An early astrolabe was made in the Hellenistic world in 150 BC. It is often attributed to Hipparchus. The astrolabe was a marriage of the planisphere (a star chart analog computing instrument) and dioptra (a tool to measure angles). It was a calculator able to work out many kinds of problems in astronomy. Theon of Alexandria wrote a complex book on the astrolabe. Lewis says that Ptolemy used an astrolabe to make the astronomical observations recorded in the "Tetrabiblos".