Calendars is a topic where ancient astronomy meets the modern world. So let's look at calendars or timekeeping, and see how deeply embedded in modern life some very ancient astronomical practices are. Astronomy still lives with us every day in our calendar. To see how deeply calendars are in time, or embedded in our everyday lives and how ancient the history of these ideas is. Look at this set of questions about calendars and time, and just see how many you could answer without looking them up. A few? Half of them? Most of them? I suspect most people could barely answer a few of these questions, yet they're simple everyday knowledge about things we deal with every day and every year. We'll answer all these questions and see how deeply they come from history and different cultures that lead to our own culture. As we can see, there are four major subdivisions of time in a year. Three of them are strictly astronomical. The days, the rotation period of the Earth. The year, is the orbital period of the Earth around the Sun. The month, is a lunar cycle or the orbital period of the moon around the Earth. The only division of time that's not naturally astronomical or obviously astronomical, is the week. Well, it turns out that the week is not always been the same division of time among-st different cultures around the world. Over thousands of years and around the world, you can find cultures that divided time with a week as little as four days, up to weeks as long as 10 days. But it does seem for various cultural reasons, that people need to divide time on a shorter amount than a month. The classic week of the modern calendar starts with the Babylonians and the Egyptians. The deepest embedding of culture and astronomy in the week, is seen if you go into a Romans language like French, or Spanish, or Italian, or Portuguese. Because the names of the days of the week in any of those languages, are quite obviously the seven moving objects in the night and daytime sky, the Sun, the Moon, and the five naked eye visible planets; Mercury, Venus, Mars, Jupiter, and Saturn. Those names go obviously into the days of the week in the Romans language, but not as obviously in Nordic languages or Germanic languages, as in England or any of the Northern European countries. Because we ended up with a cultural mashup where the Teutonic myths, and the Norse gods, overlaid four of the Romans languages, giving us Tiw for Tuesday, Woden for Wednesday, Thor for Thursday, and Fria for Friday. So the week, is an astronomical relic. As you can see, the ordering of the days in the week and their names, do not relate to their distance from the Sun. They were actually rooted in ancient astrology going back 2,000 years and historians still debate the reason why the ordering is the way it is. Let's look at the origin of our modern calendar of the year, and just see what a crazy historical construction it was, going back to Roman times. Our calendar traces back to the ancient Romans, by which I mean Rome at the time of Romulus and Remus, the seventh century BC. This is nothing like the Imperial Rome that you may be aware of, from movies and TV shows. This was Rome when Rome was a set of warring tribes, quite militaristic, that dominated their region, and occasionally would do battle with other tribes North of the Alps. They needed a calendar, but it didn't have to be a very good calendar. This was not a strictly agrarian society. So the ancient Romans started their year in what we would call March, when the snows melted enough that they could raise an army and go off and do battle. Because they started their year in March and they were following roughly a lunar calendar, they only needed 10 months in their year before they reset it again. Essentially, they didn't count the time in the dark winter. Therefore, as you can see, the last four months of our year, are named after the Roman words for seven, eight, nine, and ten. You may have wondered growing up, why are the last four months of our year named after the number seven, eight, nine, and ten, when they are clearly the ninth, tenth, eleventh, and twelfth months of the year. The answer comes from Ancient Rome. This calendar sufficed for a few centuries, and then Promiscuous, and other Emperors, decided to pad it out into a legitimate lunar calendar by adding a couple of light months at the front end, giving us the 12 months. The year they were matching with their lunar calendar consisted of 354 days, that is 12 lunar cycles. So this was a lunar calendar. If we look around the world, we'll find that most of the world's first calendars were lunar calendars. Tracking lunar cycles is the easiest thing to do, which is why we see evidence of it going back tens of millennia and the calendar sticks I've already talked about. So the ancient Romans had a lunar calendar with 12 months, two live months at the front, and 10 regular-sized months at the end, but not a very good calendar. The solar calendar, the time it takes the Earth to go around one complete orbit of the Sun, is about 365 and a quarter days. So that's 11 and a bit days longer than a lunar calendar. In other words, a lunar calendar is no good for agriculture because it goes out of sync by that 11 days a year. So the Romans with a growing and powerful society and a need for feeding their growing number of citizens, wanted a solar calendar to plant crops reliably. The lunar calendar was no longer working for them, so they started to match to a solar cycle. So the calendar moves towards 365 days. As a relic however, February is the short month, the unlucky month. The month when bad luck happens, and that continues to this day. February is still our short month. They still had a pretty strictly alternating sequence of 30 and 31 days among-st the later months. They also had a front and a back end of the year. The last months of the year named after numbers, and the first months