How Does a Sun Dial Work?

sun-dial-work Credit: Greg Hewgill/CC-BY-2.0

A sun dial works by casting a shadow that is determined by the sun's position in the sky. Sun dials come in a variety of styles and shapes. Cheap, mass-produced sun dials often have design flaws that prevent them from accurately showing the time.

  1. Find true north

    Locate true north. This is different from compass north, which is affected by the Earth's magnetic pole. GPS systems have a setting for true north. A combination analemmatic-horizontal sun dial is self-orienting. You stick a pin in today's date and turn it so that both sundials show the same time.

  2. Make your adjustments

    Solar time and standard time may be as much as 16 minutes apart because the number of hours of sunlight varies while the length of a day is always 24 hours. You can adjust solar time to standard time by consulting the Equation of Time chart, which is included with some sun dials.

  3. Read the time line

    Look for the edge of the shadow that's furthest from the gnomon (pronounced no-mon). This shadow line should be straight from the center of the sun dial to the periphery. If the time is near solar noon, there may be a bend in the line. In this case, use the outermost part of the shadow. Morning times appear on the left side of the center meridian, while afternoon hours are on the right.