Perhaps the most unexpected finds on the “Tortugas” shipwreck site were three bronze mariner’s astrolabes, an impressive number for such a small ship.
The mariner’s astrolabe was developed for shipboard use by the Portuguese at the School of Prince Henry the Navigator at Sagres in the mid-15th century. The earliest recorded use of a sea astrolabe was by the Portuguese Diogo d’Azambuja in 1481 on a voyage down the west coast of Africa. Early astrolabes were large wooden instruments suspended from a frame. Smaller brass instruments such as the “Tortugas” examples replaced the earlier wooden variety in the 16th century to become essential parts of navigational assemblages.
The forerunner of the sextant, the astrolabe was widely used by sailors in the 16th and 17th centuries as a navigational device to measure the angle of a celestial body in the sky and hence to determine latitude in conjunction with declination charts and tables. With solar tables and an accurate instrument with which to measure the height of the sun at meridian passage, latitude could be computed without reliance on viewing the Pole Star. Suspended by a ring from the thumb, the instrument was held at arms’ length, and the center alidade rotated so the sun or a specific star could be sighted through a hole in the vertical plates. The altitude of the object was read by noting the number to which the alidade pointed on a scale of degrees engraved around the perimeter of the device. A book of tables was then consulted to interpret the reading and estimate latitude.
Mariner's Astrolabes went out of use after the mid-17th century, and finds of subsequent date are rare. Despite its small size and status, the Tortugas navio was thus surprisingly well stocked with this sophisticated class of navigation equipment. Today only 101 examples are currently known to exist.