We’ll be talking about dates and time. Not the exotic time of Einstein or Steve Hawking, discussing quantum physics, but plain ordinary everyday time, such as some retirees have on their hands.
Time comes in all sizes. Seconds, minutes, hours, days, months, years. Nature provides us with some of these units and some are man made.
The day is the time for the earth to rotate once on its axis. It is our most obvious unit of time. Some people wanted to divide days into smaller, more manageable pieces .
How did we get the 24 hour day? We know it was the Egyptians, around 1300 BCE. They divided the day into day and night, and assigned 12 hours to each. Why 12? We’re not sure, but a base number of 12 is just as valid as base 10. We think we use ten because we have 10 fingers . The length of the daytime hours and nighttime hours differed, and varied with the season.
How did the ancients measure time? There were several methods – none very accurate.
Sundials, burning candles, water clocks,
The hourglass , or sandglass, wasn’t invented until the 14th century. It could quite accurately measure a time interval, and is still in use today for timing eggs, games, etc..
Sally and I visited the castle in Spain from which Queen Isabella ruled Castile. They showed us that she had a sandglass mounted on a pivot on the wall next to her desk. When a citizen was granted an audience to make a request or lodge a complaint, she would flip the sandglass over to start it, and when the sand ran out, his time was up.
Mechanical clocks began to be used in the 15th century, and it was not until then that the length of the hour became standardized and was the same day and night.
How did we get minutes and seconds? We got them from the Babylonians, around 200 BCE. For some reason, the Babylonians used a number system of base 60 for doing astrological calculations. Don’t ask. But they divided the hour into sixty minutes, and the minute into sixty seconds.. There have been proposals to change to a metric (base ten) system of dividing the hours, but the base 60 divisions are so thoroughly ingrained in our culture that they are unlikely ever to be changed.
These smaller divisions of time are man-made, so the day can be divided exactly. But the larger units of time: days. months, years were determined by nature, and are not convenient multiples of each other. Which poses a problem: how do we construct an accurate calendar?
Why do we care what day, or month, or year it is? We want to keep track of anniversaries, birthdays, weekends etc. For farmers, it was important to know the best dates for planting and harvesting their crops. An error of a few days could sometimes be catastrophic. Particularly in northern latitudes, where the growing season is short.
If you haven’t visited Stonehenge, I ‘m sure you’ve seen pictures of it. It doesn’t look much like a calendar, does it? Yet archaeologists tell us that in addition to its religious significance, it was designed to reveal the solstices. This is simple technology. In spring, the sun rises a bit farther north each day. Keep track of the shadow of a pole each day, and when the trip north reverses, that’s your solstice. Knowing the winter solstice, for example, you can count off so many days, and its time to plant the barley.
Archaeologists have found remains of several monuments similar to Stonehenge, but made of timbers, designed to pinpoint a summer or winter solstice or both. Think of them as “re-set buttons” from which to time future events in absolute synch with nature.
Everyone wants to keep track of certain historical events, business commitments, future plans, etc. Are we old enough to drive, vote, drink, retire, etc. Obviously these are modern concerns. Back in Kentucky, we used to say, ”If they’re big enough, they’re old enough.” Probably wouldn’t stand up in court.
On a personal note, during the 1970s I was sent to England on business a few times, and I would manage to get a free day to take a one-day bus tour from London. One of these included a visit to Stonehenge, when visitors could still walk in among the stone pillars.
Two other destinations were relevant to our subject. One was the Salisbury Cathedral in southern England. This is contemporary with Notre Dame Cathedral in Paris – both were completed about 1350. The building was impressive, and they were proud to show an original copy of the Magna Carta of 1215.
But the relevant item here was a clock, which they believe is the oldest in Europe. The documentation is pretty thin, but it was probably built in 1386. Large gears and mechanism are mounted in an iron frame, powered by weights, like a cuckoo clock. No face or hands – it just struck the hours. It is still workable, and when they tested it, it was accurate to within two minutes a day. So in Salisbury they were pioneers in the use of mechanical clocks. I understand they also did something with ground beef .
Another destination was the Royal Observatory at Greenwich. Here, I stood on the prime meridian – zero longitude, represented by a metal strip embedded in the courtyard paving. The British Navy and their merchant marine were vitally interested in time, because of its importance to navigation. If you knew your local time, and Greenwich Mean Time, you could calculate your longitude. At Greenwich, they have a clock and watch museum, where you can see the evolution of clocks over several centuries. Of course, for navigational purposes, the Global Positioning System supersedes them all.
What year is this? This hasn’t always been considered to be a terribly important question. Most cultures had some way of keeping track of years, at least in the short term. They would count the years of the Olympiad, or some other notable event, or of the reign of the current king. New king, new calendar.
