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A display of historical Chinese astronomical instruments on the roof of the Beijing Ancient Observatory in Beijing, China, an astronomical observatory that dates from the 1400s. From left to right: equatorial armilla (1673) to measure true solar time and right ascension and declination of celestial objects; sextant (1673) to measure angular distance between any two stars less than 60 degrees apart, as well as angular diameters of the sun and moon; azimuth theodolite (1715) to determine azimuth and altitude (elevation angle) of celestial objects; ecliptic armilla (1673) to measure ecliptic longitude difference and latitude of celestial objects.(KEYSTONE/SCIENCE PHOTO LIBRARY SPL/MARK WILLIAMSON/SCIENCE PHOTO LIBRARY)