The astrolabe is an ancient scientific instrument dating back to 170 B.C. Complex planetary astrolabes were used to measure the altitude of celestial bodies and to track their movements. From these calculations, latitude and time could also be determined. In the sixteenth century a simpler nautical or mariner's astrolabe was developed for navigational use.
The simple astrolabe consists of an outer disk with the circumference marked off in degrees. At the centre of the disk is a movable pointer called the alidade. To use, the navigator aligned the horizontal axis of the astrolabe with the horizon. He then pointed the alidade at the sun or polar star and read its position on the outer disk. This measured the angle of inclination of the sun or star from the earth's horizon. By consulting an ephemeris or astronomical tables with this reading, the navigator could fix his latitude.