What is a sundial?

The sundial works just like the stick and the stones. People put them in their gardens or on buildings and tell the time by the moving shadow. The trouble was they didn’t work in the dark!

The first device for indicating the time of day was probably the gnomon, dating from about 3500 BCE. It consisted of a vertical stick or pillar, and the length of the shadow it cast gave an indication of the time of day. By the 8th century BCE more-precise devices were in use. The earliest known sundial still preserved is an Egyptian shadow clock of green schist dating at least from this period. The shadow clock consists of a straight base with a raised crosspiece at one end. The base, on which is inscribed a scale of six time divisions, is placed in an east-west direction with the crosspiece at the east end in the morning and at the west end in the afternoon. The shadow of the crosspiece on this base indicates the time. Clocks of this kind were still in use in modern times in parts of Egypt.

Another early device was the hemispherical sundial, or hemicycle, attributed to the Greek astronomer Aristarchus of Samos about 280 BCE. Made of stone or wood, the instrument consisted of a cubical block into which a hemispherical opening was cut. To this block a pointer or style was fixed with one end at the centre of the hemispherical space. The path traveled by the tip of the pointer’s shadow during the day was, approximately, a circular arc. The length and position of the arc varied according to the seasons, so an appropriate number of arcs was inscribed on the internal surface of the hemisphere. Each arc was divided into 12 equal divisions, and each day, reckoned from sunrise to sunset, therefore had 12 equal intervals, or “hours.” Because the length of the day varied according to the season, these hours likewise varied in length from season to season and even from day to day and were consequently known as seasonal hours. Aristarchus’s sundial was widely used for many centuries and, according to the Arab astronomer al-Batt?n? (c. 858–929 CE), was still in use in Muslim countries during the 10th century. The Babylonian astronomer Berosus (flourished c. 290 BCE) invented a variant of this sundial by cutting away the part of the spherical surface south of the circular arc traced by the shadow tip on the longest day of the year.

Credit : Britannica 

Picture Credit : Google

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