Christmas has passed, and we are in the midst of our traditional rest period between the spiritually significant solstice celebrations like Christmas and Chanukah, and the more worldly revelries of New Year’s Eve.
The juxtaposition of the two holidays may seem strange until we consider how they got their start.
So let’s examine a few “astronomical” aspects of the two celebrations and the week that separates them.
Jesus of Nazareth was not born during the year we now call 1 CE or AD, as was incorrectly determined by Dionysius Exiguus, a Roman abbot who lived around 550 CE.
Jesus was born between 7 BCE and 4 BCE. References in the Bible determine those years. Augustus sent out his taxation decree in 7 BCE (Luke 2:1), and Herod, the villain in the birth story, was dead by 4 BCE.
In addition, Dec. 25 is not the day Jesus was born. In those days, if you wanted to see shepherds “abiding in the field, keeping watch over their flock by night” (Luke 2:8), you would probably do so in the springtime, not the winter. During the spring, the shepherds slept in the fields to protect newborn lambs from predators.
Early Christians most likely celebrated the birth of Jesus in the winter because they already had Easter, the most important Christian holy day, as a springtime celebration.
Also, they could encourage converts from the Roman population by celebrating Christmas during a significant Roman holiday, late December’s Saturnalia.
The Titan Saturn was associated with the winter harvest. The Saturnalia was a week or so when masters and servants sometimes exchanged places, and the standard social order temporarily broke down. The high and the low feasted and drank heavily together. They also exchanged gifts, as we do to this very day.
The Saturnalia generally lasted a week. We preserve that tradition by bracketing our end-of-year celebration between Christmas and New Year’s Day. Eventually, the end of the old Saturnalia became New Year’s Eve. Christmas took on a life of its own much later, largely thanks to the Yuletide writings of Victorian novelist Charles Dickens.
As New Year’s Eve approaches, I hope you all will find the time to thank an astronomer for stargazing’s contribution to our time-honored and time-cursed measurement of time.
Our year is based loosely on the orbit of Earth around the sun. Our month is loosely (very loosely) based on the moon’s phases. Our day is defined by the rotation of Earth on its axis and the apparent rising and setting of the sun.
The trouble is that none of these astronomical times are evenly divisible into each other. A true “moonth,” a set of lunar phases, lasts 29.53 days. A true solar (or “tropical”) year is 365.2422 days, not 365.25 days. As a result, our calendar is a messy hodgepodge of unequal months and leap days.
Furthermore, even with leap days, the calendar still doesn’t account for that pesky .0078 days in the short term. (More on that later.)
The biggest problem is that time must be artificially sliced into manageable pieces, and we can’t find a satisfactory way to do it.
Ancient peoples started by using the sun’s motion to determine the day and the moon’s phases to determine their “moonth.”
That method worked well enough if the culture wasn’t too complicated, but difficulties arose as people had to arrive at places at precise times and on specific days.
Imagine this scenario: “When does the meeting start?” “I dunno. In a day or two. Whenever everybody gets here.”
Or this one: “When does your plane land?” “I dunno. Sometime on Thursday.”
Society would be doomed.
Unfortunately, the moon takes 29.53 days to cycle through one set of phases. If you’re trying to create a calendar, what do you do with the leftover .53 of a day?
The year was probably a later invention. Since Earth orbits the sun in about 365.25 days, the sun appears to move once around the sky during that time. Unfortunately, the lunar month doesn’t divide evenly into the calendar year. Assuming 12 months in the year, we have about .4 of a month left over. What to do?
By Roman times, the lunar-based system was so messed up that Julius Caesar decided to do what any self-respecting autocrat ought to do. He assembled his wisest and most knowledgeable astronomers.
They decided to dump the lunar calendar in favor of a calendar based on the motion of the sun alone. Because of all the earlier temporal perplexities, what we now call 46 BCE consisted of 445 days divided into 15 months.
This “Year of Confusion,” as it came to be called, was a disastrous year for scheduling chariot payments. Nevertheless, once it was over, the western world settled into a serviceable 365-day calendar with a leap day every four years, at least for a while.
The trouble was that the Julian year was 11 minutes and 14 seconds longer than the solar year. That doesn’t sound like much, but it really adds up over 1,600 years. The Julian calendar gains a full day every 128 years.
By 1582, the first day of spring had moved from its intended date of March 21 to March 11. Eventually, spring would begin in February. Something had to be done — again.
Pope Gregory XIII did what Julius had done 1,600 years earlier. He called in a gaggle of astronomers to revise the calendar. For starters, they suggested dropping ten days from 1582. On Oct. 4, Gregory simply deleted the 10 days. Thus, the next day was — voilà — Oct. 15.
He also declared that century years like the year 2000, which would have been leap years, would not be, except for years divisible by 400, like the year 2000, which therefore was a leap year.
So the year 2100, which is divisible by four but not divisible by 400, will not be a leap year. Please mark your calendar accordingly.
Gregory’s changes took place immediately throughout the Roman Catholic world. Such was the power of the Papacy at the time.
However, England was not a part of the Roman Catholic world. They had recently gone through their version of the Protestant Reformation under Henry VIII.
As a result, his daughter and heir, Elizabeth I, resisted Pope Gregory’s temporal changes. Great Britain and her American colonies continued to let their calendar slide until 1752, sending the spring equinox back to March 10.
During that year, the English-speaking world finally got its calendrical act together. The dates between September 2 and September 14 simply ceased to exist.
Chaos reigned. Workers were none too happy with having to pay their monthly bills with three weekly checks. Angry mobs rioted in the streets, chanting, “Give us back our 11 days!”
Eventually, everything quieted down, but some things had to change. Poor old George Washington had to change the date of his birthday to account for the new calendar. When history teachers mention the specific dates of events before 1752, ask if they are giving the dates in the Julian or Gregorian calendar. That should give them pause.
Those steps brought the average calendar year to 365.2425 days, just .0003 days off the real solar time. That’s durned close, but not close enough for the time-obsessed 21st century.
The current Time Lords have decided that years divisible by 4,000, which would have been leap years under the “400” rule, now won’t be. That brings the average calendar year to 365.24225 days, but you’ll have to wait 1,978 years to set your watch correctly.
If all that isn’t complicated enough for you, then consider the leap second.
Astronomers have traditionally defined one second as 1/86,400 of the time required for Earth to rotate once on its axis. Unfortunately, Earth’s rotation is slowing down, primarily because of the moon’s gravitational pull.
Consequently, the Time Lords occasionally must add a leap second to the day so that our clocks will match Earth’s rotation.
However, atomic clocks can now measure time very precisely. In case you were wondering (ha!), one second equals 9,192,631,770 vibrations of one cesium 133 atom.
Thus, the Time Lords are debating whether or not they should dispense with the leap second entirely. Will the madness never cease?
In the end, time is a continuum. You will find no magic tick marks on Earth’s orbit to mark off the days. Our calendar doesn’t reflect any reality except the vagaries of solar-system motion, and it does an exceedingly poor job of that.
The most significant fact about time is that we have so little of it. Your universe began the moment you were born. It will end the day you die. The real meaning of time is what you do with the irreplaceable moments in between.
Hmm. I feel a New Year’s resolution beginning to form.
Tom Burns is the former director of the Perkins Observatory in Delaware.