What is The astrolabe ?

The astrolabe is a scientific instrument dating back to 170 B.C. Complex planetary  astrolabes were used to measure the altitude of the planets and stars to track their movements. From these calculations, latitude and time could also be determined. In the 16th century, a simpler nautical or mariner's astrolabe was developed for navigational use. The one pictured above is believed to be the astrolabe used by Christopher Columbus. He would have aligned the horizontal axis of the astrolabe with the horizon of the sky. He would then have directed the moveable pointer  ("Alidade") towards the sun or polar star and read the position on its outer dial.