(1998).ISO/media/astdict/p/graphics/prules.jpg)
Ptolemy's rules. The angle between the vertical rod and
the upper hinged rod is the star’s zenith distance, which is 90 degrees minus altitude.
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An early instrument for determining the altitude of a celestial body, also known as a triquetrum. Despite the name, it seems likely that its invention predates Ptolemy (c. AD 100-170). It consists of a vertical bar with hinged rods attached at the top and bottom (see illustration). The hinged rods are linked, so that the whole arrangement forms a triangle, but the upper arm can slide along the lower one, which is graduated. The upper arm is fitted with sights. When the object under observation is in view, the position of the upper arm against the graduated lower arm gives the measure of altitude.
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