Tuesday, January 16, 2007

Figuring out Time: The History of Calendars

We all know that George Washington was born on February 22, 1732, right? That’s what we learned in school, and that’s what the majority of texts about Washington tell us. You might be surprised to learn that the truth is slightly different.

February 22 is indeed Washington’s birthday … if you’re using the Gregorian calendar, which is the way we measure time today. But on the day Washington was actually born, the English—and thus the American colonists—were still using the Julian calendar. Consequently, when the future commander in chief of the American Army and our first United States president made his debut in the world, the calendar actually read February 11.

Let’s take a brief overview of calendars through the ages. What is a calendar? Simply put, it's a system of organizing and measuring months, days, and years. Days are based on the length of time it takes for our earth to make one complete rotation on its axis. Months are based on how long it takes for the moon to revolve around the earth. And years are based on the length of time it takes for earth to make a complete circuit around the sun. The problem is that not only do these astronomical cycles have slight variations in length, but they also don’t exactly line up with each other. That makes it impossible to come up with a formula for calculating a calendar that is completely accurate. Every calendar humans devise needs regular tweaking to keep these built-in errors from accumulating over time.

If you’re like me, well before New Year’s day, you were already searching avidly for just the right calendars for your office and home. I couldn’t function without my Daytimer. In addition to our clocks, we all depend on calendars to keep our lives organized. Not only are calendars important for helping us keep a grip on our personal business, but they also provide the basis for agricultural planning; predicting solar and lunar events such as eclipses; determining when the seasons change; and maintaining cycles of civil and religious events.

Our ancestors had many of the same needs and concerns. Throughout human existence, the sun, moon, planets, and stars have been important reference points for measuring the passage of time. From ancient times, civilizations have depended on the perceived motion of celestial bodies through the sky to determine seasons, months, and years. Although we don’t know much about how the earliest humans calculated time, ancient records and artifacts show that people have always measured and recorded the passage of time.

Over 20,000 years ago Ice Age hunters in Europe carved lines and holes in sticks and bones that modern researchers believe may calculate the days between phases of the moon. Five thousand years ago, Sumerians in the Tigris-Euphrates valley in modern-day Iraq devised a calendar that divided the year into thirty-day months, divided days into 12 periods, each corresponding to 2 of our hours, and then divided these periods into 30 parts that correspond to 4 of our minutes. The alignment of the massive stones at Stonehenge, which was built over 4000 years ago in England, strongly indicates that this ancient formation was used to determine seasonal or celestial events, such as lunar eclipses and solstices.

The earliest Egyptian calendar was based on the moon’s cycles. Later Egyptians, however, determined that the “Dog Star” in Canis Major, now called Sirius, rose next to the sun every 365 days, roughly at the same time the annual flooding of the Nile began. They used this discovery to establish a 365-day calendar that appears to have begun in 4236 BC, the earliest recorded year in history.

Before 2000 BC, Babylonia, which covered a part of modern-day Iraq, observed a year of 12 alternating 29-day and 30-day lunar months, which resulted in a 354-day year. The Mayans of Central America, who flourished from around 2000 BC until about 1500 AD, relied not only on the sun and moon, but also on the planet Venus, to calculate 260-day and 365-day calendars like the one above. According to Mayan records, they believed that the creation of the world occurred in 3113 BC. The great Aztec civilization that followed incorporated Mayan calendars into their own calendar stones, one of which is shown in the image below right.

The ancient Romans used a lunar calendar, a system that is highly inaccurate. On the advice of his astronomers, Julius Caesar established a sun-based calendar and decreed that one year would consist of 365 and a quarter days, divided between 12 months. The Romans renamed the month of Quirinus July to commemorate his reform. This Julian calendar continued in common use up until it was replaced by the Gregorian calendar.

We have a slight change of order on this week’s posts. Tomorrow I’m going to delve into how the Gregorian calendar we use today was developed. I’ll also include links to several sites where you can find historical sun and moon data. I’ve found these sites to be particularly helpful whenever I set a scene on a specific date and time and want to describe the moon or show when the sun rises or sets.

Then on Thursday we’ll have Michelle Sutton’s review of Loving Liza Jane by Sharlene Maclaren, which will hit bookstore shelves in April. Michelle is still reading it, so you’re going to get it hot off the press! Friday ... well, it's still up for grabs.

As always, be sure to get in on the action! Post a comment, and you’ll be entered in Friday’s drawing for a free copy of Deeanne Gist’s A Bride most Begrudging.

No comments: