Sky Watch: What time is it, anyway?

As one who writes and speaks about upcoming celestial events for national and international audiences, I'm constantly faced with this problem: What time will an astronomical event occur where you live?

It's not as straightforward as one might think. For example, what time is it right now? Most people find that a quick glance at their watch does the trick. For those of us in science, however, the answer often depends on several factors.

We all know that the Earth turns on its axis, and that this rotation is what causes the sun to rise in the east, drift across the sky and set in the west. If the sun appears in our sky, it cannot also be in the sky on the other side of the planet. Modern technology has shown this to be true: We can watch a live news report from the Middle East, for example, where the sky is dark, but outside our windows at home it's broad daylight.

So what time is it anyway? A simple question with no simple answer. And at the beginning of March when many of us are preparing to change our clocks from "standard" to "daylight-saving" time, it can become even more befuddling.

Time is maintained by a number of precise atomic clocks around the world. But this wasn't always so. Back in the nineteenth century, for example, time was purely a local matter. Want to know what time it is? Go check out the clock on the church steeple or local jeweler's shop window. But travel or communicate across greater distances, and you've got a serious problem. This wasn't a huge issue for most, but it became one as technology improved.

So to help keep schedules straight, the railroads in the United States and Canada split the continent into time zones on Nov. 18, 1883. And, though this was an idea not immediately embraced, its practicality soon became clear.

But then there's daylight-saving time. Benjamin Franklin first conceived of this scheme in a 1784 essay, but more than a century passed before it became reality in the United States. On March 19, 1918, U.S. law established the Standard Time Act, which not only set time zones across the country, but also established Daylight-Saving Time (DST) -- a concept that still isn't accepted by all states.

For those that do, however, DST begins this year on March 10 and ends on Saturday, Nov. 2.

As complex as time seems, for astronomers, it's relatively straightforward. By convention, we use one time zone -- that of Greenwich, England. We call this time Coordinated Universal Time or simply UTC. And if you know how many time zones you lie east or west of Greenwich, you can use basic arithmetic to calculate your corresponding local time.

It's not difficult to do, either. Each zone west of Greenwich represents a time of one hour earlier. U.S. Eastern Standard Time, for example, is five hours behind UTC. In other words, UTC minus 5 equals EST. So, if UTC is 11:00 a.m., it's only 6 a.m. in New York. And on the west coast, it's 3 a.m.

So, what time is it?

Well, that all depends. . . .