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Using the Seasonal Hour Arcs

On the back of the astrolabe is a set of curves known as the seasonal hour arcs. These curves can be used as an alternate method of determining the seasonal hour. Only two pieces of information are needed: the declination of the sun and its height above the horizon.

The declination of the sun can be measured by placing the rule against the sun on the front of the astrolabe and reading off its declination from the markings on the rule. Notice that one half of the alilade (on the back of the astrolabe) is marked with declinations from -23.5 degrees to +23.5 degrees. Align the alilade with the current height of the sun on the altitude scale and note the position of the current declination of the sun with respect to the seasonal hour arcs. This will allow you to estimate the seasonal hour.

Example: What is the seasonal hour if the sun is 20 degrees above the western horizon during the vernal equinox (September 21st)? During the vernal equinox, the sun's declination is 0 degrees. If you don't believe me, check it using the front of the astrolabe. Now align the alilade with 20 degrees on the altitude scale. Notice that 0 degrees declination as indicated on the alilade falls just after the 10th seasonal hour arc.