Surprisingly for the modest size of the “Tortugas” ship, two bronze astrolabes recovered in the stern half of the ship near the pump well and a third from the southern extremity of the site were available to its captain. The forerunner to the sextant, the astrolabe was widely utilized in the 16th and 17th centuries as a navigational device to measure the angle of a celestial body in the sky and to determine latitude in conjunction with declination charts and tables. The device was developed for shipboard use by the Portuguese in the mid-15th century. With solar tables and an accurate instrument with which to measure the height of the sun at meridian passage, latitude could be computed without viewing the Pole Star.
Suspended by a ring from the thumb, the instrument was held at arm’s length and the center alidade rotated so that the sun or specific star could be sighted through a hole in the vertical plates. The altitude of the object was read by noting the number the alidade pointed to on a scale of degrees engraved around the perimeter of the device. A book of tables was consulted to interpret the reading and to estimate latitude. Astrolabes went out of use after the mid-17th century, and finds of subsequent date are rare.