Using a Simple Astrolabe

An astrolabe can be used to measure the altitude of an object, including changes in the Sun's path over the course of the year. Tracking these changes can help explain why days are longer in the summer and shorter in the winter.

 Workers at the observatory of Taqf ad-Din in Istanbul (in the year 1577) work with an astrolabe. The painting is from Shahinshah-nama (History of the King of Kings), an epic poem by "Ala ad-din Mansur-Shirazi. Photo courtesy Johann Wolfgang Goethe Institut.
 What You'll Need 1 - astrolabe (see Activity 7, "Making a Simple Astrolabe", for directions on how to construct one). 1 - Altitude of the Sun chart 1 - pencil or pen

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 What to Do

E X P E R I M E N T     1

Measuring the Altitude of Trees and Buildings

To become familiar with how an astrolabe works, practice measuring the altitude (angular height) of trees or buildings. To make a proper measurement, look at the top of the object through the straw.

Have someone read the altitude in degrees from the side of the astrolabe. The point where the string crosses the scale is the proper measurement.

Practice using your astrolabe by measuring and recording another tree or building of a different height.

Now that you have an understanding of how an astrolabe works, you can use it to measure the motion of the Sun.

E X P E R I M E N T     2
Measuring the Altitude of the Sun

Because it is harmful to look directly at the Sun, a new method for measuring the Sun's altitude must be used. Hold the astrolabe so that the straw points in the direction of the Sun. Do not look through the straw.

Aim the straw so that you see the shadow of the straw on your hand. Move the straw slightly until a small circle of light forms on your hand. The straw is now pointing directly at the Sun.

Ask someone to read the Sun's altitude (in degrees) where the string crosses the scale. Take note of the time of day the reading was made.

One day a week, at the same time each day, measure the altitude of the Sun with your astrolabe. Make three consecutive measurements and record them in the chart provided. Be sure to include the date.

As the weeks progress, look at your measurements of the Sun's altitude. Can you detect a change? Is the altitude increasing or decreasing? Is there a pattern of change? How can you explain these changes?

What's Going On
The changes you notice will be different depending on the time of year. In the spring, the altitude of the Sun increases. In the Fall, you should notice a decrease.

The cause of the change in altitude (i.e., the location of the Sun) is the tilt of the Earth's axis, which causes the Earth to face the Sun at an angle of 23 degrees. Where the Earth is located in its orbit around the Sun will determine both the altitude of the Sun at any given point in time and the length of the day. Since the Earth's location around the Sun is changing continuously, so is the Sun's position in the sky.

 Related Websites The Astrolabe: An Instrument with a Past and a Future www.astrolabes.org "equinoxes, precession of the" From Britannica Online www.britannica.com/eb/ article?eu=33424&tocid=0

Activity 8: Using a Simple Astrolabe Derived from "Making Measurements of Objects in the Sky": from Science Resources for Schools: Doing Science, Vol 3, No. 1. Copyright 1985 by the American Association for the Advancement of Science & the Smithsonian Institution.

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