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The George Washington Sundial
at Second Presbyterian Church
Lexington, Kentucky
Our clocks differ from sun time due to 3 effects; two due to our own choices and one due to nature:
- +38 minute position in the time zone effect:
Our clocks are set to agree with clocks at the center of our standard time zone instead of agreeing with the sun. Philadelphia is near the center of the Eastern Time Zone - we call it noon here when the sun is overhead in Philadelphia - but the sun arrives overhead here 38 minutes later. So we must set our clocks ahead of the sun by 38 minutes to put them on Eastern Standard Time. (We are actually closer to St. Louis, near the center of the Central Time Zone - if we went with that time zone, we would set our clocks just 22 minutes behind the sun instead of 38 minutes ahead.)
- +60 minute daylight savings time effect:
From mid March to early November we observe Daylight Savings Time - then we set our clocks another 60 minutes ahead. This makes sunsets seem to occur later, according to our clocks.
- +/-16 minute sun fast or slow effect:
The speed of the apparent east-to-west motion of the sun around the earth is not quite constant due to the tilt of the earth's axis and the eccentricity of the earth's orbit. The length of the solar day (the time from one solar noon to the next) is sometimes more and sometimes less than 24 hours. As a result, the variable-speed sun gets ahead or behind our constant-speed clocks by up to 16 minutes.
The sum of these 3 effects is always positive here in Lexington, so our
clocks are always ahead of our sundials; way ahead during DST. Clocks are
ahead by:
as little as 22 minutes (38-16) in early November,
as much as 52 minutes (38+14) in mid February, and
as much as 1 hour and 48 minutes (38+60+10) in mid March with
DST in effect.
The dial at 12:51, sun time.
Our sundial is a copy of the one George Washington had made for him and placed in the center of the circular courtyard in front of his Mount Vernon home sometime before 1785. The badly worn original dial is now kept inside the house at Mount Vernon but a copy like ours has replaced it in the courtyard. The copies are made of cast bronze in fine detail by Virginia Metalcrafters. Since the latitude of Lexington (38.0o North) is nearly the same as Mount Vernon (38.7o North), the dial designed for George serves very well for us too.
The dial, with two-minute tick marks, is readable and accurate to about one minute. Read the time at the edge of the shadow that is cast by the top of the gnomon (the edge of the shadow away from the XII mark). Our dial is in the shadow of the church until around noon. The gnomon makes an angle of 38o from horizontal and points north so it is parallel to the earth's axis. As the earth spins, the gnomon keeps pointing in the same direction - toward the North Star. The streets in downtown Lexington don't run N-S and E-W, they are about 45o off; so our dial appears to be oriented at a diagonal in our courtyard.
Our dial is a classical horizontal sundial - as such, it reads solar time instead of clock time. What good is such a timepiece? Think of it as a true natural clock that tells us what time it really is, here where we are, instead of the adjusted time we adopt to accommodate our constant-speed clocks and our wish to synchronize with folks in other places. If you want to know clock time you probably already have that available on your watch anyway. You can check your watch using the sun: just add the amount given above to the sundial reading.
This chart shows the separate contributions to the correction for clock time throughout the year:
- The red line is the amount due to the tilt of the earth (up to +/-10 minutes). The amount is 0 at the equinox on March 20 and again at the beginning of each of the other three seasons.
- The blue line is the amount due to the eccentric orbit of the earth (up to +/- 7 minutes). The amount is 0 on January 2, when the earth is closest to the sun and again in July when the earth is farthest from the sun.
- The yellow line is the amount due to our position in the time zone (always 38 minutes).
- The orange line is the amount due to daylight savings time (60 minutes in the summer).
- The heavy black line is the total amount.
(For an excellent explanation of the effects of the earth's tilt and elliptical orbit, visit analemma.com.)
- Second Presbyterian Church is located at:
- Latitude: 38.0o North.
- Our dial's gnomon must point North, 38o above the horizon, and the dial's hour lines must be calculated and laid out especially for a dial to be used at 38o.
- Longitude: 84.5o West.
- Standard time zones are centered every 15o around the world; Eastern at 75o, Central at 90o. We are 9.5o west of the 75o center. The sun moves 15o every hour, or 1o every 4 minutes, so our EST clocks are (4 minutes/degree) x (9.5 degrees) = 38 minutes ahead of the sun.
After a decade of controversy and confusion, Lexington officially dropped "sun time" and adopted "standard time" ("railroad time") in 1896. Official clocks were set back 22 minutes, putting us on Central Standard Time. Church clocks were often slow to conform, preferring to keep "God's time" instead (and our love of sundials is no doubt a reflection that same feeling). In 1960, Lexington, along with central and eastern Kentucky, switched from Central to Eastern Standard Time ("fast time").
On the fourth day of creation, "God said 'Let there be lights in the dome of the sky to separate the day from the night; and let them be for signs and for seasons and for days and years' ... two great lights ... and the stars." Accordingly, our calendar is related mainly to the motion of the "greater light", the sun. We have dropped most of our calendar's relation to the "lesser light", the moon, except that our months are close to the length of the moon's cycle. Any relation to the stars has practically vanished from our lives. We still base one significant calendar "sign" however on both the sun and the moon: we celebrate Easter on the Sunday after the first full moon after the first day of spring. In 2010, the sun rises directly east on Saturday, March 20; the next full moon is on Monday, March 29; and we celebrate Easter on the next Sunday, April 4. See the Calendar.
In 2008, the sun rose directly east on Thursday, March 20, the moon was full the very next day, Friday, March 21, and Sunday followed just two days later, on March 23. So Easter came very early in 2008: March 23. It hadn't been that early since 1913 and won't be that early again until 2160. Easter could possibly come even one day earlier, on a March 22 (when Spring, the full moon, and a Sunday fall on 3 days in a row - March 20, 21 and 22), but that hasn't happened since 1818 and won't happen again until 2285. Easter 2008 was the earliest one most of us get to see!
Sundial Dedication Service - May 22, 2005