If you read my last astrolabe post and swung by one of the links you can find there, you’ll have seen that these astrolabes work like a charm. As well as telling time, an astrolabe can be used for surveying and navigation. Of course, it wouldn’t work at all without Ptolemy’s accurate mapping of the sky.
The astrolabe was used for centuries before clocks came along. Even after clocks it was used for predicting when sunup or sundown would occur. This was important in the Muslim world, where the faithful need to pray at exact times, like sunup. The Koran says that it’s sunup when it’s light enough to tell the difference between a black thread and a white thread, but an astrolabe tells you when sunup will happen beforehand—by looking at the stars.
Notice that around the rim of the astrolabe the circle is divided into 24 hours of the day. Each hour takes up 15 degrees of the 360-degree circle. If you’ve been following this blog for the past year, you’ll remember the Sumerians came up with that idea—a 360-degree circle uses the Base 60 system of counting. This is an example of using distance to calculate time. In this instance, not miles traveled but degrees around a circle. The hours are distributed equally along the circle of Earth’s horizon.
Back to the beginning of The Western Civ User’s Guide to Time & Space