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The Mathematics of Sundials
The shadow cast by the gnomon at a given hour points in different directions, depending on the seasons. The angle covered by the shadow during a certain time interval depends on the seasons. Different Classification of HoursIn the course of history, classification of time has changed several times.
Unequal or Temporary HoursThe time from sunrise until sunset, as well as the night arc, was divided into twelve equally long parts. The "hours" of any one day were equal, but the "hours" of the winter were short and the "hours" of summer long. Equinoctial HoursThe time beginning with the passing of the low meridian, about midnight, until the passing of the next low meridian, was divided into 24 hours of equal length.
Italian HoursCounting of the 24 equal hours began at sunset. Babylonian HoursCounting of the 24 equal hours began at sunrise. Equation of Time -AnalemmaThe equation of time is the difference in time between what your watch reads and the position of the Sun. The analemma is the path that the Sun takes if we mark the position of the Sun in the sky at the same time everyday, say sometime around noon.
There are two independent reasons why the Sun takes this strange path The Earth is tilted on its axis The Earth does not orbit the Sun in a circle, but in an ellipse. It is simply the sum of these two effects that causes the analemma.
Different Types of SundialsThere are two ways to classify sundials. The first way is to group sundials into three main classes Sundials having gnomon parallel to Earth's axis 2. Sundials depending on altitude of the Sun 3. Sundials depending on azimuth of the SunThe second way is to group sundials into two main classes: 1. Pole-style sundials 2.
Nodal sundials A pole-style sundial has a gnomon which casts a line-shaped shadow on a set of hour lines. Equatorial, horizontal and vertical sundials belong to this class.
For a nodal sundial, the time is read from the shadow of a fixed point or node. Equatorial SundialsThe dial plate is in the plane of the celestial equator.
They can be classified into two types: 1. Equatorial disk dials 2. Armillary dials Equatorial Disk DialsThis is the only dial that has faces on both upper and lower sides. Armillary DialsThe armillary dial is an extension of the equatorial dial, with two rings representing the celestial equator and the meridian. A third ring is usually added to represent the horizon. Horizontal Sundials.
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Sundials: their theory and construction
Handbook of Practical Astronomy pp Cite as. Sundials have become a favorite ornament in houses and gardens. Design and construction are not difficult with some knowledge of spherical astronomy. The basic parts of a sundial are the shadow-casting style or gnomon, and the dial plate. Advice on computation and the graphical construction is given. Designs for higher accuracy are outlined. Unable to display preview.
The Mathematics of Sundials
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