Noon today is REALLY noon
Dennis Barnes is setting up a beautiful instrument in his garden in Abingdon. It's called an armillary Galileo sundial. He wrote to me yesterday because he is preparing to "set" the sundial today, taking advantage of the fact that today - April 15 - is one of only four days on the annual calendar when solar noon - the moment when the sun is highest overhead - is the same as noon according to "mean solar time," or clock time.
To astronomers, it's the day when the "equation of time" equals zero. At other times of the year, the sun can be as much as 14 minutes "fast" or 16 minutes "slow" relative to clock time.
But of course this is astronomy, so nothing is as simple as we'd like.
Here's Dennis's problem: First, the equation of time (the difference between mean sun time and clock time) is only zero today along the "standard meridian" in each time zone. For us here in Eastern Time, that's 75 degrees west longitude, which runs north and south just off the beaches at Ocean City. So, all of Maryland is actually west of the standard meridian. Solar noon reaches us late as the sun moves across the sky from east to west.
Still with me? Dennis was aware of the problem, but he wasn't sure how far west of 75 degrees he is, or how that would affect his "local solar noon." I took the question to Geoff Chester, at the U.S. Naval Observatory in Washington. He told me that the sun is about 4 minutes "late" for every degree of longitude west of the standard meridian.
Dennis used Google Earth to determine that his garden is at 76 degrees, 19 minutes and 45 seconds west longitude. So, Geoff did the math and reported that solar noon will reach Abingdon at 5 minutes 19 seconds after 12 noon Eastern Standard Time. But we have to correct for Daylight Time, pushing solar noon over Dennis's sundial to 1:05:19 p.m. EDT
But wait! There's more.
Since this is a Leap Year, the sun on the 15th is actually 5 seconds later than it would be on a non-Leap Year. So solar noon in Dennis's garden will actually occur at 1:05:24 p.m. EDT, according to Geoff's calculations. That's Dennis's new sundial above, photographed at precisely 1:05:24 p.m. Tuesday. You can see the arrow's shadow, smack on the XII.
All of this is, of course, way too fussy for setting a sundial. But it points out the quirks of timekeeping owing to irregularities in the Earth's tilt and orbit that astronomers have noted for thousands of years.
For the record, the equation of time will be "zero" again - bringing sun time on the standard meridians into step with clock time - on June 13, Sept. 1 and Dec. 25.
(Photo by Dennis Barnes, used with permission)
Comments
Why in April? The other dates are approx 3 months apart (as expected when considering 4 times during the year), but April is only 2 months prior to June and 4 months after December.
FR: It's because the Earth's orbit around the sun is not circular, but elliptical, so its speed around the sun varies during the year; and because of the inclination of the Earth's axis to the plane of the solar system. Both help to skew these dates. I don't pretend to have mastered the subject, but I'm learning. You can read more about it here: http://en.wikipedia.org/wiki/Equation_of_time
Posted by: mapuser | April 15, 2008 8:42 AM
Hey, this was cool! I learned something new today, even if it is a little obscure. And this is the type of info that tends to "stick" in my brain (unlike people's names, which must be made of mental teflon), so I can "amuse" others with it at some indefinate point in the future...
Posted by: Corine | April 15, 2008 10:07 AM
Nice Ticks! or Tocks.....
Posted by: Jay in Canton | April 16, 2008 6:49 AM