Sundials

Before modern inventions and the age of industrialization, people all over the world developed methods of measuring time through astronomy. One method of keeping time was the use of a sundial. A sundial is rightly named after its measurement based on the position of the sun relative to the earth. This involves a complex understanding of geometry and astronomy to keep accurate measurements. The earth completes a rotation about its axis in about 24 hours and therefore turns by about 15 degrees per hour (curious.astro.cornell.edu). With the positioning of the sundial, the sunlight hits it in a way that you would be able to tell the time by the shadow it produces.

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A Sundial in the Horticulture Center in Fairmount Park, Philadelphia wikimediacommons.com