Horrebow-Talcott Method

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The Horrebow-Talcott method is a precision method in astronomy and geodesy for determining the polar height (astronomical or geographical latitude ). It was developed in the 18th century by the Danish astronomer Peder Horrebow and rediscovered by Andrew Talcott in 1833 .

A special ( optical- mechanical) eyepiece micrometer is used to measure the zenith distance difference between several pairs of stars when passing through the meridian ( upper culmination ). The two stars of each pair must culminate in the south and in the north and have approximately the same zenith distance in order to be visible in the telescope on both sides (after pivoting by 180 °).

The measurement with the micrometer avoids possible small circle pitch errors as well as those of the circle reading , because the telescope is set to a constant (mean) zenith distance for each star pair. A special, high-precision vial is used for this ("Horrebow level" or double second vial ), which is clamped directly to the tilting axis . The measuring arrangement also eliminates other small instrument errors such as telescope bending.

The following relationships exist between the latitude  φ , the star declination  δ and the zenith distance:

  • at southern culmination φ = δ + z
  • at northern culmination φ = δ - z ,

so that the latitude results from the mean value of  δ and the height difference Δz of the north and south star measured on the micrometer  .

To be added to Δz :

  • the differential astronomical refraction
  • the difference between the dragonfly readings (usually less than 0.5  pars )
  • the possible influence of the different telescope bending.

Towards the end of the 19th century, the International Broad Service introduced this method to monitor the precise polar movement of the earth's body. For this purpose, five observatories were selected at 39.1 ° north latitude, which were almost evenly distributed on this parallel . The constant latitude ensured that the observatories could observe the same fundamental stars , so that any errors in the star locations had no influence on the measured latitude changes. The measurements of the International Polar Motion Service  (IPMS) achieved accuracies of a few 0.01 ″ , so that the polar coordinates and their changes (see Chandler period ) could be determined with an accuracy of a few decimeters.

Towards the end of the 1970s, the methods of satellite geodesy became more precise than those of astrometry , so that today - in the IERS earth rotation service - visual methods are no longer used, but a combination of satellite and VLBI measurements.

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