What, then, was the astrolabe? It was an instrument used for taking the altitude of the sun and stars. For two hundred years before it was used by the Christian seamen of the Mediterranean, it had been employed by the Arabian pilots in the eastern seas. The derivation of such a curious word is not without interest. The Arabic is “asthar-lab,” and this in turn came from the two Greek words, ἀστήρ and λαμβανω, meaning “to take a star.” It consisted of a flat brass ring, some 15 inches in diameter, of which an excellent illustration can here be seen. It was graduated along the rim in degrees and minutes, fitted with two sights. There was a movable index which turned on the centre and marked the angle of elevation. When the mariner wished to take the height of the sun with this instrument he proceeded as follows: The sun being near the meridian or south, the pilot observed the same until it reached its greatest height. Then, holding the ring on one of his fingers, he turned the alhidada up and down until he saw the shadow of the sun pass through both the sights thereof, being sure that the astrolabe hung upright. The astrolabe was best for taking the height of the sun when the sun was very high at 60, 70, or 80 degrees; for the sun, coming near “unto your zenith,” has great power of light for piercing the two sights of the alhidada of the astrolabe, and then it was not good to use the cross-staff (reference to which will be made below), because the sun hurt a man’s eyes and was also too high for the cross-staff. Furthermore the astrolabe, was a more correct method than that of the cross-staff.