What Time Is It Anyhow?
I brought a sundial to the mountains when we sold the Big House in Cary and finally got around to locating it in the iris garden by the driveway.
Once installed, I twisted the timepiece until the reading matched my watch. Trouble was that every time I took a look at it thereafter it gave the wrong time ... except at the time of day I had set it.
Now I knew that folks have been using sundials for quite some time and would not tolerate one as inaccurate as this one. Here, in fact, is the sundial on the side of a building in Hochstad, Germany, the town from which my ancestors immigrated to America in 1723.
It looks quite a bit different and goes from eleven o'clock in the morning until ten o'clock in the evening (they are a bit north of us), but it is the town's sundial. So it was that I was obliged to make a small study of sundials.
Seems that sundials have no understanding whatsoever for daylight savings time. Accordingly, you can either subtract an hour from the reading or physically rotate the numerals (but not the sundial itself) by an hour. Daylight savings time being a relative new invention, sundials typically don't have mechanical adjustments.
Then comes the problem that sundials don't understand time zones. They give local time based on the sun being directly overhead at high noon and unless you are located at some longitude that is a multiple of 15° (the earth rotates 360° in 24 hours or 15° per hour), your sundial is going to be slow by the fraction of an hour (fraction of 15°) you are west of the multiple. In the case of Air Bellows, located at a longitude of 81.15°W, we are 6.15° west of 75°W (5 X 15°), making the local time sundials here read perpetually 25 minutes behind Eastern Standard Time or an hour and 25 minutes behind Eastern Daylight Savings Time. (The 0° longitude goes through Greenwich, England, thus giving us Greenwich Mean Time, the stardard throughout the world that all airplanes, for example, use to avoid crashing into one an other. EST is 5 hours behind GMT.)
Then there is the problem with the sundial's style, the edge of the part that causes the shadow to appear on the dial. To be perfectly accurate, it must be aligned with the earth's north-south axis (point toward true north) and be perpendicular to the sun's rays. Provided the sundial has been properly oriented toward the north and the edge is inclined at an angle equal to the earth's latitude where you are located (36.4°N for Air Bellows), this condition will be met only four times each solar year (365.25636042 solar days). Because of the earth's wobble about its north-south axis that causes summer and winter and its non-circular (elliptical) orbit about the sun, local time can be a fraction of another 17 minutes fast or 14 minutes slow, depending upon the time of year. This error can be corrected by several means, including the use of a single-hour "noon" sundial to determine the deviation once a day. Or, it can be calculated as shown here.
All this has gotten me interested in sundials and I now have a new project to "sculpt" a few of them for which there are dozens of interesting fundamental designs. Creating the dials themselves is an interesting process in itself and can be done either empirically (marking shadows at intervals of time) or numerically using equations involving transcendental functions (sin, cos, tan, arctan, etc.) And, of course, each and every sundial must be designed for the particular spot on earth (both longitude and latitude) where it will be used. Unlike a wrist watch which can be correct by a simple adjustment of the hour hand as it moves around the world, sundials don't travel very well.
Once installed, I twisted the timepiece until the reading matched my watch. Trouble was that every time I took a look at it thereafter it gave the wrong time ... except at the time of day I had set it.
Now I knew that folks have been using sundials for quite some time and would not tolerate one as inaccurate as this one. Here, in fact, is the sundial on the side of a building in Hochstad, Germany, the town from which my ancestors immigrated to America in 1723.
It looks quite a bit different and goes from eleven o'clock in the morning until ten o'clock in the evening (they are a bit north of us), but it is the town's sundial. So it was that I was obliged to make a small study of sundials.
Seems that sundials have no understanding whatsoever for daylight savings time. Accordingly, you can either subtract an hour from the reading or physically rotate the numerals (but not the sundial itself) by an hour. Daylight savings time being a relative new invention, sundials typically don't have mechanical adjustments.
Then comes the problem that sundials don't understand time zones. They give local time based on the sun being directly overhead at high noon and unless you are located at some longitude that is a multiple of 15° (the earth rotates 360° in 24 hours or 15° per hour), your sundial is going to be slow by the fraction of an hour (fraction of 15°) you are west of the multiple. In the case of Air Bellows, located at a longitude of 81.15°W, we are 6.15° west of 75°W (5 X 15°), making the local time sundials here read perpetually 25 minutes behind Eastern Standard Time or an hour and 25 minutes behind Eastern Daylight Savings Time. (The 0° longitude goes through Greenwich, England, thus giving us Greenwich Mean Time, the stardard throughout the world that all airplanes, for example, use to avoid crashing into one an other. EST is 5 hours behind GMT.)
Then there is the problem with the sundial's style, the edge of the part that causes the shadow to appear on the dial. To be perfectly accurate, it must be aligned with the earth's north-south axis (point toward true north) and be perpendicular to the sun's rays. Provided the sundial has been properly oriented toward the north and the edge is inclined at an angle equal to the earth's latitude where you are located (36.4°N for Air Bellows), this condition will be met only four times each solar year (365.25636042 solar days). Because of the earth's wobble about its north-south axis that causes summer and winter and its non-circular (elliptical) orbit about the sun, local time can be a fraction of another 17 minutes fast or 14 minutes slow, depending upon the time of year. This error can be corrected by several means, including the use of a single-hour "noon" sundial to determine the deviation once a day. Or, it can be calculated as shown here.
All this has gotten me interested in sundials and I now have a new project to "sculpt" a few of them for which there are dozens of interesting fundamental designs. Creating the dials themselves is an interesting process in itself and can be done either empirically (marking shadows at intervals of time) or numerically using equations involving transcendental functions (sin, cos, tan, arctan, etc.) And, of course, each and every sundial must be designed for the particular spot on earth (both longitude and latitude) where it will be used. Unlike a wrist watch which can be correct by a simple adjustment of the hour hand as it moves around the world, sundials don't travel very well.
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