Tag Archives: Ptolemy

And it doesn’t need batteries

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

Hey, what time is it?

We’ve talked about telling time with sundials and water clocks and hourglasses. Those things are a headache to carry around. Mechanical clocks, like a pendulum clock, wouldn’t be invented until 1637. What if you’re traveling around in ad 800—how do you know what time it is?

One way to tell time was this fantastic little device called an astrolabe.

This sketch is based on a beautiful antique brass astrolabe.

Wherever you happen to be, if you can see the Sun or the stars, you can tell the time if you’re carrying an astrolabe with you. The main feature of an astrolabe is a flat map of the sky—with the stars and planets on a grid. The grid—called a climate—shows the sky as it appears in your part of the world. It’s circular and fits into a circular frame, called the mater (Latin for ‘mother’). On top of the climate is the rete (Latin for ‘net’), an openwork circular plate with pointers that you can line up to point at the Sun or a specific star on the climate. On top of that is a sighter—a straight arrow kind of piece. All these spin on the same axis. You pick a star, adjust the rete to point at your star on the climate, and hold up the astrolabe and sight the actual star along the sighter. When the sighter lines up with the star, you can read the time with remarkable accuracy. Here’s a video showing how it’s done. This guy even made his own astrolabe. And here’s more.

Here’s a website that explains how to use an astrolabe and even gives you pdfs you can download and print to make your own.