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Finding the length of time a star will be above the horizon

There are several ways to solve this problem. Perhaps the most efficient way is as follows: Place the star on the eastern horizon and align the rule with XII at the top of the astrolabe. Now place your thumb on the rule to hold it tight against the rete. Rotate the rule and the rete at the same time until the star is on the western horizon. Whatever number the rule indicates (as measured by the Roman numerals on the outer edge of the astrolabe) is the number of hours that star will be above the horizon. If the rule has passed XII again in the course of rotating the rete, then you need to add twelve to whatever the rule indicates. For example, if the rule has rotates all the way past XII to I on the bottom of the astrolabe, the star will be above the horizon for thirteen hours.

Example: How long will Spica be above the horizon each day? Begin by rotating the rete until Spica is on the eastern horizon. Now position to the rule so it is pointing straight up to XII (see the figure in the star rise example). Keeping your thumb on the rule so that it rotates with the rete, move Spica to the western horizon. According to the rule, Spica will be above the horizon for about 10.5 hours (see below).