Folding sundial
The folding sundial is a sundial in which the shadow pointer or shadow thrower ( gnomon , pole rod , wise man) can be folded into the plane of the dial in order to avoid bulky volumes. The Common Authority File (GND) defines: “Portable sundial that can be opened with hinges and has a small compass for north-south alignment. A gnomon serves as a shadow thrower. ”It shows the true local time (also called“ true solar time ”). It was intended for travelers from the late 15th century. Their invention is attributed to the Austrian astronomer Georg von Peuerbach .
Special requirements for folding sundials
In order to be able to display the true local time with a sundial regardless of the season, the shadow object must be parallel to the earth's axis. Since the folding sundial is intended for travel, the problem arises of how the shadow projector can be aligned parallel to the earth's axis with as little effort as possible. In addition, the dimensions and weight should be correspondingly small, but the material should be durable. In general, the smaller the sundial, the more difficult it is to align the sundial precisely and the more difficult it is to read the time, which results from an interpolation of the shadow between the hours marked on the dial, estimated with a sense of proportion . This means that a compromise has to be found between size and the required reading accuracy.
There are numerous different versions of folding sundials. They can be of the type horizontal sundial ( Fig.Paul Reinmann, Nuremberg 1557), vertical sundial (Fig.Marine sundial, both vertical and horizontal sundial, depending on the latitude in which the ship is located) or equatorial sundials (Fig. Ring sundial) be. You may or may not have a built-in compass , the declination is taken into account or not. Rods, angles or tensioned threads are used as shadows; in the case of the ring sundial shown, a node is used to project the sun onto the dial. Possibilities to adapt the shadow object to the respective geographical latitude may exist.
The accuracy of the reading depends on the size of the sundial. On the Stokes sundial, markings for intervals of 5 minutes are visible, on the much smaller pocket sundial only hour lines are engraved on the dial.
Compendium sundial, Christoph Schissler Senior, Germany 1556
Paul Reinmann, Nuremberg 1557
Sundial for 47 ° north. FA Knitti, Austria, early 18th century
Marine sundial, Nuremberg
Gabriel Stokes, Dublin 1742
Ring sundial in use, light spot between 1 p.m. and 2 p.m.
Construction principle of the pocket sundial
In order to display the true local time , the shadow object of a horizontal sundial must be aligned parallel to the earth's axis and the dial must be horizontal. With a sundial intended for travel, it should be possible to do both as quickly and easily as possible at the time of measurement. In the case of the folding sundial shown, this is done with the help of a compass and plumb bob.
Both the direction of the earth's axis and the exact horizontal are initially unknown at the location of the measurement. The basic idea is that the point on the earth's surface where the compass is located spans a geometric plane with the earth's axis. In this plane, at the location of the sundial, both the horizontal to the earth's surface and the parallel to the earth's axis must be found. The vertical to the center of the earth and the straight lines pointing north are also in the same plane. This geometric plane is therefore also spanned by a vertical line (by the plumb line) and by the north direction (determined by the compass needle). As soon as this plane has been determined, the horizontal must be determined and the shadow object must be erected on this horizontal with an angle of inclination that corresponds to the geographical latitude.
Neglecting the deviation of the magnetic from the geographic north pole (see also declination ), a compass needle points north. However, it points neither to the celestial pole nor to the geographic north pole , but rather follows the magnetic field lines at the location of the measurement and is therefore only exceptionally horizontal. The extension of this imaginary line cuts the earth's axis to a sufficient approximation . A line has thus been found to determine the desired level at the location of the sundial.
The second required line is formed by the plumb line that is fixed to the case of the folding sundial. The perpendicular cuts the earth's axis in the center of the earth. The two straight lines are thus obtained in order to span the plane in question.
