An astrolabe, or "star grasper," was an ancient handheld computer that determined the altitude of objects in the sky, like the stars and the sun. The most common astrolabe had a celestial sphere projected on the plane of the equator. It was used to calculate the latitude by determining the angle between its position and Polaris, the North Star. It was also used for finding the time during the day or night, the time of sunrise or sunset and also the position of the stars.
The idea behind the astrolabe was developed more than 2,000 years ago in Greece. Muslim scholars learned about the astrolabe through the translation of Greek texts. The astrolabe was further developed by Muslim scientists in the eighth and ninth centuries. The oldest existing astrolabes are Arabic and from the tenth century.
Muslims were using astrolabes to aid in worship. Prayer times and the direction to Mecca could be made using the astrolabe. The calculation of the beginning of Ramadan and the Eid festivals were also made using the astrolabe. In fact, calculation of the Islamic calendar (a lunar calendar) was much easier with this instrument. The astrolabe was being used all over the Islamic world: from Persia to Spain, from North Africa to India.
Eventually, it was introduced to European culture through Moorish Spain (Andalusia). The earliest astrolabes used in Europe were imported from Andalusia with Latin words engraved beside the original Arabic. It is possible that the use of Arabic names for stars by Europeans was because of these imports. When Europe began producing their own astrolabes, they got rid of the features from the Islamic astrolabes related to the prayers.
Astrolabes were the most popular astronomical instruments until about 1650, when more specialized and accurate instruments replaced them. But their importance for astronomy education is still recognized today. You can see some of these old instruments in several museums across the United States. The Adler Planetarium and Astronomy Museum in Chicago, Illinois has the largest collection of astrolabes in the US.