Mariners were using astrolabes like this one to determine latitude by the late fifteenth century. A mariner would hold the astrolabe up at noon so
the sun shone through its two sights. He would then read the scale to determine the sun’s altitude and consult a table of the sun’s daily declination to
determine the latitude of his ship. Astrolabes with bigger pinholes could also
be used to sight stars. Because it had to be suspended for use, the astrolabe
was affected by the rocking of the ship and the wind. It was eventually replaced by the cross staff and then the back staff, both of which were cheaper to build and easier to use.
This astrolabe was reportedly found at the bottom of Manila Harbor in 1917
and may have been used on a Portuguese ship. The alidade, axis pin, and
butterfly nut may not be original. The numbered scale is inaccurate, with
"40" stamped twice and the "0" position marked as
"1." Though the instrument is relatively small, it is quite heavy,
weighing approximately 5.5 pounds.
Ref: Alan Stimson. The Mariner’s Astrolabe (Utrecht, 1988), p. 70-71.