The horizontal is obtained during the folding sundial characterized in that firstly two by opening the thread tensioner leg of a right triangle constructed, one leg of the compass housing and the dial is formed, the other leg by the thread tensioner. To ensure a right angle , the swivel joint is designed so that it can be opened exactly up to an angle of 90 ° with normal handling (see side view). As soon as the vertical leg is set up vertically with the help of the built-in plumb bob , the housing must be horizontal . Up to this point we have achieved that by means of a compass and plumb line a horizontal line has been established exactly in a north-south direction.
If you know the geographical latitude in which you are, a pole rod must take exactly this angle to the horizontal and lie in a plane that also goes through the earth's axis in order to be parallel to the earth's axis. You therefore now need a device in order to be able to erect the shadow projector at this same angle to the horizontal that has just been obtained, which is already aligned in the north-south direction.
The function of the pole rod is taken over by a thread in the shown folding sundial. To do this, the thread is stretched as a hypotenuse of the right-angled triangle, the right angle of which is formed by the hinge of the flap. The corner point on the horizontal is fixed because all engraved hour lines on the dial must run through this point . The inclination of the hypotenuse with respect to the horizontal is set by passing the thread through one of the prepared drill holes on the thread holder. To make it easier for the user, five drill holes are provided in the present case, with the drill holes being labeled for 48 °, 50 ° and 52 °.
handling
As soon as the folding sundial is open and aligned in a north-south direction, the plumb line points exactly to its base point in the frame of the thread holder and the thread has been stretched through the hole on the thread holder corresponding to the respective latitude, the sundial is ready for use. If there are no major changes in the geographical latitude as a result of traveling, then it makes sense to fix the thread directly in the corresponding hole in the thread holder so that the thread is as taut as possible when the folding sundial is open, but without its tension opening the folding sundial up to to prevent right angles or, conversely, to tear.
When the sundial is folded up, the other knob at the upper end of the thread holder engages in a hole between the compass housing and the fastening eyelet, so that the folding sundial does not open unintentionally during transport (the two knobs are on the right in the figure, folding sundial, closed for transport, side view to be seen at the height of the 5 cm mark.).
The pocket sundial shown is sold under the name "Folding Sundial".
Web links
- Folding sundials - Nuremberg's handiest export item.
- Georg v. Peuerbach exhibition
- Nuremberg compass maker and folding sundials, pp. 57-59, in: '' Astronomy in Nuremberg: on the occasion of the 500th anniversary of the death of Bernhard Walther and the 300th anniversary of the death of Georg Chr. Eimmart '', pp. 57-59.
Individual evidence
- ↑ Folding sundial with reference to Manfred Ballweg: Bruckmann's Uhren-Lexikon / Manfred Ballweg. Among employees by GO Weiland . Bruckmann, Munich 1980, ISBN 3-7654-1825-0 .
Remarks
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↑ Different names have been established for the object that casts the shadow. Meyers Konversations-Lexikon in the 6th edition of 1908 defines in the eighth volume on page 66: "Gnomon (Greek," indicator "), ancient astronomical instrument for determining the height of the sun and the time of noon (the highest sun height)" and " Gnomonics, the art of making sundials. ”In the eighteenth volume from 1909 on page 609:“ Sundial, a device which indicates the time by means of the position of the shadow, which is a rod illuminated by the sun and parallel to the world axis (gnomon or sage ) on a generally flat surface, the dial. ”What is sometimes referred to as the“ pole rod ”was called“ Gnomon ”or“ Weiser ”at the time this work was printed. The term “pole rod” is not a lemma in this lexicon and does not appear as a term in either “gnomon” or “sundial”.
In the case of the pocket sundial, a thread serves as a shadow thrower. Calling a thread a " rod " (pole rod) can lead to misunderstandings. - ↑ A picture of his folding sundial can be found here: Georg von Peuerbach (1423–1461), court astronomer and astrologer, 1455, folding sundial, cast brass, engraved. City of Peuerbach, accessed on May 1, 2016 .
- ↑ Zurich is at 47.38 ° N, Berlin at 52.51 ° N.