ZacQuicksilver t1_jaabhtu wrote on February 28, 2023 at 12:09 AM

It's worth noting here that the modern 365-day calendar is somewhat recent.

Many older cultures - notably, both the Jews and Chinese, who retain their older calendars - were what are called "Lunisolar" calendars. In these calendars, you have a set cycle of months, and introduce a leap month into the calendar if a specific month would happen before some yearly event. While I can't find that for the Chinese Calendar right now, the Hebrew calendar is normally 354 days, but adds a 13th leap month if the new year would start before the Spring Equinox.

Other people have already described how these people determined the solstices; and the equinoxes are pretty easy to figure out as well (halfway between the solstices - both in terms of time; and in terms of where the sun rises/sets).

merlin401 t1_jab2dly wrote on February 28, 2023 at 3:28 AM

Yeah very surprised at all the top comments explaining how the ancients had it all figured out! Even at the end of the Roman Republic they had the seasons and the calendars all out of synch until the Julian Calender figured out how to have it work out with leap years and such. And of course even that wasn’t quite right… getting a working calendar was absolutely not a trivial task (and figuring out why it worked wasn’t trivial either!)

ZacQuicksilver t1_jab6myo wrote on February 28, 2023 at 4:04 AM

Except that while it's very hard to get it exactly right, it's really easy to get close.

Getting to 360ish days is relatively trivial - as best as we can tell, a 12-month, 360 (+- 10) day year was developed independently in at least India, China, Mesopotamia, and Mesoamerica - and that's a minimum: there's reason to believe that northern Europe, Mediterranean Europe and/or North Africa, Southeast Asia/Polynesia, South America, North America, and southern Africa also independently developed calendars that were not far off from 360 days. From a 360-day calendar, all you have to do is throw in a few non-year Holy Days before New Year (which, for many calendars, happens at one of the four main days - the solstices and equinoxes) until it's the right time, and move on. Alternatively, if your calendar is lunisolar, you add a month if New Year is too early.

Yes, getting things exact is really hard - even the Julian Calendar is off by a little bit, and technically the Gregorian Calendar is off by a little bit (it's off by .0003 days per year - or about one day in 3000 years). But I think you're selling human intelligence short saying it's not trivial to get a working calendar - as long as you understand the calendar isn't perfect, being a few days off isn't a problem. And both ancient calendars still in use (Chinese, Hebrew) understood that, and had rules to make sure that the calendar was not off by more than one month - and it's safe to assume they weren't the only ones like that.