The first astrolabes were constructed in the Islamic countries of the Middle East which had absorbed and applied the remnants of Greek science and technology beginning in the 9th century C.E. The astronomer's astrolabe was a complex and costly affair. In essence it was a mechanical computer which could be rotated over a plate on which a network of azimuths and altitudes dividing the heavens were engraved. It was used to determine the elevation of stars above the horizon. The sea astrolabe was an adaptation of the astronomical type, used to calculate latitude. It was introduced about 1460, but did not see general use until the beginning of the 16th century. Its use persisted until after 1670, particularly in the fleets of the Spanish and Portuguese, where it was in evidence early into the 18th century.