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How to Set a Sundial

The sundial is an instrument for measuring time by using the shadow of the sun. They were quite common in ancient times before clocks and watches were invented. At the present time they are used more as an ornamentation than as a means of measuring time, although they are quite accurate if properly constructed.
There are several different designs of sundials, but the most common, and the one we shall describe in this article, is the horizontal dial. It consists of a flat circular table, placed firmly on a solid pedestal and having a triangular plate of metal, Fig. 1, called the gnomon, rising from its center and inclined toward the meridian line of the dial at an angle equal to the latitude of the place where the dial is to be used.
The shadow of the edge of the triangular plate moves around the northern part of the dial from morning to afternoon, and thus supplies a rough measurement of the hour of the day.
The style or gnomon, as it always equals the latitude of the place, can be laid out as follows: Draw a line AB, Fig. 1, 5 in. long and at the one end erect a perpendicular BC, the height of which is taken from table No. 1. It may be necessary to interpolate for a given latitude, as for example, lat. 41 degrees-30'. From table No. 1 lat. 42 degrees is 4.5 in. and for lat. 40 degrees, the next smallest, it is 4.2 in.
Their difference is .3 in. for 2 degrees, and for 1 degrees it would be .15 in. For 30' it would be 1/2 of 1 degrees or .075 in. All added to the lesser or 40 degrees, we have 4.2+.15+.075 in.= 4.42 in. as the height of the line BC for lat. 41 degrees-30'. If you have a table of natural functions, the height of the line BC, or the style, is the base (5 in. in this case) times the tangent of the degree of latitude. Draw the line AD, and the angle BAD is the correct angle for the style for the given latitude. Its thickness, if of metal, may be conveniently from 1/8 to 1/4 in. ; or if of stone, an inch or two, or more, according to the size of the dial. Usually for neatness of appearance the back of the style is hollowed as shown.
The upper edges which cast the shadows must be sharp and straight, and for this size dial (10 in. in diameter) they should be about 7-1/2 in. long.

Details of Dial

TABLE No. 1.
Height of stile in inches for a 5in. base, for various latitudes

To layout the hour circle, draw two parallel lines AB and CD, Fig. 2, which will represent the base in length and thickness. Draw two semi-circles, using the points A and C as centers, with a radius of 5 in. The points of intersection with the lines AB and CD will be the 12 o'clock marks. A line EF drawn through the points A and C, and perpendicular to the base or style, and intersecting the semicircles, gives the 6 o'clock points. The point marked X is to be used as the center of the dial. The intermediate hour and half-hour lines can be plotted by using table No. 2 for given latitudes, placing them to the right or left of the 12-o'clock points. For latitudes not given, interpolate in the same manner as for the height of the style. The 1/4-hour and the 5 and 10-minute divisions may be spaced with the' eye or they may be computed.

Table No. 2.
Chords in inches for a 10 in. circle Sundial.

When placing the dial in position, care must be taken to get it perfectly level and have the style at right angles to the dial face, with its sloping side pointing to the North Pole. An ordinary compass, after allowing for the declination, will enable one to set the dial, or it may be set by placing it as near north and south as one may judge and comparing with a watch set at standard time. The dial time and the watch time should agree after the watch has been corrected for the equation of time from table No. 3, and for the difference between standard and local time, changing the position of the dial until an agreement is reached. Sun time and standard time agree only four times a year, April 16, June 15, Sept. 2 and Dec. 25, and on these dates the dial needs no correction. The corrections for the various days of the month can be taken from Table 3. The + means that the clock is faster, and the means that the dial is faster than the sun. Still another correction must be made which is constant for each given locality. Standard time is the correct time for longitude 750 New York, 900 Chicago, 1050 Denver and 1200 for San Francisco. Ascertain in degrees of longitude how far your dial is east or west of the nearest standard meridian and divide this by 15, reducing the answer to minutes and seconds, which will be the correction in minutes and seconds of time. If the dial is east of the meridian chosen, then the watch is slower; if west, it will be faster. This correction can be added to the values in table No. 3, making each value slower when it is east of the standard meridian and faster when it is west.

The style or gnomon with its base can be made in cement and set on a cement pedestal which has sufficient base placed in the ground to make it solid.

The design of the sundial is left to the ingenuity of the maker.

Table No. 3
Corrections in minutes to change.
Sun time to local mean time,- add those marked + subtract those
Marked - from Sundial lime.

—Contributed by J. E. Mitchell, Sioux City, Iowa.

Excerpt from the book:

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