of the year named after primordial gods. Some of these gods such as Maya, which gives us the name for May, are found worldwide and were not simply Roman gods. Others such as Janus for January, the god of doorways and beginnings, we can see the two faced god in archways in ancient Rome, you can visit them these days, were particular to the Roman culture. Rationalizing the Roman calendar fell to Julius Caesar. This around the time of Christ, also corresponded to Christianity. The Romans had moved to sun worship, but Christianity was also becoming a part of their culture. Julius Caesar presided over one of the most extensive and powerful regimes ever known in the world history. It's stretched all the way from Africa to Scotland. Regulating an empire this large, required messages to be passed around thousands of miles, communications, synchronization of military movements, and the movement of goods and services. It could not be done without a good calendar. So Caesar recognized that the old Roman calendar was not up for the job. So we have the Julian calendar. The first revolution in calendars where Caesar adds a leap year, that is an extra day every fourth year, giving a calendar with a mean length of 365.25 days. That's close to a solar cycle. So this was a pretty decent calendar. Because Caesar had a fairly large ego, he also decided to take the first unnamed month in sequence and name it after himself, hence July. Coming soon after Caesar, Augustus didn't make any innovation to the calendar, but he had a similarly large ego, so he named the next numbered month after himself, hence August. But he also made the disconcerting reference to the fact that Julius Caesar's month was longer than his month, so he added a day to his month, hence messing up the pattern. If you wonder why you have to use the knuckles of your hand to figure out which days have 30 and 31 days it's because of Julius Caesar, and Augustus who followed him and their big egos. So we immediately have the idea that the calendar we use now, is a mash up of ancient Roman and imperial Roman practices, and somewhat arbitrary things like the bad luck of February being the lightest month. There things stood for quite a while. The Julian calendar is quite successful. However, the exact time that it takes the Earth to go around the sun is 100th of a day different from 365.25. It's actually 365.24199. That difference is trivial. From one year to the next, is truly trivial, but of course it adds up. So the Julian's calendar started to get out of sync by 100th of a day per year. After a century, that's a day. By medieval times, the Julian calendar was off by a week or so, and agricultural planting cycles were starting to be affected. So in the era of the great Popes of the Italian Renaissance, we have the Gregorian calendar, the last adjustment to the modern calendar made by Pope Gregory. Pope Gregory recognized the problem with the calendar. He had quite astronomers making careful measurements of the length of the day and the year. So he made a final adjustment which is in built into our calendars, although we rarely have to think about it, where the leap year is skipped in years divisible by 400. That tweaks the average length of the calendar to very close to the true time it takes the Earth to go around the Sun. The Gregorian calendar is basically only off by less than a ten thousandth of a day per year, and therefore will be good for tens of millennia. This is the calendar we live with, and it's the history of Rome going through the Catholic times. An interesting sidelight on this is that the Protestant countries after the Reformation, did not want to follow the Pope's guidance on the calendar, so they resisted making the adjustment that Gregory made for another 150 or 200 years. In France, this happened quickly, but in England and the United States were Protestants were dominant, the calender was not adjusted until the gap between the solar year and the calendar year had approached 13 days. So by theat, on one day, April fourth turned into April 15th. Benjamin Franklin wrote in Poor Richard's Almanack, widely read by Americans of the time," Do not fret over the loss of those days". But people did fret, they worried they'd be ripped off rent, and bad things would happen although the Earth continued in its orbit. The French supposedly created the April Fool holiday to mock the Americans and the British for taking so long to get the right calendar. So calendars have been part of our culture for a long time. Did we ever have a rational calendar? Yes. For a very short time in the French Revolution, Napoleon as he was spreading his empire almost to the heights of Caesar's extent at the height of his powers, decided on a more rational calendar. He decided that he didn't want idolatry or ancient gods and so he named the festivals after harvest and more civil names. He also had the idea of decimal time, dividing the day into 100 hours, an hour into 100 minutes, and a minute into 100 seconds, which interestingly makes the second about the same as it is in our time keeping. This calendar was not popular because only France followed this calendar, and even then only because Napoleon was at the height of his powers. So as Napoleon was about to fall, France reverted to the Gregorian calendar. But this one brief moment of decimal time was the only time that calendars have been truly rational. Calendars and their history, offer us a quirky and sometimes entertaining embedding of astronomy in modern culture. Calendars and timekeeping have taken place in different ways around the world, originally relating to sun worship, giving us Pagan holidays. Then with the Romans through a Western tradition, a solar calendar designed to keep agricultural timekeeping and the provision of crops for growing societies. In other parts of the world, especially the Islamic world, a lunar calendar is still followed.