Facts of the Matter
Facts of the Matter
The calendar
from the skyThe calendar is such a basic thing, we take it for granted. It consists of 52 weeks of seven days, plus one extra day, for a total of 365 days. But it is complicated by leap year every four years, except in years that are divisible by 400, except the year 2000, which was a leap year anyway. It was the source of much confusion for our ancestors when trying to figure out how to make a calendar based on both the sun and the moon. Despite its relative simplicity, it wasn't until the 16th century that an accurate calendar was established.
The complications arise because the motions of the earth, sun and moon are not synchronized, and it's not easy even to determine the number of days in a year.
A month is based on the time for one orbit of the moon around the earth. It is slightly more than 29 1/2 days. There are 12 moon cycles plus a little more than 11 days in one year.
A year is the time for one orbit of earth around the sun. It is 365.2422 days, slightly less than 365.25 days. A year of 365.25 days is 11 minutes and 15 seconds too long, which amounts to about three days every 400 years. So three of every four years ending in "00" are not leap years. The years 1700, 1800 and 1900 were, but 2000 was not.
The Gregorian calendar that we use today is based on a cycle of 400 years, which is 146,097 days. Dividing 146,097 by 400 yields an average length of 365.2425 days per calendar year, which is close but not exact. The difference amounts to about one day in 2,500 years.
Finding the simplest way to keep the calendar in synch with the cosmos took thousands of years. The ancient Egyptians almost had it thousands of years ago, but their calendar had no leap year. Their year consisted of 12 months of 30 days each, with five days added at the end. The error of about one-fourth day per year caused the starting date of the year to drift slowly forward with respect to the seasons, and it took 1,460 years to return to where it started. All they had to do was add a day every four years, and the problem would have been mostly solved. So close but yet so far!
A sophisticated but extremely complicated system of keeping track of the motions of both the sun and moon in a single calendar was developed in Mesopotamia 1,000 years later. It added seven extra months over 19 years. The extra month was added in years three, six, eight, 11, 14, 17 and 19. It was added in the 12th month in most of those years, except in the 17th year it was added after the sixth month. Just try and keep track of the date using that system!
The Romans were not as good at astronomy as were the Egyptians who preceded them by 2,000 years, but they were able to develop a calendar with leap years. According to legend, the Roman calendar began when Romulus, the founder and first king of Rome, divided the year of only 304 days into 10 months. Numa, his successor, added two extra months. The 12 months consisted of alternating 29- and 30-day months to roughly correspond with the moon cycle. Originally, the year contained 354 days, but an extra day was added to make the number odd, which was considered more astrologically favorable, so the year had 355 days. This differed from the solar year by 10 days and a fraction. The ancient Roman year began with March, as is indicated by the names September, October, November, December (months seven, eight, nine and 10). July and August were called Quintillis (month five) and Sextillis (month six); they were changed to their present names to honor Julius Caesar and Augustus.
To further complicate things, the Romans did not number the days of the month sequentially. Instead they had three named days in each month: "Calendae" was the first day of the month (the origin of the word "calendar"). "Idus" was the 13th day of January, February, April, June, August, September, November and December, and the 15th day of March, May, July and October. "Nonae" was the ninth day before Idus. The days between Calendae and Nonae were called "the fifth day before Nonae," "the fourth day before Nonae," "the third day before Nonae," etc." Similarly, the days between Nonae and Idus were called "the Xth day before Idus," and the days after Idus were called "the Xth day before Calendae (of the next month)."
To keep in synch with the seasons, an extra month was inserted every second year, consisting of 22 and 23 days alternately, so that four years constituted 1,465 days, and the average length of the year was 366.25 days. This made the year too long by one day, which made another correction necessary.
Since the one extra day added 24 days in as many years, every third eight-year period had only three extra months instead of four, consisting of 22 days each. This finally brought the average length of the year to 365.25 days.
Then another problem arose. Priests called pontiffs were given the power to add days as needed when the calendar was found to differ from the celestial motions. Bribes quickly caused abuses of this power as days were added instead for political interests to prolong or shorten a political term or to hasten annual elections. The nicely designed calendar was thrown into confusion. By the time of Julius Caesar, the calendar was off by more than 60 days.
To put an end to the disorder, Caesar abolished this system in 45 B.C. in a decree that every fourth year should have 366 days, the other years having 365 each, as in our modern leap year. In order to calibrate the calendar, he ordered a one-time insertion of two months between November and December. The first consisted of 33 days, the second of 34 days. Caesar's calendar with its leap year began on Jan. 1 and is known as the Julian calendar.
This should have solved the major part of the problem, but things are never that simple. Due to a misunderstanding, every third year instead of every fourth was made a leap year for some years, and because leap years were considered unlucky, they were avoided in times of crisis.
Even when administered correctly, the Julian calendar left a small error, so every 128 years, the seasons shifted one day backward with respect to the calendar.
The solution was the Gregorian calendar, introduced in 1582 by Pope Gregory I, as part of the sweeping changes to the Roman Catholic Church in the face of the Protestant Revolution that was sweeping Europe.
By then the vernal equinox had drifted backward, and it was felt that March 21 was the proper day for it because that was the date established during the Council of Nicaea in 325 A.D. Besides that, the Julian calendar had Easter sometimes falling before the equinox, which cannot happen. The Gregorian calendar was therefore brought into synch with the heavens, and 10 days had to be dropped. Many people at the time were offended, angered or downright scared, convinced that the change would shorten their lives by 10 days.
The Gregorian calendar is about as accurate as we can get it, but a day will still need to be added every now and then since the earth's orbit varies a few nanoseconds per year. The first one will happen around the year 4082. Don't hold your breath!
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu