How Does a Sundial Tell Time?

As Earth rotates, the Sun appears to move across the sky at about 15 degrees per hour. A sundial's gnomon casts a shadow onto a dial marked with hour lines. For accuracy, the gnomon's edge should point to the celestial pole and be tilted at an angle equal to the local latitude. Sundials show local solar time, which can differ from clock time because of time zones, daylight saving and the 'equation of time', a variation of up to about 16 minutes through the year. Ancient Egyptians used shadow clocks over 3,000 years ago.