Did you lose your smartphone? No worries, just pull out your astrolabe to find the time of day or night. During the day, you would base your calculations on the altitude of the sun. At night, you would use the altitude of a visible star. As an example, let's walk through the steps to determine the time of night:
- First, you must convert the current calendar date to a zodiac date, as we did in the previous exercise. Turn the alidade until it points to the date on the calendar scale, then read the corresponding value from the zodiac scale.
- Now you need to find a suitable reference star. Remember, people in the ancient world would have been intimately familiar with the night sky and would have known the major constellations. As the eighth brightest star in the sky and the brightest in the constellation Canis Minor, Procyon would have been a familiar beacon and would have appeared on most astrolabes. As a result, we'll use Procyon as our reference star.
- Find the altitude of Procyon following the same steps given in the first exercise.
- Now turn the astrolabe over and find the target star on the rete.
- Rotate the rete until the target star touches the altitude line matching the value you calculated in step 3.
- To find the time, rotate the rule until it touches the specific zodiac value. Then read the time from the outer rim.
The Web site of the Oxford Museum of the History of Science provides an excellent interactive demo of this last exercise, using a replica of an ancient astrolabe. As it steps you through the process, it also shows the markings on the instrument's various scales.