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Declination Lines of Sundials

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DECLINATION LINES OF SUNDIALS - Page 6

Note that the Tropic of Cancer, which is the line nearest the center of the horizontal sundial, is farthest from the center of the south vertical sundial. It is much easier to visualize the position of these lines if they are labelled (north or south declination), and the direction of the celestial pole properly. Declining sundials are constructed in exactly the same manner as those for the south vertical sundial, Plate XII. It must be remembered that the height of the style is always its elevation above the sundial plate. The location of the horizontal line is derived in the same manner as for the equatorial sundial, Plate XI, and it is perpendicular to the substyle line. There are four declining sundials. Of these the south sundial declining east is typical. The example shows the lines of declination as they appear on a south vertical sundial declining 2o° east, in latitude 4o°N. The Construction On this sundial, AF is the substyle line; AB the meridian or 12 o'clock line; AN is the style; angle NAF is the height of the style; N is the nodus; NF and F the height and foot of the perpendicular style, respectively. In order to construct the lines of declination, first, lay out the hour lines for an horizontal sundial, whose meridian or 12 o'clock line is the substyle line AF; and whose height of style is equal to the angle NAF. Then, with all parts of the gnomon for the horizontal sundial equal to those of the declining sundial, proceed to lay out the lines of declination. Thus will the lines for the declining sundial be properly constructed. The horizontal line on declining sundials will pass through the point where the equinoctial line intersects the 6 o'clock line, as at K. From K, it is drawn at right angles to the 12 o'clock line AB. It also passes through the foot of the perpendicular style, F. Lines of Declination on the north vertical sundial. The north vertical sundial, owing to the short period that the sun shines upon it, is generally used only on a pillar sundial. During its period of usefulness the sun has but a small elevation above the horizon. It is, therefore, not necessary to draw the lines of declination on it. However, certain lines (depending upon the latitude of the place) may be drawn upon the sundial plate if the work is done in the same manner as for the south vertical sundial.

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