For the week ending February 5, 2011

Sundials aren’t very accurate clocks by today’s standards. They were good enough when there were no other clocks to synchronize to them and while mechanical clocks weren’t so accurate either.

Today’s clocks are much more regular than a sundial. A quartz controlled timepiece, even when off a little, is off by a regular, predictable amount. Maybe your digital watch gains six seconds every week. That’s every week. A sundial is fast sometimes and slow sometimes.

We all know a solar day lasts 24 hours. So we expect exactly 24 hours to pass from high noon to high noon. Trouble is, we’ve been misled. Most days, it’s a few seconds more or less than 24 hours and that changes throughout the year. Those seconds add up to the sun usually being several minutes fast or slow compared to manufactured timepieces.

From February 1 to February 22 this year, the sun is 14 minutes behind. If you’re on the meridian for your time zone, the sun will be at its highest point at 12:14pm. That’s as late as the sun gets. It’ll catch up and be right on time around April 15. A month later, the sun will run a little fast by about four minutes. In mid-June, it will seem to have adjusted itself to be correct again.

There’s a back-and-forth adjustment like that to be made nearly all year if you want to tell time by sundial. In late October and early November, high noon will be at 11:44am. That’s the fastest the sundial runs.

There are more adjustments to be made for not being on the time zone meridian and for the summer time shifts used in many countries. For every degree east of the meridian, your local time will be four minutes ahead of time zone time. Likewise, being west of the meridian means being behind the time zone time. Daylight Saving Time, as it’s known in the United States, is a one-hour adjustment. Those are easy to remember.

The day-to-day adjustment for the sun seeming to run fast or slow is called the equation of time. It’s predictable, recurring in a one-year cycle. Just like the sun rising and setting at the same time a year from today, the time of high noon will also be the same (unless you change locations). That one-year cycle, you can probably guess, has everything to do with where the earth is in its slightly elliptical orbit around the sun. If the earth’s orbit were a perfect circle, sundials would be accurate every day.

We have many reasons for not telling time by sundials anymore, besides needing to know the time at night and on cloudy days. Standard time zones are a necessity for conducting business across many miles. If it’s 5:00 local time for me and you’re 350 miles to the west, to you it’s about 4:30. I could be wondering why you didn’t call me at 4:45 while you’re thinking it’s not time to call yet. A conference call would be real tough to schedule.