Tag Archives: Ptolemy

Good news for canaries

Ptolemy published a book titled Geography which gave the latitude and longitude for over 8,000 places.

Ptolemy’s map* shows all of Earth he knew back then: Europe, the north coast of Africa, India and most of Asia. The Prime Meridian of Ptolemy’s map runs through the Canary Islands just west of Africa. Everything else is east.

As mapmaking took off as a business in the Age of Exploration, mapmakers in every little country, state and principality used a different Prime Meridian. You can see how this might get confusing.

1200px-PtolemyWorldMap

 

* Maybe Ptolemy drew this map, maybe he didn’t. The locations on the map, though, are based on their coordinates given in Ptolemy’s book.

Back to the beginning of The Western Civ User’s Guide to Time & Space

The Equator

Human brains are marvelous. Something we humans are able to do is look at a round object, like the Earth, and draw it flattened out as a map. Then, get this: other humans are able to look at the flat map and comprehend a round Earth. Yay, us!

Ptolemy was the first to think of drawing lines of longitude and latitude on a map. Longitude lines travel from north to south poles. Latitude lines travel east to west and are parallel to the Equator. Together they make a grid on a flat map. Longitude and latitude lines are man-made; they don’t exist except on maps. Longitude and latitude lines are the way we organize the Earth’s surface so we can navigate on it. You assign the lines names or numbers.

Latitude lines (also called parallels) are circles that run around the circumference of the Earth and are measured in degrees. The Equator divides the Earth into northern and southern hemispheres (hemi = half, sphere = globe). It’s at 0 degrees. The North & South poles are at 90 degrees.

Back to the beginning of The Western Civ User’s Guide to Time & Space

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.