Back to Astrolabes

Contents

• The Front of the Astrolabe
• The Latitude Plate
• The Rete
• The Back of the Astrolabe
• The Rule and Alilade

Overview

The astrolabe is nothing more than a flattened version of the celestial sphere. Just as an atlas of the earth is a projection of the globe, the astrolabe is a projection of the celestial sphere and our local horizon. The particular type of projection used to construct an astrolabe is known as a stereographic projection. Roughly speaking, on the clear plastic rete the northern half of the celestial sphere is flattened into a disk while most of the southern hemisphere (which we can't see much of anyway from our latitude) disappears off the edge of the disk. The local horizon is projected in the same way onto the latitude plate beneath the rete. If you happen to live in the southern hemisphere, be patient. I'm currently working on a southern hemisphere version of the Astrolabe software.

The front of the astrolabe (below) is a representation of our local view of the night sky. It has a clear plastic disk (the rete) representing the heavens and a rotating object resembling a ruler (the rule). This side of the astrolabe is used to locate the sun and stars in the sky and to read the local time. The central portion underneath the rete (the latitude plate) is drawn for a specific latitude, in this case for Stevens Point, Wisconsin. The outer ring labeled with Roman numerals is called the mater (the mother) because in a traditional astrolabe it was designed to hold a series of removable latitude plates. At the very top is the throne containing a hole from which the astrolabe can be vertically suspended by a ring.

Front

The back of the astrolabe (below) is simply an analog computer for calculating the position of the sun in the zodiac for a given day. It also contains markings that allow one to make simple trigonometric calculations (the shadow boxes). Finally, it has a cross bar similar to the rule called the alilade which is both a calculating instrument and an observing instrument. It can be fitted with vanes that look like gun sights for actually observing the height of astronomical objects. The vanes are not shown in this diagram.

Back