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How to set up a horizontal sundial
This page has been translated into Polish
A horizontal sundial consists of the dial plate, marked off in hours, and the gnomon which sits on the noon line and projects out from the dial plate.
In order to tell the correct local time the gnomon must be parallel with the earths axis, or, in other words, that it should point towards the celestial pole. In the northern hemisphere, this means, for practical purposes, that the gnomon should point at the Pole Star. One should first check whether or not the sundial is correctly made for the place at which it is to be set up. If it is not, the base plate of the dial must be corrected so that the gnomon is pointing correctly true north, towards the celestial pole.
Finding the direction of true NorthVarious methods are suggested in the literature, and are summarised here, with references to published sources if you need them
Checking the angle of the gnomonSince horizontal sundials are often mass-produced, they have to be made for just one latitude. Many are made in Birmingham, where the latitude is around 52½ deg.N, so the angle between the gnomon and the dial plate is also 52½ Quite often, people bring back a sundial when they have been on holiday, so the angle may be very different. For example, a sundial made for the south of Spain will have an angle around 37 deg. and will not tell the correct time if it is set up with the dial plate horizontal in Southern England where the latitude is 51 deg. Fortunately, this can be compensated for.
First, measure the angle of the gnomon with a protractor.
Second, you can if you wish cross-check this measurement and check that the hour lines have been laid out correctly, by "back-calculating the gnomon angle from the angles of the hour lines. (The book by Waugh gives an example of this calculation on p.48, and also a table showing the correct angles of the hour lines for each degree of latitude. For example, the angle of the 9am and 3pm hour lines from the noon line is 26º24 at 30ºN, 29º50 at 35ºN, 32º44 at 40ºN, 35º16 at 45ºN, 37º27 at 50ºN, and 39º20 at 55ºN.)
Compensating for an incorrect gnomon angleThird, provide a wedge to bring the gnomon parallel to the earth's axis. For example, the holiday sundial brought back from Spain (lat 37ºN) to be set up in Southern England (lat 51ºN) would have to be wedged up by 14º, so that the gnomon is at 51º to the horizontal. You can either measure this angle with a protractor, or you can calculate the height of the wedge by multiplying the length of the dial plate by the sine of the correction angle. In this case, the wedge required for a square sundial with a side of 10 cm would be 2.4 cm.
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