The Use of the Astrolabe by 15th Century Mariners

Mariners of the 15th-century used astrolabes to determine their position at sea.

The first European metal astrolabe was produced in Lisbon in the 15th century, although the device was invented by the Greeks around 160 B.C. Mariners used the astrolabe to find their position at sea in relation to the stars. Mariners used the device for navigation until the 18th century, when it was replaced by the sextant.

Astrolabe History and Description

After its invention by the Greeks, the astrolabe went through a series of modifications before the Muslims developed the brass astrolabe in the 8th century. That astrolabe was a circular instrument with degrees marked off around its circumference. It had a pointer with two sighting vanes used to align the instrument with the stars, making celestial navigation possible. The Europeans adopted the device in the 15th century and used it to launch the Age of Discovery.

Mariner's Astrolabe

Traditional astrolabes were difficult to use from a rolling ship's deck, so mariner's developed a smaller and heavier version of the instrument designed to be more stable at sea. Unfortunately, the smaller astrolabes were not as accurate as the original devices. Mariner's astrolabes required at least three sailors when used on deck: one to hold the instrument, one to sight the star through the points and one to take the reading.

Vertical Uses of Mariner's Astrolabe

Sailors used the mariner's astrolabe in two ways. They could hang it vertically from a ring in one hand and rotate the pointer until a particular star was seen through both sights. They then took a reading that could be converted into the degrees latitude of their position. In the vertical mode, they could use the astrolabe to find the height of mountains, buildings and other objects.

Horizontal Uses of Mariner's Astrolabe

In a flat position, sailors could use the astrolabe to find the "azimuth"--or bearing from true north--of a celestial object. Sailors used this information to establish lines of position. In addition to the other changes mariners made to the astrolabe, they added two degree arcs on either side, each with a zero degree mark at the horizon and a 90 degree mark at the zenith. Sailors used these additional features to determine altitude.

References

• Brittanica.com: Astrolabe
• Immigrant Ships: Navigation Tools
• The Pirate King: Astrolabe