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2 Observations

All the observations were made visually by the author of this paper and each one consists in timing the transit of one border of the Sun through the corresponding small circle of fix altitude. During one day one observes the transits of the upper and lower border at the east, and the transits of the lower and upper border at the west. The fix zenith distances of $30\hbox{$^\circ$}$ and $60\hbox{$^\circ$}$ are defined by means of CERVIT reflecting prism that are interchangeable rather easily in a few minutes without further adjustment of the telescope. Therefore, observations at at both zenith distances are possible during the same day. According to the latitude of the astrolabe ($-33\hbox{$^\circ$}23\hbox{$^\prime$}56\hbox{$.\!\!^{\prime\prime}$}976$) the Sun can be observed during the whole year at $60\hbox{$^\circ$}$ zenith distance and from October 6 untill March 7 at $30\hbox{$^\circ$}$.

The transit times are registered by a PC microcomputer adapted as chronograph with $0\hbox{$.\!\!^{\rm s}$}0001$ resolution. The precise UTC reference is provided by a GPS timer receiver at the astrolabe pavillion, or alternatively, by a Cesium atomic standard of the time service of Cerro Calán Observatory.


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