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SUNDIAL SECTIONS: Developing and Making Sundials Interesting Sundials of the World
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PORTABLE SUN sundials - Page 9Two small vertical uprights are mounted on the upper edge; the one on the right has a narrow slit. When in use, the bead is adjusted to the date, which is marked on a scale at the left edge. Then the whole plate is held in a vertical position and slowly tilted up and turned to the left or right until a ray of light passing- through the slot in one of the upright pins, is centered on the other, at which point the time may be read from the position of the bead. These sundials are often made of cardboard. The wood pillar sundials in the lower left and right corners have been previously described and need no elaboration here. Portable sundials make use of every possible means of telling time. Some are even fitted with gears that divide the hours into minutes. Most of them are based on one of three positions of the sun. First, the sun's hour angle from the meridian as in equatorial, horizontal, etc.; second the altitude of the sun or its height above the horizon; and third, its azimuth. All the information necessary for constructing practically any kind of a sundial is given either in previous chapters or in the appendix. Our aim has been to provide you with an incentive and the proper knowledge. The Mensing Collection of astronomical instruments is one of the finest in the world. It is located in the Adler Planetarium, Chicago, Illinois. More than four hundred instruments are on display, about one hundred and seventy-five of which are sundials of all sorts. Many of them are intricate devices containing gears, lenses, etc. for determining time by the sun. It is representive of the finest work done in the period from 1479 to 1579. If you are ever in or near Chicago, don't miss this collection.
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