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# Astrolabe: Medieval Measurements

By John Wiley & Sons, Inc.
Updated on Dec 15, 2010

In ancient times astronomers used a very simple tool to measure the height of a star in the sky. This instrument, the astrolabe, enabled scientists to create the first star charts. People relied on the astrolabe to determine the altitude of objects above the horizon. Using measurements made with an astrolabe, navigators could calculate their latitude. Until recently the astrolabe has been pivotal in helping ships find their way through the seas. Today the Global Positioning System (GPS) has taken the place of the astrolabe. In this activity you will make and test an astrolabe.

### Materials

String (1 meter long)
Protractor with a tiny hole in the center of the flat side
Drinking straw
Metal washer
Tape

### Activity

1. Tie the washer to one end of the string. Tie the other end of the string to the protractor by threading it through the hole in the center of the flat side.
2. Tape the drinking straw to the flat section of the protractor so the straw extends over both edges of the protractor.
3. Hold the astrolabe in one hand so that the curved side of the protractor is pointing downward. The string should be able to swing freely.
4. Point one end of the straw at an object near the top of your classroom, such as a wall clock or top of a screen. Look at the object through the straw. When you have it sighted, press the string against the protractor. Read the angle (degrees of the protractor) that the string falls across.
5. Subtract the angle you just measured (number of degrees) from 90 degrees. This is the altitude of the object you are sighting.

### Follow-Up Questions

1. What is the purpose of an astrolabe? What was its primary purpose in medieval times?
2. What was the altitude of the object you observed today?

1. The purpose of the astrolabe is to find the altitude of an object above the horizon. In medieval times, it was used to determine latitude.

### Extension

Go outside and use the astrolabe to sight a tall object such as a treetop or a roof. Repeat the procedure in the activity to find the angle of altitude.