Astronomical instruments

Before telescopes, astronomers measured angles: how high a star stands above the horizon. The astrolabe, a brass disc with a scale of degrees and a sighting rule, did this, and with it astronomers told the time at night, made calendars and found directions. Observatories such as the one Nasir al-Din Tusi founded at Maragheh in 1259 gathered big fixed instruments, libraries and astronomers. In 1609 Galileo turned a telescope to the sky and saw the Moon's mountains and Jupiter's moons. Today there are great optical telescopes on mountain tops, radio telescopes that catch radio waves from space, and space telescopes, such as Hubble and James Webb, above the blurring air.