By Tom Burns
December 23, 2013
When the ball drops on Dec. 31, time will be on our collective mind.
Astronomers can measure these temporal milestones with incredible accuracy, of course. In fact, there’s this guy at the Naval Observatory who is in charge of adding or subtracting a leap second every year based on tiny perturbations of Earth’s orbit.
All you time aficionados will probably note that the ball will drop a few seconds too late or too early. It always does.
In fact, astronomers invented time, or at least our way of measuring its passage. Time and astronomy are thus inextricably intertwined, much to the detriment of time, I am sorry to say. The year, for example, is determined by a single orbit of the Earth around the sun, about 365.25 days.
The “moonth, ” or “month” as you Earthlings call it, was originally measured by the orbit of the moon around the Earth. A set of lunar phases, the ancient way of determining a month, takes about 29.5305882 days. The trouble comes because 29.5305882 days doesn’t divide evenly into the 365.25-day year. We simply can’t have months of equal days. Also, what do we do with that pesky .25 of a day when we measure out the year on a calendar?
Adding to the mind-numbing complexity, Earth’s orbit is a seamless path. There are no convenient tick marks to tell us when the new year begins. Why then do we begin the year in the middle of the blessed winter? Time is a kluge. We do the best we can.
The logical time to begin the calendar year is on the first day of spring, when the world begins to come alive again after its winter dormancy. In fact, the ancient Romans, from whom we get our basic calendar, began the year in March.
The Roman calendar was only 355 days long, in other words, 12 months of, on average, 29.58 days each — pretty close if all you care about is the length of the month. Unfortunately, that left the problem of that 365-day solar year.
Thus, the priest-astronomers who were in charge of the calendar periodically added an extra month after February 23. The trouble was that they often did so for political reasons. If some favored bureaucrat wanted to lengthen his term in office, the priests would add a month.
Tom Burns is the director of Perkins Observatory. He can be reached at firstname.lastname@example.org.
By 46 B.C., the calendar year was a couple of months out of sync with the solar year. Their month of March was happening during the dead of winter in the month we call January. At the advice of astronomers, Julius Caesar, the current Roman dictator, added 67 days to November that year, essentially stopping the motion of the calendar for two months. Voila! The year now began in January, but at least spring began in March again.
To eliminate future problems, the wise Julius standardized the year at 365.25 days, adding a leap day every four years. Unfortunately, the solar year is actually 365.242199 days. Julius’s year was 11 minutes and 14 seconds too long.
The extra 11 minutes may not seem like much, but they add up over the centuries. By 1582, the first day of spring was 10 days removed from the traditional date of March 21.
So Gregory, the Pope at the time, decreed that years that begin centuries, like 1900, would not be leap years. Exceptions would be made for years divisible by 400. Thus, the year 2000 WAS a leap year.
In addition, Gregory removed 10 days from October 1582 to bring the calendar back into sync with the seasons, a decision that caused considerable consternation, especially among folks who had to pay a full month’s rent on their land for an October that lasted only 21 days.
Recall, these problems happened because Earth doesn’t take an even number of days to orbit the sun, and the moon takes an uneven number of days to orbit the Earth. The way we measure time is a constant reminder that we live on a planet in space orbiting a star.
Please remember those cosmic facts when the ball drops a few seconds too late on Dec. 31. Things could be worse.