Drawing of an astrolabe from Medina's Arte de Navegar, 1545. The seaman's astrolabe was a graduated ring or disc fitted with a sighting rule pivoted at the center. The instrument was suspended so that it hung vertically. The user then turned the sighting rule so that the sun or star could be sighted along it and the altitude read off the ring. It dates back to ancient Greece and was heavily used in the Islamic world. The first records of a seagoing version date to around 1481.
For sun sights, the ring was pivoted so that the spot of sunlight from the top sighting hole fell on on the lower hole.