December 31, 2004 in Nation/World

New Year’s gives astronomy its due

Harry Levins St. Louis Post-Dispatch
 
File/Associated Press photo

The Earth’s trip around the sun isn’t a tidy 365 days, which is how we ended up with the concept of leap years.
(Full-size photo)

ST. LOUIS – When you pop a champagne cork just before midnight tonight, forget about toasting “Auld Lang Syne.” Instead, lift a glass to the heavens, because the New Year’s celebration is really a salute to astronomy.

Among all the divisions of time, only the year and the day are determined by nature – specifically, by the movement of heavenly bodies.

All of the other measurements, from the minute to the millennium, are man-made and arbitrary.

So when the clock strikes 12 tonight, take two sips of New Year’s Day champagne – one for the day and one for the year. It’s the least you can do to honor the occasion.

The heavenly choreography works with graceful regularity.

Earth rotates on its axis once every 24 hours. That rotation gives us the day.

In turn, the rotating Earth orbits the sun every 365 1/4 days. That orbit gives us the year.

Thus, astrophysics solemnly sets the length of the day and the year.

Everything else – everything – is artificial. Consider that:

The second and the minute were devised by the ancient Babylonians.

The hour was likewise invented by the Babylonians, or maybe the Egyptians.

The week was decreed in the Old Testament by the ancient Jews.

The month is a jury-rigged notion – a failed attempt to match the calendar to the phases of the moon.

The decade stems from the simple fact that we’re born with 10 fingers. As a result, we count by tens. Naturally, we subdivide some periods of time by tens – the decade (and its 10-based multiples, the century and the millennium).

But humans had nothing to do with the day or the year, except to note their passing.

Breaking time down

The passage of a day was obvious from the start: Sun rises, sun sets, sun rises again.

Getting a grasp on a year was trickier. But the coming and going of the seasons gave early humans some notion of a year’s length.

And curious humans who studied the stars at dawn or dusk noticed that as the days plodded on, the sun seemed to move eastward among the stars. When it got back to its starting point, a year had passed.

But as society grew more organized, people felt the need to break both the year and the day into smaller chunks.

At first, the smallest piece was the hour. Some sources say the Egyptians divided daylight into 10 equal parts – then stuck an 11th part on for dawn, and a 12th for dusk. And if daylight had 12 parts, so did darkness. Thus, the 24-hour day.

Others credit the Babylonians, who lived between the Tigris and the Euphrates, in today’s Iraq. By this reckoning, the Babylonians divided the daylight course of the sun by 12. Throw in 12 more for night, and you have the 24-hour day.

Why 12 parts instead of 10? In part, because 12 goes evenly into 60 – and the Babylonians counted not by tens but by sixties.

Given our 10 fingers, 60 may seem like an odd system. But it has some advantages. “My guess is that they used 60 because it’s so easy to divide,” says Michael Friedlander, who teachers physics and astronomy at Washington University.

The seven-day week traces all the way back to the Book of Genesis, in which God labors creatively for six days and rests on the seventh. The Book of Exodus puts humans on the same schedule:

“Remember the sabbath day, to keep it holy. Six days shalt thou labor, and do all thy work: But the seventh day is the sabbath of the Lord thy God.”

The month is a trickier piece of work. Throughout time, humans saw the moon wax and wane every 29 1/2 days. So a subdivision of the year into moon-months seemed logical.

Trouble is, the moon’s math fails to mesh with the sun’s. So our system of 30- and 31-day months (plus February’s 28) is an awkward compromise.

Lunar calendars come up short

True, some societies gear their calendar to the moon. “If the moon is important, you need a unit of time that describes the lunar phases,” says Thomas Madden, chairman of the history department at St. Louis University. He adds: “All the calendars in the ancient world were run by the priestly classes.”

Even today, the Islamic calendar is among those geared to the moon, not to the sun. Trouble is, that calendar has only 354 days. So for 11 years out of each 30, the Islamic calendar pumps in an extra day, just to keep things straight.

Still, the date of the Islamic New Year moves backward through the seasons, returning to where it started over 32 1/2 years. Similarly, the Islamic holy month of Ramadan rotates through the seasons.

But around the world, the decade is obvious: 10 fingers, 10 years, one decade.

Even so, nothing in nature runs in decade-long cycles. Although we speak of “the Gay ‘90s” and “the Roaring ‘20s,” Earth and the sun pay no attention.

The decade is an artificial construct, a man-made convenience. So are the century (10 decades) and the millennium (10 centuries).

And in a small way, humans have also added an artificial touch to the year.

The fresh calendar that you’ll hang up Saturday has 365 days, from Jan. 1 to Dec. 31. It’s the Gregorian calendar, worked out in the 1580s at the order of Pope Gregory XIII.

Most years in the Gregorian calendar have 365 days. But nature’s new year – like all of nature’s years – will have 365 1/4 days.

If humans let nature have its way entirely, that fraction would be nettlesome.

In four years, human calendars would have accounted for 1,460 days. But nature would have chalked up 1,461.

In four decades, things would be a full 10 days out of whack. And in four centuries, the calendar would lag more than three months behind the seasons. The calendar would say New Year’s, but tree buds and songbirds would say mid-spring.

Thus, the leap year. Every four years, we add a day to February. That way, we take into account the one-fourth fraction tacked on to the other three years.

To leap or not to leap

It’s all very tidy – too tidy, in fact.

What we describe as a 365 1/4 -day year is actually a year of 365 days, five hours, 48 minutes and 46 seconds. If every fourth year were a leap year, the small difference (11 minutes and 14 seconds) would eventually get bigger and bigger, with the calendar eventually outpacing the sun.

So every four centuries, we drop three leap years. Most years that end in “00” – 1700, 1800 and 1900 – should be leap years but are not.

Unless they’re evenly divisible by four. Thus, 1600 was a leap year. So was 2000. And so will 2400 be.

That tinkering aside, nature runs the show for New Year’s Day.

So if tonight’s imbibing dizzies you, blame it on nature’s spins – all that rotating, all that orbiting.


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