Sundial
Beside the banner marking, black on white, the morning hours (VII to IX, left) and evening (III to VI, right), are the reverse of the banner marking the hours close to noon (X and XII I-II) and drawn in white on black background, the signs of the zodiac (six left and six right), whose boundaries correspond to the path of 7 diurnal arcs (which are arcs of hyperbole). The specified time is 1 hour and 50 minutes in the afternoon, July 16, 25 days after the summer solstice.
Coordinates: 50 03 '41 "N 19 56' 24" E / 50.0614, 19.9400.
A sundial is an instrument silent and still showing the solar time by the movement of the shadow of an object variable in shape, the gnomon or style, on a surface, the table of the dial associated with a set of graduations plotted on this surface. The table is generally flat but may also be concave , convex , spherical , cylindrical , etc..
The gnomon is a vertical element that indicates the time by the length or direction of his shadow. Currents on the dials, the shadow-holder element is usually a pin (or edge of a plane) tilted parallel to the axis of rotation of the Earth's axis or the world. He took the name of "style". This inclination, the angle depends on the latitude of the place, can tell the time throughout the year directly on the same set of scales: the range of hour lines.
Summary |
History
The sundial is considered, because of its simplicity, as one of the earliest objects used by man to measure the passage of time. The oldest known models , simple dials in height, were found in Egypt. A new model (the Scaph), based on the sphere, was introduced by Berosus in Greece in the third century BC. AD ; other models resulted from this (hemispherical, conical, plane, ...).
These models indicated unequal hours of that divided the day from sunrise to sunset, 12 hours, summer or winter: summer hours were long, shorter winter hours. Around the seventh century dials canonical appeared in Europe following the work of the Venerable Bede. They secured a transition to sundials inclined to style as we know them today, and whose principle, from the Arab civilization, appeared around the thirteenth and fourteenth centuries. The inclination of the style has to draw a diagram showing the hour lines of equal hours, that is to say, as we use it one day, a midday following is divided into 24 hours, regardless is the time of the year.
The emergence and spread of the clock , from the late fourteenth century , led to the development of this type of sundial, since its readings could be directly compared with those of clocks: the sundial said earlier , dependent on the clock to keep it. The dials then made the subject of science, gnomonic , branch of astronomy , which reached its apogee in the eighteenth century and an art practiced by the sundial.
Principle and use
Coordinates: 50 25 '23 "N 6 12' 06" E / 50.4231, 6.2017 (Belgium)
The displacement of the shadow during the day is linked to the apparent displacement of Sun in the sky, which reflects the rotation of the Earth. It is measured by the apparent solar coordinates: hour angle , height and azimuth. We will therefore dials hour angle, by far the most common, dials height ( dial shepherd for example), and dials azimuth ("spiders"). The time shown by a sundial is solar time, or true time, the place where it is located: in the past, it was appropriate for everyone, since the movements were slow and where there was no way to broadcast time.
This time is different from the legal hour of every day for several reasons:
- the solar day is not strictly constant with the seasons: the variation is reflected in the equation of time ;
- must take into account the longitude of the place and, of course, now, any changes DST / Standard Time.
To switch to solar time to standard time, we must apply at the time indicated by the sundial three successive corrections. The first is the correction corresponding to the equation of time : it gives the average time. The second is the correction of longitude in minutes of time equal to four times the longitude in degrees, in effect, the Earth went around on itself, or 360 degrees in 24 hours, or 15 degrees in one hour , 1 in 4 minutes. This correction gives the universal time and is counted as negative for east longitudes and positive for the West: the dials are located to the east ahead of time of Greenwich. Finally, we must add the time difference related to time zone to time zone location of the dial: for example, for France, Belgium or Switzerland, add one hour in winter, as these countries are in the zone UTC +1 , and two hours in summer, to reflect the DST :
- Legal Time = Time + Equation of Time Solar Longitude + + Summer / Winter
For example, March 10, 2010 a sundial located in Brussels (longitude 4 21 '09 "E) indicates 15 h 40; legal time will be determined by the following calculation: 15 h 40 min 18 s + 10 (value the equation of time to March 10) - (4 x 4 21 ') is - 17 min 24 s + 1 hours (Brussels is in timezone GMT +1, and we're on winter time ), or 16 h 32 min 54s. Meanwhile in Brest (longitude 4 29 'West), a sundial will indicate 15 h 04 min Types of sundials The most common gauges are dials for hour angles. Several software are now freely available on websites. They provide fast drawing a clock according to the location, type and dial the desired component (diurnal arcs, equation of time) for this course. It remains then to the person wishing to make a dial to see this track on the medium of their choice, providing the address in detail the theory and calculations. One can cite Shadows , Cadsol and Solarium . 
But there are many other types of dials much more original. 
Software
Bibliography
See also
Internal Links
External Links
References
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