Armillary spheres were developed by the Greeks and were used as teaching tools already in the 3rd century B.C.. In larger and more precise forms they were also used as observational instruments. No material advance was made on Ptolemy's instrument until Tycho Brahe, whose elaborate armillary spheres passing into astrolabes are figured in his Astronomiae Instauratae Mechanica. Armillary spheres became popular again in the Late Middle Ages; the Danish astronomer Tycho Brahe (1546-1601) constructed several of such instruments. Renaissance scientists and public figures often had their portraits painted showing them with one hand on an armillary sphere, which represented the height of wisdom and knowledge. Armillary spheres were among the first complex mechanical devices. Their development led to many improvements in techniques and design of all mechanical devices.