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SUNDIAL SECTIONS: Developing and Making Sundials Interesting Sundials of the World
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Declination Lines of Sundials
It is obvious, that in all types of sundials, the size, shape, and all the parts must be known, before the lines of declination can be drawn. The equatorial sundial may be drawn on both faces. In the example it is assumed that only the upper or north face is to be used. Since the plane of the equatorial sundial lies in the plane of the celestial equator, it is evident that all the lines of declination cannot be placed upon it. When the sun has a declination of 0° the shadow of the nodus will not fall upon the sundial; and when the sun is south of the equator no shadow will be cast on the upper or north face. If the location of the shadow of the nodus is marked when it reaches each hour line throughout any particular day and a line drawn through those points, a portion of a circle would result, with the foot of the perpendicular style as its center. For this reason it is much easier to draw the lines on an equatorial sundial, than on other types. The Construction AB represents the plane of the horizon: CD the sundial plate; N the nodus; MN the height of the perpendicular style; and M the foot of the perpendicular style. For the Horizontal Line: Through N draw EF parallel to AB, intersecting CD at P (angle PNM is equal to the latitude of the place). Take the distance MP and lay it off from the foot of the perpendicular style M to Q, on the 12 o'clock line. Through Q draw the line PR perpendicular to the 12 o'clock line. Then, PR will be the horizontal line for this sundial. For the Lines of Declination: Since the equinoxes cannot be shown on the equatorial sundial, the line of declination for April: 19 will be substituted. From the table the greatest northern declination is found to be 23° 27', on June 21; and 6'N' on April 19. Then, draw the line GH parallel to CD. This represents the plane of the celestial equator. With a protractor lay off the angle GNV 11°6'N; and the angle GNT = 230271N. Produce the lines VN and TN until they cut the sundial plate, as at X and U. Take the distances MU and MX and lay them off from the foot of the perpendicular style, to T and W, respectively. With M as a center and radii MT and MW, describe the arcs STU and VWX, respectively. Thus will the arcs STU and VWX be the desired lines of declination. For all other lines of declination repeat the work precisely as shown. Sunrise and Sunset By means of the horizontal line and the lines of declination it is possible to tell at what time the sun rises or sets.
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This section shows the construction of the lines of declination. Rather than clutter the reader's mind with many diagrams and lines, only those necessary for a proper understanding of the method have been used. Although the fundamental principle of plotting a line of declination on a sundial plate is the same for all sundials, each type of sundial will be treated separately, so that the reader will have no difficulty. In addition, the horizontal line for each type is shown. Each example shows the construction of the lines representing the path of the shadow cast by the nodus when the sun has a declination of 0°, and when the sun reaches its greatest northern and southern declination. The first is often referred to as the equinoctial line, because the shadow of the nodus falls upon it when the sun is at the equinoxes, marking the beginning of spring and fall, when the day and night are said to be of equal length. The lines showing the sun's greatest northern and southern declination were called the Tropics; and on old sundials they were often labeled the Tropic of Cancer and Capricorn, respectively. They note the longest day (beginning of summer) and the shortest day (beginning of winter) of the year. These lines may also be referred to as limiting lines, for between them all other lines of declination must fall.Copyright © 2005-2006 The Sundial Store. All Rights Reserved. |