[00:00.00]Listen to part of a lecture in an anthropology class.
[00:04.12]Professor: OK, I, I want to begin today by talking about calendars, [00:09.24]I know, some of you are thinking it’s not all that fascinating, right? [00:14.75]But listen, the next time you look at a calendar, I want you to keep something in mind. [00:21.35]There are at least three natural ways of measuring the ... the passage of time-by day, by month, and by year. [00:31.34]And these are all pretty easy to see, right? [00:34.42]I mean a day is based on one rotation of Earth. [00:38.25]A month is how long the Moon takes to move around the Earth. [00:42.99]And a year is the time it takes for Earth to move around the Sun, right? [00:47.86]So they’re all based on natural events. [00:50.82]But the natural clocks of Earth, the Moon, and the Sun run on different times, and you can't divide any one of these time periods by another one without having some messy fraction left over. [01:06.68]I mean one lunar month -that’s the time it takes for the Moon to go around Earth-one month is about 29 and a half days ... not really a nice round number. [01:18.87]And one year is a little more than 365 days. [01:24.56]So these are obviously numbers that don't divide into each other very neatly. [01:29.48]And this makes it pretty difficult to create some sort of tidy calendar that really works.
[01:35.56]Not that different cultures haven't tried. [01:38.39]Have any of you ever been to Stonehenge? [01:41.78]No ... you know, that amazing circle of giant stones in England? [01:47.36]Well, if you ever go, and find yourself wondering why this culture way back in prehistoric England would go to so much work to construct this monumental ring of enormous stones,… [01:58.66]well, keep in mind that a lot of us think it was designed, at least partially, as a calendar-to mark when the seasons of the year begin, according to the exact day when the Sun comes up from a particular direction. [02:12.44]I have colleagues who insist it's a temple, maybe, or a tomb … but they can’t deny that it was also used as a calendar ...probably to help figure out, for example, when farmers should begin their planting each year.
[02:27.34]The Mayans, in Central America, also invented a calendar, but for a different purpose. [02:33.54]The Mayans, especially the royalty and priests, wanted to look at long cycles of history-so the calendar they used had to be able to count far into the future as well as far into the past. [02:47.97]And not only were the Mayans keeping track of the natural time - keepers we mentioned before-Earth, the Moon, and the Sun-but another natural timekeeper: the planet Venus.
[03:00.75]Venus rises in the sky as the morning star every 584 days, and the Venus cycle was incorporated in the Mayan calendar. [03:11.78]So the Mayans kept track of long periods of time, and they did it so accurately, in fact, that their calendar is considered about as complicated and sophisticated as any in the world.
[03:22.73]Now, the ancient Chinese believed very strongly in astrology一the idea that you can predict future events based on the positions of the stars and planets like, say, Jupiter. [03:34.62]Incidentally, the whole Chinese system of astrology was based on the fact that the planet Jupiter goes around the Sun once every 12 years, so one orbit of Jupiter lasts 12 of our Earth years. [03:49.68]Apparently, that’s why the Chinese calendar has a cycle of 12 years. [03:54.66]You know, like, "The Year of the Dragon,” "The Year of the Tiger,” and so on ...all parts of a 12-year astrological cycle, that we get from the orbit of Jupiter.
[04:05.18]Calendars based on the orbits of other planets, though, are a lot less common than those based on the cycle of the Moon-the lunar month. [04:14.10]I could mention any number of important cultures around the world that have depended on lunar calendars, but there really isn't time. [04:22.60]So let's go right to the calendar that's now used throughout most of the world-a solar calendar-based on the number of days in a year. [04:32.65]This calendar’s mainly derived from the one the ancient Romans devised a couple thousand years ago. [04:39.99]I mean the Romans-with more than a little help from the Greeks一realized that a year actually lasts about 365 and one-quarter days. [04:49.95]And so they decided to round off most years to 365 days but make every fourth year into a leap year. [04:58.78]I mean, somehow, you have to account for that extra one-fourth of a day each year, so every four years, they made the calendar one day longer. [05:08.40]By adding the leap year, the Romans were able to make a calendar that worked so well -that, with a few minor adjustments, this calendar is still widely used today.