The winter solstice, a split second on Earth’s analog clock

December 22, 2011

At the winter solstice the sun is a layabout, so late to get rolling that it misses most of the rush hour. It hangs low in the sky, lurking behind bare trees. Only for a few hours at midday is the sun high enough to survey the world it supposedly warms. Then it retreats below the treetops, declares that it’s beer-thirty and calls it a day.

This winter solstice is a single moment in time, marking the moment when it is officially safe to utter the phrase “obliquity of the ecliptic.” It occurred at 12:30 a.m. Eastern Thursday.

The weather changed right on schedule. Wednesday’s ocean of gloom had drained away by Thursday. The first day of astronomical winter was unreasonably warm, dawning with a clear sky etched horizon to horizon in vapor trails. It looked like an air-route map in the back of an inflight magazine.

We will emerge from the solar recession rather slowly — not until Jan. 25 will we have a full 10 hours of sunlight in Washington. Winter is not our best season. This part of the world has too much sleet, freezing rain, “wintry mix.” But there will be fine days, too, when frozen ground greets legitimate snow. It is a striking season visually, with the landscape starker, the architecture more vivid. The vegetation vanishes and the surface reveals its secrets, the old chimneys and foundations. The clear night sky explodes in big, young, blue stars.

The Earth is a giant clock moving through space. The spring inside was wound up billions of years ago. The planet spins, and simultaneously falls around a star. It is an old star, stuck in its ways, fortuitously dull, and residing in a galactic cul-de-sac where not much happens. That’s a desirable location in a universe where things explode, collide, collapse into black holes, etc.

The galaxy itself moves, as does the whole galactic neighborhood – everything going somewhere. The entire universe is expanding. Change is a cosmic imperative. The clock can’t stop.

The problem with the Earth-clock is that it’s analog, not digital. This is a conundrum for an increasingly technological, wired civilization. Time can be measured very precisely with atomic clocks. A second is defined internationally as 9,192,631,770 oscillations of a cesium atom. Whereas the Earth, spinning its way through space, seems kind of old-fashioned and Pontiac-like by comparison.

And it is wobblier than you think. Listen to Brian Luzum, astronomer at the U.S. Naval Observatory and head of the Earth Orientation Department.

“There are five components that we measure,” he says. “There are two polar-motion components, there is one earth-rotation component, and then there are two precession-nutation components.”

Precession mutation?

“Nutation. ‘N.’ Nancy.”

If there were a long, straight rod through the axis of the Earth, from the Antarctic to the Arctic, it wouldn’t stick out at the geographically designated North Pole, but from a spot hundreds of yards away. That spot would wander over time. As it moved, we’d notice that the earth is wobbling on its axis the way a spinning top wobbles.

Meanwhile, although a day is very precisely defined by civil authorities as 86,400 seconds, the astronomical day is not so exact. There are “solid earth tides” that make the earth spin slightly faster or slower on any given day — a millisecond here, a millisecond there.

To understand where the Earth is in space and how it is moving at any given moment, astronomers around the world aim radio telescopes at quasars — very bright objects billions of light-years away — and feed the observations into computers that chew the data and spit out something that looks like a fact.

But even if we understand where the Earth is, the fact remains that its rotation is slowing down. A day, astronomically measured, is now about 2 milliseconds longer than it was a century ago. The atomic clocks were calibrated in the 1950s. Universal Coordinated Time is based on an average among atomic clocks (because no two will ever agree).

That means that, for decades, a day according to UTC has been just a bit shorter than a day according to UT1 — astronomical time. In the course of a couple of years, UTC and UT1 can become off by a full second.

“This really plays havoc when you’re trying to synchronize large-scale computer systems,” says Naval Observatory spokesman Geoff Chester.

The international community has repeatedly tried to fix the divergence with the insertion of “leap seconds.” The authorities in charge of time have added 24 leap seconds since 1972. In January they will meet to decide whether to do away with the leap second altogether. It’s become a burden, keeping the time of the universe synchronized with the time of human civilization.

If the two time systems are allowed to diverge, eventually the sun will reach its zenith in the sky at 1 p.m. instead of “high noon.”

“It’ll be like daylight savings time,” says retired Naval Observatory Director of Time Dennis McCarthy.

It’s enough to make your head spin as fast as the Earth itself. And we haven’t even started talking about Calendar Reform.

Joel Achenbach writes on science and politics for the Post's national desk and on the "Achenblog."
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