Origins

The origins of the Astrolabe were in classical Greece and it was originally an elaborate astronomer’s instrument used for finding the position of celestial bodies. In about 1480, the astrolabe was adapted for maritime use. The most influential individual on the theory of the astrolabe projection was Hipparchus who was born in Nicaea in Asia Minor (now Iznik in Turkey) about 180 BC. He discovered the precession of the equinoxes, redefined and formalized the projection as a method for solving complex astronomical problems. However, more recently, the use of the instrument is said to dates back to Eudoxus of Cnidus (409 – 356 B.C.).

How it works

Astrolabe was originally used by astronomers to find the position of the stars and planets. Position of various stars and planets are marked on the face of the astrolabe and by setting the moveable parts of the astrolabe to a specific dates and times, the entire sky, both visible and invisible, is represented on the face of the instrument. Typical uses of the Astrolabe include finding the time during the day or night, finding the time of a celestial event such as sunrise or sunset and as a handy reference of celestial positions. It was also one of the basic astronomy education tools in the late Middle Ages.

Diagram of an Astrolabe

The Mariner's Astrolabe (left) is the adapted version of the Astrolabe used solely for navigation. It is a much simpler device compared to the typical Astrolabe, consisting of a heavy ring suspended from a thumb ring. The circumference of the ring is marked in degrees and the alidade is pivoted in the center of the ring with pointers against the scale for measuring celestial altitudes. Using the thumb ring, the astrolabe is held above eye level. The alidade is then rotated until the Sun or star is visible and the altitude is then read off the scale.  

When using the Sun to measure altitude, the observer, instead of holding the astrolabe above eye level, he can just rotate the alidade till the sunray, through the upper aperture of the upper sight throw a spot of light onto the lower aperture.

Remarks 

Similar to the Quadrant, the angle measured by the Astrolabe is with respect to the vertical, and thus, the reading will be affected by the deflection of the ship. However, it is more stable than the Quadrant because of its weighted base and hence measurements can be made more quickly. On the other hand, it is more inconvenient to hold the instrument above one’s head when one need to find out one's position using the star.

Another similar instrument was the Ring (below) which is a heavy metal ring with graduation on one side of the inner arc and a small hole on the other side, 45 degree from the top. This instrument can only be used for measuring altitude using the sun as it requires the sun ray. The instrument is held vertically with the small hole facing the sun. The spot of light that enter the hole will fall on the inner arc of the ring and the altitude can be read off the scale. The advantage of the Ring to the Astrolabe is that the angle that can be measured by the Ring is bigger.

Diagram of the Ring