The gnomon was basically a stick stuck vertically into the ground. It was used by ancient civilisations to measure the length of the Sun’s shadow at different times. At the winter solstice the shadows are longest and at the summer solstice the shadows are the shortest during the year. At the equinoxes the path of the shadow traced by the upper tip of the gnomon is a straight line.
Between AD 721 and 725 during the Tang Dynasty (618-907), the Buddhist monk Yi Xing (673-727) and the Astronomer Royal Nangong Yue attempted to measure the circumference of Earth. Using gnomons to make simultaneous measurements of the shadows at the summer solstices at nine stations covering more than 3500 km (≈2200 miles) on a nearly north-south axis, they determined that one degree latitude on Earth corresponded to almost 155 km (≈96.3 miles), today’s value is about 111 km (≈69 miles).
In the tenth century AD, to obtain greater accuracy in their measurements, Arab astronomers began to build huge stone instruments because the larger intervals engraved on them could be read more accurately. Arab influence is thought to be present in the building by Guo Shoujing (1231-1316) at Gaocheng in 1276 during the Yuan Dynasty (1271-1368) of a masonry tower with a forty-foot gnomon.
The armillary sphere was invented to show the apparent movement of the Sun, Moon and the planets around Earth. It is essentially a cut-open celestial globe with rings to represent the orbits of the celestial bodies. Many armillaries include rings for the horizon, meridian, equator, the tropics, the polar circles and the ecliptic. The sight was pointed at a star and the angles read off the rings to identify its position.
In China the armillary sphere first appeared during the Western Han Dynasty (206-00-09). Luoxia Hong (c.130-c.70 BC) made an equatorial armillary sphere to make observations for his new calendar. In 52 BC Geng Shouchang (fl.75-49 BC) introduced the first permanently fixed equatorial ring. During the Eastern Han Dynasty (AD 25-220) the astronomers Fu An and Chia Kmuei added the ecliptic ring by AD 84. Zhang Heng (AD 78-139) added rings to represent the horizon and a meridian of altitude about AD 125.
Zhang Heng connected an armillary sphere to a water drive to make it rotate so that observers were able to read the position of the planets at any time. Yi Xing (683-727) applied the earliest known escapement mechanism to a water-powered celestial globe to regulate its motion. Su Song (1020-1101), a Northern Song Dynasty (960-1127) scientist, invented a water-driven astronomy clock tower which combined the functions of observing the stars, recording astronomy data and telling the time.
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