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How Astrolabes Work

Using an Astrolabe: Are You Ready?

In the 10th century, Persian astronomer (and astrolabe adorer) Abd al-Rahman al-Sufi wrote a book claiming 1,000 uses for an astrolabe. The Persian may have been exaggerating slightly, but in the hands of a skilled practitioner, the instrument could provide answers to many problems. With an astrolabe, astronomers could calculate the position of celestial objects, the time of day (or night), the time of year, the altitude of any object, the latitude and much more.

One of the easiest calculations to make with an astrolabe is the altitude of an object above the horizon. The object could be anything -- a tree, a mountain peak, a star. To find its altitude, follow these steps:

  1. First, attach a short length of rope, string or twine through the ring at the top of the astrolabe.
  2. Hold the rope so that the instrument hangs vertically.
  3. Turn the astrolabe so that its edge points toward your target.
  4. Rotate the alidade (the clocklike hand on the back of the astrolabe) until the object lines up with both ends, or vanes, of the dial. (Note of caution: Don't sight the sun by looking directly at it. Instead, you should adjust the alidade until the shadow of the upper vane falls on the lower vane.)
  5. Read the altitude of the object using the outermost elevation scale marked on the back rim of the astrolabe. This should tell you the object's elevation, in degrees.

Cool, huh? Now let's say you want to use the astrolabe to determine when the sun will set on a particular day. Here's what you do:

  1. Find the position of the sun on the target date. To do this, use the alidade on the back of the astrolabe, turning the dial until it points to the date on the calendar scale.
  2. Read the corresponding value from the zodiac scale.
  3. On the front of the instrument, rotate the rete until the specific zodiac value obtained in step 2 touches the western (right) horizon.
  4. Rotate the rule (the clock-dial on the front) until it touches the specific zodiac value.
  5. Read the local solar time from the time scale on the limb.

More fun hands-on stuff next.