A Quantitative Study of Accuracies in Positions of Star Markers on Historical Astrolabes
The relative accuracies of the positions of star markers on the retes of some historical astrolabes, studied using images of these retes, is presented. These images are superposed on planetarium software sky views with fisheye stereographic projections. Almost all astrolabes studied, other than a modern electronic one, do show some inaccuracies in the positions of star markers on the retes. This would likely arise from their small sizes, the complications of metalwork involved and transfer of technology to artisans. Two historical astrolabes, presumably from Jai Singh’s Jaipur karkhana for astrolabes, are studied for depiction of the accuracies in star markers. The analysis can be extended to tympans for studying historical astrolabe functional accuracies. The method can be utilised for studies of any historical instrument which has laminar structure, and which may currently be too fragile for actual handling and quantitative measurement of any kind.
We are grateful to the Oxford Museum of History of Science for making a rich library of astrolabe images available for researchers; to Skyskan for the planetarium software Digitalsky; to Image-magick (http://imagemagick.org/); and to Fred Weinhause for providing software used in our analysis.
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