The astrolabe was known by the sixteenth century. The marine astrolabe is based on the astronomical astrolabe. It is very heavy and, in use, is suspended from a strong swivel thumb-ring. The sights line up the pointer arm with a star or the sun, and the angle is read on the outer ring.

     The stamp below features al-Zarqali of Cordoba, a geometer, astronomer and maker of instruments. He produced a set of trigonometric tables for use in astronomy.

     The astrolabe can either be sighted directly at the sun, or the shadow of the sun can be measured indirectly. The illustration is from Willem Blaeu's The Light of Navigation, 1612.

      The problem with these instruments is that the navigator may have to look directly into the sun, with the result that over time eyes are damaged. Some people say this is how pirates came to wear their famous black patches – they went blind staring at the sun! 

     "The Persian mathematician and astronomer Jamshid al-Kashani (or al-Kashi) (d. 1429) made extensive calculations with decimal fractions and established a notation for them, using a vertical line to separate the integer and fractional parts. A prodigious calculator, he determined π to 16 decimal places and obtained a very precise value for the sine of 1°, from which many other trigonometrical values can be determined."

Robin J. Wilson, Stamping through Mathematics, Springer-Verlag, 2001, p. 26