Introduction

The sundial is one of mankind's oldest instruments for telling the time during daylight hours. The earliest surviving dials come from Egypt. There we find dials from as early as the 15th century B.C. with a short vertical block (called a gnomon) of finished stone at the end of a horizontal stone ruler marked with a scale of hours [6, p. 59]. When the device is turned so the gnomon faces the sun and casts its shadow on the ruler, the end of the shadow shows the hour of the day according to an approximate arithmetic scheme.

The first people to find exact methods for using shadows to tell time were the ancient Greeks, who used geometry and a geometrical model of the cosmos to construct sundials. Aristarchus of Samos invented a sundial called “the bowl” or “hemisphere,” which consisted of a block of stone with a hemisphere hollowed out of the top. From the top rim of the hemisphere a thin rod extended to the center. When the sun was shining (as it often does in Greece!) the tip of the rod's shadow moved across a net of lines on the hemisphere below and told the time as the sun moved across the hemisphere of the sky. Berosus the Chaldean modified this to form a dial by cutting away the part of the hemisphere south of the course of the shadow tip at the summer solstice. (Figures 1a and 1b show the original bowl dial and Berosus's cut-away version.)