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Sundials measure time as it is. Watches measure time as we would like it to be, because it is both easier and more convenient.
Sundials are based on the idea of measuring time before and after noon. Noon is when the sun is at its highest in the sky. People tend to assume that the time between noon one day and noon the next is exactly 24 hours.In fact, the length of a day varies slightly throughout the year. The shortest days are some 23 hours, 59 minutes and 40 seconds long and occur around 15 September, while the longest days around Christmas are some 24 hours and 30 seconds long. The reasons for these variations are complex, and are explained in details in our companion website, Sundials on the Internet, on the Equation of Time page).
Watches are based on the convenient assumption that all days in the year are exactly 24 hours long. Sundials take the days as they are, varying in length from 24 hours and 30 seconds on December 22 and 23 hours 59 minutes and 40 seconds on 15 September as stated above.
1. Find out the longitude of the place where the sundial will
be installed You can do this from any good atlas - longitude
lines run from north to south on the atlas. Many atlases give the
latitude and longitude against the place names in the index. The
latitude and longitude obtained in this way will not be the exact
latitude and longitude of your sundial, although it will usually
be good enough for these purposes unless you live in a large city.
2. Find out the longitude of your standard meridian The world is divided into time zones 15 deg. apart, measured from longitude 0 in Greenwich, England. Thus, the standard meridian for the United Kingdom, Ireland and Portugal is the prime meridian of 0 deg, while the rest of continental Europe keeps European Time, for which the standard meridian is 15 deg. E of Greenwich (which passes through Prague, the capital of the Czech Republic). The time zones of North America are:
3. Calculate the difference between your longitude and the prime meridian (e.g 4 deg 10 secs West for Plymouth), Subtract the longitude of the prime meridian of your time zone (eg 0 for Greenwich). Since the sun takes 1 hour to traverse 15 degrees, the sun crosses the longitude of Plymouth later than it is at Greenwich. It will be 4 minutes later for each degree of longitude, and 4 seconds later for each minute of longitude. So 4 deg 10 secs means that solar noon in Plymouth will be 16 minutes and 40 seconds later than it is at Greenwich.
This example and three others are given as worked examples below. + indicates that the place is West of its prime meridian (and thus the sun is overhead later than it is at the prime meridian); - indicates that the place is east of its prime meridian, and thus the sun is overhead earlier than it is at the prime meridian)
4. Look up the time of solar noon at your prime meridian in the table below
Time of solar noon at the prime meridian
5. Calculate the time of solar noon at your location (if you think it will be sunny that day at the time of solar noon).
For example, on 11th March, solar noon is at 12:10:18 at the prime meridian, so solar noon at Plymouth is 12:10:18 plus 16:40 which gives 12:28:50 by your watch. Similarly, solar noon at Barcelona is 12:10:18 - 51:26 which gives 11:19:52 on your watch. The calculation is tabulated below..
In the last line of the table above, the times are noted as "apparently" fast or slow because it is the watch which is keeping a purely artificial time, and the sun which is continuing in its course as it has done for millennia. As a practical day-to-day matter, however, it may be helpful to think of the sundial being, say, just about 27 mins. slow in Plymouth on 11th March. Then we know that at 9:27 by our watch, the sundial will be reading 9 am exactly, at 10:27 by our watch it will read 10 am exactly, and so on.
5. If your country operates Daylight Saving Time or Summer Time when all clocks are altered to read an hour ahead of the standard time, add 1 hour to your calculated time of solar noon.
For example, on 11th August, solar noon occurs at 12:03:13.at the prime meridian, so solar noon at Plymouth is 12:03:13 plus 16:40 plus 1 hour which gives 1:19:53 on your watch. Similarly, solar noon at Barcelona is 12:03:13 - 51:26 plus 1 hour which gives 12:11:47 on your watch
6. Set your watch accurately by a radio time signal.
7. Set up your Spot-On Sundial. At the exact time of solar noon which you have calculated, rotate the sundial on the baseplate until the ray of sunlight falling between through the slot in the gnomon is exactly over the dotted noon line.
Note that the Spot-On Sundial has the hours marked in two ways. Roman numerals indicate the winter hours, and Arabic numerals indicate Daylight Saving Time.
You may find it more convenient to set the Sundial at some other time than solar noon. It is nearly as accurate to set the shadow when it is exactly on one of the hour lines, using the number of minutes apparently fast or slow from your calculations to know the corresponding time on your watch.
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