Sundial

A sundial is a quiet and motionless instrument which indicates the solar Temps by the displacement of the shade of a variable object of form, the Gnomon or the style, on a surface, the table of the dial, associated with a whole of graduations traced on this surface. The table is generally plane but can also be concave, convex, spherical, cylindrical…

The Gnomon is in the beginning a vertical element which indicates the hour by the length or the direction of its shade. On the current dials the element carry-shade is generally an axis (or the edge of a plan) parallel to tilted the axis of rotation of the Ground or centers world. It then takes the name of “style”. This slope, whose angle depends on the Latitude of the place, makes it possible to read the hour during all the year directly on the same whole of graduations: the range of the time lines.

History

The sundial is regarded as one of the very first objects used by the man to measure the flow of time because of its simplicity. The oldest known models, simple dial height, were found in Egypt. A new model (scaphé) based on the sphere was introduced by Bérose in Greece with before our era; other models resulted from this (hemispherical, conical, plane,…). These models indicated unequal hours which divided the day, of the rising to laying down sun, in 12 hours, be like winter: the summer-time were long, the short hours of winter. About the 7th century the canonic dials appeared in Europe following work of Bède Worthy the. They ensured a transition towards the sundials to inclined style such as we know them today, and whose principle, coming from Arab civilization, appeared about the 13th-14th centuries. The slope of the style made it possible to plot a diagram of time lines indicating of the equal hours, i.e. such as we use them: one day, of a Midi with the following is divided of 24 hours whatever the time of the year. The appearance and the diffusion of the Horloge starting from the end of the 14th century involved the development of this type of sundial since its indications could be directly compared with those of the clocks: the sundial said the hour, with load for the clock to preserve it. The dials were the subject then of a science, the Gnomonique, connects Astronomie, which knew its apogee at the 18th century.

Principle and Use

The displacement of the shade during the day is related to the apparent displacement of the Sun in the sky, which reflects the rotation of the Ground. It is measurable by the apparent solar coordinates: time Angle, Height or Azimuth. There will be thus dials of time angle, by far the most spread, dials height (Cadran of shepherd for example), and dials of azimuth (" araignées"). The hour indicated by a sundial is the solar hour or true hour of the place where it is established: formerly that was appropriate for everyone, insofar as displacements were slow and where it was possible no to diffuse the hour. At our time this hour differs from the standard time of the every day, for several reasons: the solar day is not rigorously constant according to the seasons, this variation is translated by the equation of time; it is necessary moreover take account of the Longitude place (system of the time zones) and of course from now on possible changes Summer-time/Heure of winter. However, these variations can be directly taken into account on a little worked out dials, for example with a style whose form compensates for the equation of time or with time lines which include the corrections: they take a corrugated form then reflecting famous the curve into 8 and in more they are shifted to take account of the Longitude.

A sundial can also indicate the seasons with the diurnal arcs, the hours passed since the rising of the sun (babylonic hours), those remaining to be run until laying down it (Italic hours), the sidereal hours, etc

Of course, a sundial does not function when the sun is not visible, during the night and when the weather is been overcast. Very early, almost all civilizations developed instruments which could then take over sundial, in particular the Clepsydre, then the Horloge.

The organization of a sundial, whose concrete forms are innumerable, allowed, especially at the end of the 20th century, to develop a whole art of the dial by very sophisticated decoration sometimes its surface and by the often fine work of the axis. Certain dials are true works of art, sculptures or paintings sometimes monumental, sometimes without any plane surface, in particular the spheres armillaires.

A currency or a proverb often decorates the dial. It can be in Latin: Carp diem (Picking the day) or Wounding omnes, ultima necat (Every hour wounds, the keep silent last hour), Horas not number, nisi serenas (I count only the serene hours); in French: With which can like the hours are luminous , Souviens to live ; or in another language: Sevel has ruffle year heol ewid year oll (Breton: the sun rises for everyone), Lou tems passo passo lou Ben (of Provence: time passes, passes the good), Καλη η ωρα (Greek: that the hour is beautiful).

They can be humorous:

  • I do not indicate the hour that beautiful days
  • I will sound when you sing (dial decorated with a cock)

Types of dials

The most frequent dials are time dials of angles.

But there exist many other types of dials much more original.

  • two-wire Dial
  • Dial analemmatic
  • Dials of azimuth
    • the Spider of azimuth
  • Dials height
    • the Dial of shepherd
    • Navicula, who is the generalized version of the Cadran of the capuchin
  • Cadrans of azimuth and height
    • the filter Chapeau
    • the Cadran of Freeman
  • Cadrans with several styles which are parts of time plans
    • the Cadran of Skinflint

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