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Finding the time of day using a star

Using a star to tell time is not a trivial exercise. First of all, one must identify a star that appears on the astrolabe's rete. This, of course, takes practice. On this particular astrolabe, the brightest stars are labeled by name, and a few familiar constellations are included as well. Practice finding these stars and constellations whenever you have a chance. Second, the naive approach to telling time at night is to place the rule against the star and read off this time. Don't forget that the sun is the keeper of time. Once you position the star correctly on your astrolabe, you are still going to have to place the rule against the sun to determine the time.

The purpose of observing a star is to set the rete of the astrolabe so that we can later use the sun to tell time. Once the height of a star is determined by sighting it with the alilade, place it at the correct height above the appropriate horizon (east or west.) Remember that the almucantars help you determine the height of an object from the horizon. Now that we have set the astrolabe, we simply need to place the rule against the sun. The rule will point to the time of day. (If you get 11:00 AM as an answer, you did it wrong.)

Example: What time is it if Rigel is 25 degrees above the eastern horizon on January 29th? Place Rigel at 25 degrees above the eastern horizon, halfway between the 20 and 30 degree almucantars. Now set the rule against the sun (i.e. the 10th degrees of Aquarius.) One can see that the rule points to 5:45 PM. It is interesting to note that the sun had not yet reached the astronomical twilight almucantar in this example. Thus, only the brighter stars are visible (fortunately, Rigel is one of the brightest.)