the same speed and angle, and some comets steadily move through the solar system in a similar fashion. So astronomers can actually rewind the clock and figure say, how many years ago an eclipse would have been visible in Egypt, and then match those dates up to your ancient Egyptian text.
Ancient civilizations tended to use astronomy-based calendar systems, and *particularly* lunar calendar systems, all of which are extremely accurate methods of timekeeping because of how stable the Moon’s motion is. Thus, it’s surprisingly easy to correlate given events to the modern Western BC/AD calendar system because it’s relatively straightforward to tie that system back to older lunar calendars.
I have yet to see radio Carbon dating mentioned. It’s widely used and is more common than traditional dating through the use of references is Radio carbon dating. This tequnique is very precise, and works by comparing the ratios of Carbon-14 to Carbon-12 (and less so Carbon-13). although is limited to things that lived less than 50,000 years ago. If anything was alive at some point (e.g. the fibers used to make papyrus that writing is on) it took in co2, some of which had radioactive Carbon-14 in it. Carbon-14 is made at a very steady rate in the atmosphere by cosmic rays transmuting Nitrogen. Once the organism dies the Carbon-14 starts to slowly decay. And since we know the initial C-12/C-14 ratio and the half life of Carbon-14 (~5000 years if I remember correctly), we can calculate to impressive precision what time period the paper or leather or wood, or any other once living piece of history.
The Romans dated years by the names of the two consuls that year. Caesar, for example, begins his *Commentaries* with the year “Marcus Messala and Marcus Piso were consuls”. Since we have pretty complete records from that time, we know that’s 61 BC.
Pharaohs are a bit different. We have complete “kings lists” the dynasties, which tell us the order and length of each pharaoh’s reign. But we don’t have a date to anchor those relative years. The Old Dynasty begins somewhere in the 27th Century BC, but if you read specific years, they’re just estimates.
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