How to Calculate With a Sundial

Determine the sundial type. While there are more than 200 types of sundials, many can be grouped into a few broad categories, such as a horizontal dial (found on a flat surface), a vertical dial (90 degrees from the earth), a solar dial (equatorial) and magnetic dial (azimuth), the latter of which tells the day of year and not necessarily the time of day. Horizontal and vertical dials are most common in the southern and northern hemispheres, while equatorial sundials are only found at the equator.

Confirm that the sundial is scientifically sound by looking at the precise angle of the gnomon, which is the pillar or rod that casts the shadow onto the ellipse. For instance, a properly constructed horizontal dial will be made with the gnomon parallel to the earth's axis (which is tilted approximately 23 degrees). To determine if this has been done, measure the angle between the gnomon and the ground, which should be the same as the latitude of the location. Other indicators include size (smaller sundials can be more difficult to construct and read than larger dials), and position (the dial should be positioned on a north/south plane).

Look to see where the shadow falls on the ellipse to tell solar time. Sundials are built on the principle that people on earth perceive the sun to be moving through the sky at an arc of 15 degrees every hour. When every 15 degrees represents one hour on a sundial, it is possible to calculate the hour based on which degree the shadow from the gnomon is falling.

Consult the "Equation of Time" graph (see Resources) to determine how many minutes should be added or subtracted from the solar time read to determine clock time thanks to seasonal axis shifts of the earth. Depending on the season, clock time and solar time can differ by as much as one hour, depending on the latitudinal location.

Convert solar time to clock time. Time was standardized as a concept and social practice in Britain in 1847 in Greenwich, England, with the need for the British railroad to operate on a shared temporal schedule. To get standard clock time from a sundial, add four minutes to every longitudinal degree west of Greenwich (which is 0) or subtract four minutes for every degree to the east.