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Finding the time of day using the sun

This, by the way, was the principle use of the astrolabe during Medieval times. Since there were few mechanical clocks during that period (the water clock was unreliable and the pendulum clock had not yet been invented), the only reasonable way of telling time was by observation of the heavens. This is where the idea of the astrolabe as an observational instrument came in. The vanes on the alilade could be used to measure either the height of the sun or the height of the star. Once this was done, it was simply a matter of rotating the rete until the artificial "sky" of the rete resembled the actual heavens.

Let's say we made an observation of the sun and determined that it was a certain number of degrees above the horizon. Since the almucantar lines indicate height above the horizon, we simply rotate the rete until the sun is at the correct height as indicated by the almucantars. (Remember, we first need to locate the sun in the sky using the back of the astrolabe.) Now that we have set the astrolabe, we simply need to place the rule against the sun. As in the problem involving sunrise, the rule will point to the time of day.

Example: What time is it if the sun is 10 degrees above the eastern horizon on January 29th? By now, you should know that the sun is in the 10th degree of Aquarius on January 29th. Place the sun on the almucantar just above the horizon (if you're not sure which almucantar corresponds to which degree, look at the labels written along the meridian.) Thus the sun is now 10 degrees above the horizon. To find the time, place the rule against the sun and read off the time on the edge of the astrolabe. In this case it's about 8:20 AM. Note that if the sun were 10 degrees above the western horizon, it would be afternoon, and you would have to place the sun on the right side of the astrolabe. If this were the case, it would be about 3:40 PM.