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Hipparchus' Celestial Globe

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Bianca
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« on: December 08, 2007, 08:59:15 am »








                                                     3. EPOCH AND LATITUDE





The constellations in the sky move slowly with respect to the declination circles and the colures, as a result of precession. The epoch for the observations that were incorporated into the Farnese Atlas is near that year for which the constellation positions on the globe most closely match those in the sky.

The latitude of the observer is related to the declination of the Ant/Arctic Circles as presented on the globe. Both of these calculations require that we get the positions of the constellations in the reference frame of the globe’s coordinate system. In principle this could be performed by taking a tape measure to the globe, but in practice an equivalent method is to take pictures of the globe and then carefully measure the pictures.

I took 49 pictures of the Farnese Atlas in Naples on 1 June 2004. I used a digital camera that allowed for good recording of the details without special lighting. For photogrammetry, it is important to know the distance between the camera and the near surface of the globe, as this is required to transform positions on the photograph to spherical coordinates on the globe. (All previous published photographs were taken at unknown distances, and that is why a new set of photographs was required.)

My photographs were all taken with the camera at a distance of either 6 or 20 feet from the surface
of the globe. There was substantial duplication and some pictures were not useable for various
reasons, so I ended up doing photogrammetry on twelve pictures. Detailed explanations and examples
for my photogrammetry techniques are presented in Appendix 1. Detailed results and analysis of the constellation positions and the declinations of the tropics and Ant/Arctic Circles are presented in Appendix 2.
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