seen in the lower right illustration of the Persian astrolabe above. When the astrolabe is held vertically, the alidade can be rotated and the sun or a star sighted along its length, so that its altitude in degrees can be read ("taken") from the graduated edge of the astrolabe; hence the word's Greek roots: "astron" (ἄστρον) = star + "lab-" (λαβ-) = to take. A shadow square also appears on the back of some astrolabes, developed by Muslim astrologists in the 9th Century, whereas devices of the Ancient Greek tradition featured only altitude scales on the back of the devices. This was used to convert shadow lengths and the altitude of the sun, the uses of which were various from surveying to measuring inaccessible heights. Devices were usually signed by their maker with an inscription appearing on the back of the astrolabe, and if there was a patron of the object, their name would appear inscribed on the front, or in some cases, the name of the reigning sultan or the teacher of the astrolabist has also been found to appear inscribed in this place. The date of the astrolabe's construction was often also signed, which has allowed historians to determine that these devices are the second oldest scientific instrument in the world. The inscriptions on astrolabes also allowed historians to conclude that astronomers tended to make their own astrolabes, but that many were also made to order and kept in stock to sell, suggesting there was some contemporary market for the devices. Parts of the astrolabe consisted of a circular disk, a sighting tube, an alidade, and arms that held graduated scales. The circular disk is the main part of the astrolabe. It was mainly used to view figures in the sky. The sighting tube is placed on top of the circular disk and was used to observe stars or planets. The alidade had vertical and horizontal cross-hairs which plots locations on an azimuthal ring called an almucantar (altitude-distance circle). An arm called a radius connects from the center of the astrolabe to the optical axis which is parallel with another arm also called a radius. The other radius contains graduations of altitude and distance measurements. See also Mariner's astrolabe Astronomy in the medieval Islamic world List of astronomical instruments Philippe Danfrie, designer and maker of mathematical instruments, globes and astrolabes Sextant References Footnotes Notes Bibliography . . .