The units of time nature gave us are not nice neat multiples. One day is the time for one rotation of earth on its axis. A lunar month is one orbit of the moon around the earth, about 29 ½ days. One year is the time for earth to make one orbit around the sun, about 365 ¼ days. Problem: How to represent the day, month and year in an orderly fashion. Some calendars have done this better than others.
Let’s look at some of these calendars. For example:
The Hebrew calendar supposedly dates from the creation of the earth, and we’re up to the year 5771. That calendar has 12 lunar months per year, except when it has 13. In seven years over a 19 year span, they insert an extra month, so over 19 years the year averages out to 365 days..
The Roman republic had a calendar (known as AUC, Anno Urbis Conditae ) which started with the year they believed the city of Rome was founded,753 BCE. Modern historians say that they use that calendar more than the ancient Romans ever did.
In the early Roman Republic the years were often identified by the name of the consuls, who were appointed for one year terms.
When emperors took power,of course it was common to number the years of the reign of the current emperor.
The Chinese have a calendar which puts the year as 4707, but they mainly use the animal designation such as the year of the tiger, or year of the rat. The cycle is repeated every 12 years.
I’ll only mention the Mayan calendar to bring up the fact that their calendar ends with the year 2012, at least in the copy we have.. Once again, there are people saying that means the world will end.
The Muslims, of course decided to go with their own system, dating from the Hegira, or flight of Mohammed from Mecca to Medina. This is their year 1432. Actually the Muslim calendar is similar to the Hebrew calendar, having 12 lunar months, except that they don’t have the “leap month” system, so their year is about 11 days short of the solar year. The seasons keep shifting. Every 33 years they are back in synch (for one year). Muslims seem comfortable with both systems. We did notice that auto license plates in Egypt used the Muslim dating system.
Julius Caesar consulted with an astronomer named Sosigenes of Alexandria and came up with changes to the Roman AUC calendar which would synchronize the calendar year with the astronomical year so that, for example, the solstices and equinoxes would occur on the same dates each year. They adjusted the length of the months, so they no longer kept pace with the moon, and gave us the 365 day calendar with the leap year, which added an extra day each fourth year
The Julian calendar was off by 11 minutes per year . Not much, but this was enough to accumulate a 10 day error by the 1500s. Pope Gregory XIII in 1582, on the advice of Aloysius Lilius, introduced a modification, which skipped the leap year on the years which can be exactly divided by 100. except for years which can be exactly divided by 400. This is known as the Gregorian calendar and is 365.2425 days in length on average , which is accurate to within a few seconds. . Some countries initially refused to adopt a “Catholic” calendar. However, it was the most accurate calendar available, and was eventually adopted by nearly every country. Some call it “the standard Western Calendar.”
Acceptance of the Gregorian calendar wasn’t universal however. In the 1500s, England counted the years “Elizabeth year 1, Elizabeth year 2” etc. However, with increases in international trade, merchants wanted a common international system, and our present system became widely accepted. In England and the American colonies, the new calendar was adopted in 1752, when Wednesday, the 2nd of September was followed by Thursday the 14th of September. In some places, there were riots by people who claimed the government had robbed them of 11 days of their lives. George Washington was born Feb 11th, old style, but they moved his birthday 11 days to Feb 22nd new style.
Today those other calendars are used mainly to calculate the dates of religious holidays and feasts. A Jewish man asked a friend “When is Chanukkah this year?” The answer was, “Same as always, the 25th of Kislev.” But of course, it’s different every year by the Western calendar.
In what would be our year 525 CE, a monk named Dionysius Exiguus proposed to number the years from the birth of Christ. This didn’t get much support at first, but Europe was largely Christian, and during the next three centuries this system began to be adopted by more and more countries. Dionysius messed up the calculations, though, and now they say Jesus was born in 4 BCE.
As the year 1000 approached, even though it was an arbitrary numbering system, some religious figures predicted that would be the end of the world. As the year 2000 approached (Y2K) some of these prediction re-appeared. But mostly we were concerned that our computers wouldn’t recognize the new century.
I should mention one more calendar, one used by astronomers, the JD calendar.. They don’t count years. They count days starting with January 1, 4713 BCE. These are called Julian Days. An astronomer came up with the idea in 1583. The idea was to avoid the problem of humans constantly re-inventing the calendar. It’s about Julian Day 2,455,770.
Then there’s Unix Time, used on some computers. It counts seconds from Jan 1st, 1970. Unix time is about 1,309,032,000. A couple of years ago, there were parties given at what Wikkipedia calls “highly technical subcultures”, meaning nerds, to celebrate a certain milestone. They had Unix time displayed on huge screens, and there was a countdown as the time approached 1 234 567 890. Then suddenly, there it was! Everyone cheered and applauded. But it was only up there for a second: Hardly time enough for anyone to enjoy it.