Powder lightning signal
Powder lightning signals are a method of signal transmission that was used between around 1800 and 1920 to synchronize precise clocks and simultaneous measurements . It can be seen as a special form of optical telegraphy .
The need for exactly simultaneous measurements is primarily in astronomy for determining the length between two observatories and in geodesy , for example for measuring degrees to determine the exact shape of the earth. With these methods, the synchronization must be significantly better than tenths of a second . Today this is achieved - within a fraction of a millisecond - with GPS or other radio signals ; previously wired or wireless telegraphy was used . Before their invention, sound was used for short distances , for longer strong light signals , for example with heliotropes , a special type of sun mirror .
Survey signals with lightning powder
In 1803, the German-Austrian astronomer and mathematician Franz Xaver von Zach developed a method using lightning powder signals to cover long distances for land surveying, even at night. First, the mutual view ( visor ) was checked and the best location was explored. By simultaneously observing the light signals, the geodesists (or later the observers from the observatories involved) were able to synchronize their astronomical clocks , which was usually done using the eye-ear method . Then the respective local sidereal time was determined, the difference of which gave the difference in length .
Various substances were experimented with for the lightning powder. Magnesium compounds turned out to be the best , for which even small amounts were sufficient when observed at night. The powder was ignited on long poles.
For special purposes, similar but field-compatible methods were developed over the years with which individual positions of moving objects could be measured using forward cuts. (In the first half of the 20th century, high-target triangulation was also used for large-scale geometra- tion , but this was already synchronized with radio signals, and around 1960 the method of flashlight satellites ).
Background to the Zach method
The reason for Zach's idea with powder flashes was the request of the King of Prussia in 1802 to draw up a military map of Thuringia . Zach saw this as a good opportunity to use Prussian geodesists to start precise degree measurements over several degrees of longitude and latitude. The ingenious organizer wanted to combine the cartographic assignment with a research project to determine the exact flattening of the earth and the shape of the geoid , and was able to inspire his sovereign Ernst II of Saxony-Gotha-Altenburg .
The Duke's influence enabled Zach and his helpers to triangulate in all of the neighboring countries . At the same time, Zach worked out a very efficient process for the major project. The signaling of the surveying points with the powder lightning bolts that can be seen from afar was intended, on the one hand, for the astrogeodetic comparison of local times and, on the other, for the early exploration of an extensive fixed point network .
In 1803 flash signals were emitted from several mountains and successfully evaluated. In this way, a geodetic baseline, necessary for the subsequent triangulation, was quickly established and the exact length of the route determined. Zach planned this base in the meridian of his observatory and had it precisely aligned with its main telescope. His most important colleague was the Prussian officer Karl Freiherr von Müffling (1775-1851), his later successor.