Serle’s Ruler

R.L. Kellogg, PhD

Noon

6pmx

Measuring The Latitude of A SundialIf you have a sundial, then youcan use a protractor to measure the gnomon’s angle and determine the dial’s latitude.

Commercial dials usually havea “one size fits all” approach, using a generic latitude of 40 or 45degrees.

Specially built sundials havea gnomon tailor made for theirplaced location. If the dial ismoved to a different latitude, thedial no longer keeps precise solartime.

Some dials have “reworked” gnomons for their new, displaced homes. Theowner mistakenly things that by just altering the angle of the gnomon, thedial will tell correct time at its new latitude. But as you know, the dial plateis also made for a specific latitude.

= latitude

Measuring the Latitude from a Dial Plate

Although we could measure the various dial hour line angles and workour mathematics backward, there is a simple way to both test dialsand create new ones.

The tool is called Serle’s Ruler. A copy of the ruler reproduced by theNorth 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

Serle’s Ruler – Step OneStart with the Dial Plate (or a copytransferred to paper). Align theruler so that the ends alwayslie on the noon line and the 6pmhour lines (arrows)

Carefully tilt and slide the rulerkeeping the end points on thenoon and 6pm hour lines until the hour line scale marks from 1pm to 5pm match up with thecorresponding 1pm to 5pm dial hour lines.

When aligned, mark the point where the ruler touches the 6pm hour line (red X).

6pmNoon

Noon

6pmx

Serle’s Ruler – Step Two

x

Now place Serle’s Ruleralong the dial’s 6pm line, withthe latitude scale starting atthe dial’s south point.

At the mark on the 6pm lineread the dial’s latitude (thisdial here has a reading of about 34°).

The measurements of thegnomon angle and the dialplate latitude should agree.

If not, it could be a “generic”dial that was commerciallyassembled for quick and lowcost sale;

Or the dial could have been moved from its originalsite and the gnomon refitted (under the falseassumption that reshaping corrects the dial’s abilityto tell time … there are a number these “discordant” dials with non-matching gnomon anddial plate, and usually an interesting story behind the dial and its owners.

Noon

6pm