Highly versatile instruments, astrolabes could also be used to tell the time by day or by night, based on direct measurement of the height of the Sun or of a star. Astrolabes had been developed to the highest levels by medieval Islamic astronomers. In the 16th century, devices of European design were produced for measuring time by night. Nocturnals, for instance, determined the hour by the position of the last two stars in the Big Dipper in relation to the Pole Star. These instruments were useless when the sky was overcast.