An astrolabe consists of a circle marked in degrees (similar to a protractor) with a rotating arm attached at its center. When the 0° mark on the circle is aligned with the horizon, and a star (or other celestial body) "sighted" at the end of the movable arm, the position (in degrees) of the star can be read ("taken") off the calibrated circle (hence, "astro" = star + "labe" = to take).
On the plate (mater) are engraved coordinate lines which represent a stereographic projection of the celestial sphere (climate), valid for places at a specific geographic latitude. Over this coordinate grid rotates the rete, which is a framework with spikes whose points represent fixed stars. After adjusting the instrument for the current time, the position of a star can be read from the coordinate grid. Conversely, the instrument can be adjusted to fit the measured position, and the time can be read off the scale.
The astrolabe therefore is a predecessor of the modern planisphere.
The astrolabe was probably invented by Hipparchus, and developed further in the Islamic world, before reaching Europe in about the 14th century. The English author Geoffrey Chaucer (~1343 - 1400) wrote a treatise on the astrolabe for his son. In the 15th century, the French instrument-maker Jean Fusoris (~1365 - 1436) started selling them in his shop in Paris, along with portable sundials and other popular scientific gadgets of the day.\n