Lunchtime
A noon indicator uses the hour angle of the sun to indicate the time of noon or the passage of the sun through the meridian. The sun is shown on a generally flat surface - mostly in central projection. It is also often referred to as the midday sundial , although the mere display of midday ( ways of midday ) is not a function of a clock . A sundial shows the hour angle of the sun, which changes over the day, not just the moment for its selected noon value.
A measurement takes place, however, if at noon every day the elevation angle of the sun, which changes over the year, e.g. B. is read in the coding as a calendar date. A common name for this is the zodiac clock . The emperor Augustus' meridian instrument served this purpose. It has been called Meridian since its excavation , a term that has meanwhile become a synonym for at least great noon wise men.
The scale is generally a straight line (straight meridian line ). With the help of an analemma-like scale loop, the middle noon can be displayed instead of the moment of the true noon . This was used to adjust mechanical clocks - for example at train stations - until the beginning of the 20th century.
Gnomon
The oldest noon wise man was the gnomon . It was used both to determine the direction of the compass and as the main component of a permanently erected astronomical instrument . In the first case ( portable noon wiser , Indian circle) it only needed to be stuck in the ground for a sunny day. In the second case it was permanently fixed in the ground and the north-south direction determined by it was marked on the ground as a noon line “smudge-proof”.
The astronomical measurement takes place at the moment of the highest point of the sun. The shadow length of the gnomon is read off as a value for the elevation angle h of the sun, from which the days for the solstices and equinoxes can be recognized. The angle of the ecliptic results from the extreme values of the elevation angles . The elevation angle of the sun on equinox days is a measure of the geographical latitude of the location.
The astronomical instrument gnomon later became a complete sundial, from which all times of the day can be read apart from the moment of noon. With Gnomonik teaching today is meant by the sundial in the first place.
Meridiana or Méridienne
From the 16th to the 18th century, monumental noon signs were installed in some Italian and French cathedrals in order to carry out astronomical measurements. Such a midday meal is called Meridiana in Italian and Méridienne in French . It was the time before and after the Gregorian calendar reform that the Catholic Church began to give up its anti-science attitude.
The image of the sun is done with a hole gnomon , which is located high under the ceiling in a south wall. The scale (midday line) extends on the floor over almost the entire length of the nave. A hole gnomon is necessary because a light spot can be created more concentrated than a shadow point at great distances.
In Italy, meridianas were numerous. Those in the domes of Florence (furnished by Toscanelli ) and Milan were established in 1475 and 1786. The Dominican Egnatio Danti constructed several Meridianas, the most famous of which was the one established in the Basilica of San Petronio in Bologna around 1575 . At noon, a ray of sun fell through a hole gnomon a few millimeters in size in the dome of the basilica onto a brass band set into the floor, about 70 meters long. On the tape you could read the calendar and the signs of the zodiac. The data read by Danti on such a noon line made the deviation of the Julian calendar from the course of the sun clear and led directly to the calendar reform by Pope Gregory XIII. Danti's Meridiana in Bologna was flawed. On the occasion of renovations, the astronomer Giovanni Domenico Cassini had a new Meridiana installed there in 1655, which still exists today.
Méridiennes for the same purpose are known from the churches of Saint Sulpice in Paris and Saint Michel in Brussels and from the old hospital in Tonnere / F. Confusion is possible with a large number of simplified sundials on outer walls, which were made in Alsace in the 18th and 19th centuries. They are also referred to as Méridiennes in the literature.
Noon wise man for setting up wheel clocks
The mechanical clock could not displace despite their great advantage of the independent functioning of the weather the sundial long. The most important reason was that the sundial measured more precisely. For a long time it was necessary to adjust the clocks that were considerably ahead or behind with the help of a sundial.
When judging was only necessary once a day, noon wedges were used, which were mostly located in public buildings such as churches, town halls and train stations. They were in use until the advent of electrical communications in the 19th century. After that, the time of day indicated by a sufficiently precise central wheel clock could be made known nationwide. The use of the middle period became necessary at the latest with the advent of the railroad in the middle of the 19th century. Midday was also preferred because sundials, which are designed for a mean time over the whole day, were ruled out due to their confusion. These noon wise men were also provided with Loch Gnomon. It cannot be used on sundials because you would have to turn it during the day so that it is perpendicular to the sun.
See also
literature
- John L. Heilbron: The sun in the church. Cathedrals as solar observatories. Harvard University Press, Cambridge MA 1999, ISBN 0-674-85433-0 .
- Giampiero Negretti, Paolo de Vecchi: The fascination of clocks. A history of timekeeping. Callwey, Munich 1996, ISBN 3-7667-1214-4 .
- René RJ Rohr: The sundial. History, theory, function. Callwey, Munich 1982, ISBN 3-7667-0610-1
- Michael Schütz: Cassini's Meridian in Bologna. In: Stars and Space. 28, 6, 1989, ISSN 0039-1263 , pp. 362-366.
- Volker Witt (Hrsg.): Astronomical travel destinations for traveling. Observatories, museums and sites of astronomy. Elsevier, Munich et al. 2004, ISBN 3-8274-1414-8 .
Web links
Individual evidence
- ↑ a b Karlheinz Schaldach: A rare form of antique sundials: The Meridian of Chios , Archäologisches Korrespondenzblatt. 2011, issue 1, p. 73.
- ↑ See Bassermann-Jordan / Bertele: Uhren , Verlag Klinkhardt & Biermann, 1961, p. 101
- ↑ René RJ Rohr: The sundial . Pp. 12-13.
- ^ A b René RJ Rohr: The sundial . P. 30.
- ↑ Weblink focus.de/wissen "Ray of Knowledge"
- ^ John L. Heilbron: The Sun in the Church . Pp. 70 and 268.
- ^ John L. Heilbron: The Sun in the Church . P. 72.
- ^ Giampiero Negretti, Paolo de Vecchi: Fascination Clock .
- ^ John L. Heilbron: The Sun in the Church . P. 91.
- ^ A b René RJ Rohr: The sundial . P. 188.
- ^ Ernst Zinner: Astronomische Instrumente , Beck'sche Verlagsbuchhandlung, 1956, page 75 and 230