Danjon astrolabe

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The Danjon Astrolabe ( Astrolabe Impersonnel ) is an astro-geodetic precision instrument for the simultaneous determination of latitude and longitude . Because of its high weight, it can only be used on fixed locations or on a solid measuring pillar .

The instrument, which is about half a meter high and rotatable on foot screws , is a further development of the prism astrolabe by Claude and Driencourt . What is essential is an improved beam path and that the personal equation of the observer is largely eliminated.

Like other astrolabes and the circumzenital , the Danjon astrolabe is used to precisely measure star passages through a constant zenith distance , from which the two parameters of the vertical direction ( astronomical latitude and longitude ) are calculated. The evaluation is carried out according to the method of equal heights .

The star passages are not observed on a thread network, but rather through the coincidence of the direct constellation and the constellation mirrored on a mercury horizon . The almucantarat (zenith distance 30 °) is defined by the center of the invisible sectional images of the two mirrored images projected on top of each other in the telescope's field of view . At the moment when the star passes through this Almucantarat, the traces of the direct and the mirror image meet at the same height.

A Wollaston prism coupled with a micrometer screw , with which the observer can maintain the coincidence over a longer period of time, is used to increase the accuracy . The times are registered electronically at regular intervals .

The achievable accuracy is better than 0.1 ", on the average of several observation nights even below 0.05" (see also evening error ). In the last decades the instrument was u. a. Used for determining the longitude of the world , but also for the precise determination of star coordinates .

literature

  • André Danjon : L'Astrolabe Impersonnel de L'Observatoire de Paris , Bulletin astronomique Vol. XV, pp. 251-281, Paris 1954
  • Karl Ramsayer : Geodetic Astronomy ( Handbook of Surveying Volume 2a), Chapter Prism Astrolabia . Tenth, completely revised edition, JB Metzler-Verlag, Stuttgart 1970
  • Ernst Buschmann: Time and latitude determination with the impersonal prism astrolab from Danjon . Surveying technology Jg.X, p.140-146, Berlin 1959