Mercury horizon
The mercury horizon is used in some precise measurement methods in astronomy and astrogeodesy . By mirroring the measuring beam incident from a star , the mirrored image can be superimposed with the direct image of the star and the star passage through the zenith angle specified by the instrument can be measured. The moment of coincidence is recorded by manual stop or automatic time registration in relation to UT1 .
The measuring principle was developed in the 1950s for the prism astrolab attachment of the triangulation theodolite Wild T3 , in which a 30 ° prism was attached in front of the telescope objective of the horizontally aligned theodolite . Wild & Co later also implemented the measurement method for second theodolites . From around 1970 it was also used in two high-precision measuring instruments, the Danjon astrolabe and the circumzenital .
The mercury surface must remain vibration-free, which is not easily achievable for measurements outdoors. In the last-mentioned instruments, the mercury is located as deep as possible inside the device, so that it is protected from the wind and experiences only a few vibrations from the observer. These can be further reduced by an automatically moving micrometer - analogous to the recording micrometer on passenger instruments.
With the same height method, the measurement is carried out by setting the pre-calculated direction in which a star reaches the selected zenith angle, and then stopping or registering the star passage based on the coincidence (with Danjon astrolabe or circumzenital) or on a special thread network (with smaller Prism astrolabia).
literature
- August Oertling: The Artificial Mercury Horizon , 1849
- Karl Ramsayer : Geodetic Astronomy (Handbook of Surveying Volume 2a), Chapter Prism Astrolabia . Tenth, completely revised edition, JB Metzler-Verlag, Stuttgart 1970
- Astronomical notes, volumes 45–46, p. 71, notes on the median circles digitized at the Altona observatory
- Magazine for the latest in physics and natural history , Volume 11, p. 101f, digitized description of the structure of a mercury horizon