Heliotrope (measuring device)
A heliotrope (from ancient Greek ἡλιότροπος hēliótropos , German , “ solstice [plant which aligns leaves and flowers according to the course of the sun]” ) is a sun mirror developed by Carl Friedrich Gauß to make distant measurement points visible . For this purpose, the sunlight is reflected at the target point with a mirror in the direction of the distant theodolite standpoint.
For precise alignment, the heliotrope has a telescopic sight that is aimed at the observer at the measuring point (theodolite). There are two mirrors in front of the riflescope that are inclined at 90 ° to each other. When one mirror reflects the image of the sun through the telescope, the other mirror throws the sunlight onto the point (the observer on the theodolite) that was aimed at by the telescope.
background
In the survey of the Kingdom of Hanover , which began in 1821, Carl Friedrich Gauß used the method of least squares to compensate for observation errors , as in the Braunschweig survey from 1802 to 1807 . This work, in which he took part personally from 1821 to 1825, lasted until 1844. With a theodolite by Georg Friedrich von Reichenbach and the heliotrope he invented to use sunlight for measurement signals over long distances, Gauss achieved a measurement accuracy that is still known today is exemplary.
The heliotrope used by Gauß is now kept in the historical collection of the Faculty of Physics at the University of Göttingen .
functionality
During the construction, Gauss remembered an observation he had made in 1818 when he was in Lüneburg . In a letter to Olbers in the same year he reported that "at a distance of 6 miles [44.5 km] the window of the top cabinet in the Michaelist Tower in Hamburg, which happened to be struck by a sunbeam, was an extremely shining point of light". Two years later, Gauss developed a device with which he could use mirrors to direct sunlight in a certain direction.
Gauß made some demands on the new instrument: It should enable measurements over a distance of up to 100 km and be easy to transport. In addition, it had to offer a way of counteracting the apparent movement of the sun in the sky caused by the rotation of the earth, so as not to influence the direction of reflection of the sunlight.
While the first two drafts of the heliotrope were being constructed, Gauss had a third idea, which consisted of placing a sextant on a stable tripod and attaching a third plane mirror to it, which reflects the sunlight in the desired direction. Gauss called this device the vice-heliotrope .
In front of the telescope objective there are two plane mirrors that are firmly attached to each other at an angle of 90 °. This mirror arrangement can be rotated about the telescope axis and pivoted about an axis transverse to it. If the mirrors are set up in such a way that the larger main mirror reflects the sunlight in the direction of the target aimed at by the telescope, the small semi-transparent, less reflective auxiliary mirror, which is arranged at 90 ° to the main mirror, reflects part of the sunlight in the opposite direction, i.e. in the axis of the Telescope into it. The operator therefore sees an image of the sun in the telescope that is projected onto the targeted target. (He cannot see the light spot cast by the main mirror on the target because it is too faint.) The operator must first align the telescope firmly with the target and then adjust the mirror combination by turning and swiveling so that the image of the sun in the telescope covers the observer position in the target. Then the heliotrope directs a solar reflex onto the observer, which the observer can see and aim at as a bright point of light. Since the sun appears to be moving in the sky, the mirror alignment needs to be corrected occasionally; the telescope setting remains unchanged. Since the operators in Gauss's time were not in speech communication over the great distances, they had to communicate otherwise, by means of agreed signs, about the successful completion of the measurement. (It is not known whether Gauss used his Morse code. However, he began telegraphing and sent 21 characters in seven minutes.)
When measuring the angle between the sun and the target point, the said mirror directs the sunlight in the desired direction. Any point on the sun can be chosen. This position can be maintained for about two minutes before correcting the alignment based on the apparent movement of the sun. This time can be justified by the fact that the reflected light beam is conical and has an opening angle of 0.5 ° corresponding to the apparent diameter of the sun.
From the target point, the reflected light appears like a star that can be easily aimed at using theodolite. About an experiment that Gauß and his assistant Hartmann carried out in mid-1821, Gauß wrote the following to his friend Heinrich Wilhelm Olbers on July 1, 1821 : “Yesterday, the fifth attempt, at the location of the southern meridian to be built in the future , […] distance 11,890 meters . Mr. Lieutenant Hartmann calls the light glorious again. [...] In the theodolite telescope, the thread seemed completely cut at the point of this sharp point of light. "
The vice-heliotrope, but also the two actual heliotrope, have proven to be very effective in surveying work, with Gauss favoring the second construction of the heliotrope because it is easier to use. Especially with very long triangle sides, there was no alternative to the heliotrope. It has been shown that the only length limitation when measuring with the heliotrope is the curvature of the earth.
The only disadvantage of the device was the dependence on sunshine. At that time, Gauss had no modern, bright, sharply focused artificial light sources available. Today the laser is indispensable as a light source in surveying technology.
Individual evidence
- ^ Wilhelm Pape , Max Sengebusch (arrangement): Concise dictionary of the Greek language . 3rd edition, 6th impression. Vieweg & Sohn, Braunschweig 1914 ( zeno.org [accessed on January 21, 2020]).
- ↑ Heliotrop on the Gauß pages of the Göttingen State and University Library
- ↑ Vice-Heliotrope on the Gauß pages of the Göttingen State and University Library
- ^ Letter to Humboldt
- ↑ See also the meridian sign in the Friedländer Forest ( archive link ( memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice . )
- ^ Letter to Olbers dated July 1, 1821
