Sandburg Center for Sky Awareness
A Fairfax County Public Schools Planetarium

How to Set Up & Use a Horizontal Sundial

A horizontal sundial consists of the dial plate, marked off in hours, and the gnomon (including the style) that sits on the meridian (noon hour line) and sticks up from the dial plate at a right angle relative to the plane of the dial face. Place the sundial on a horizontal surface (hence the name, "horizontal sundial") and align the gnomon with true north (on the dial face, 12 noon should point north).

Finding the Direction of True North

  1. Use a compass (compensate for magnetic declination).
  2. Align with Polaris, the North Star.

For more information, visit the How to set up a horizontal sundial Web page:

Visit the SCSA Equatorial Sundials Web page for an explanation of how the sundial may be used to align itself with your local meridian:

Find your meridian by using a self-orienting Combination Analemmatic-Horizontal Sundial:

Telling Time Using the Sundial

Morning times appear on the dial face to the left side of the meridian; afternoon times to the right. Most days, Solar Time and Standard Time are slightly different (by as much as 16 minutes). This time difference is known as the Equation of Time. Add or subtract the Equation of Time as appropriate (see reference table showing daily Equation of Time).

The Sandburg Sundial features a correction for longitude (note that the gnomon is offset slightly from the meridian) so that Solar Time will coincide with Standard Time (after adding/subtracting Equation of Time). Add one hour for Daylight Saving Time.