Serle’s Ruler

1. Serle’s Ruler Noon x 6pm R.L. Kellogg, PhD

2. Measuring The Latitude of A Sundial If you have a sundial, then you can use a protractor to measure the gnomon’s angle and determine the dial’s latitude. Commercial dials usually have a “one size fits all” approach, using a generic latitude of 40 or 45 degrees. Specially built sundials have a gnomon tailor made for their placed location. If the dial is moved to a different latitude, the dial no longer keeps precise solar time. Some dials have “reworked” gnomons for their new, displaced homes. The owner mistakenly things that by just altering the angle of the gnomon, the dial will tell correct time at its new latitude. But as you know, the dial plate is also made for a specific latitude.  = latitude

3. Measuring the Latitude from a Dial Plate Although we could measure the various dial hour line angles and work our mathematics backward, there is a simple way to both test dials and create new ones. The tool is called Serle’s Ruler. A copy of the ruler reproduced by the North American Sundial Society (NASS) is shown below. Make a copy of this page and cut out the ruler for your use. or use this smaller version

4. Serle’s Ruler – Step One Start with the Dial Plate (or a copy transferred to paper). Align the ruler so that the ends always lie on the noon line and the 6pm hour lines (arrows) Carefully tilt and slide the ruler keeping the end points on the noon and 6pm hour lines until the hour line scale marks from 1pm to 5pm match up with the corresponding 1pm to 5pm dial hour lines. When aligned, mark the point where the ruler touches the 6pm hour line (red X). Noon x 6pm Noon 6pm

5. x Serle’s Ruler – Step Two Noon Now place Serle’s Ruler along the dial’s 6pm line, with the latitude scale starting at the dial’s south point. At the mark on the 6pm line read the dial’s latitude (this dial here has a reading of about 34°). The measurements of the gnomon angle and the dial plate latitude should agree. If not, it could be a “generic” dial that was commercially assembled for quick and low cost sale; Or the dial could have been moved from its original site and the gnomon refitted (under the false assumption that reshaping corrects the dial’s ability to tell time … there are a number these “discordant” dials with non-matching gnomon and dial plate, and usually an interesting story behind the dial and its owners. 6pm