Sun Compass

Using a Horizontal Sundial as a Sun Compass - by John C. Fakan, Ph.D.

In each image the magnetic compass is adjusted for the 5-deg W local magnetic variation.

(Click on the thumbnails for a larger image.)

Sun Compass set for

17 Jan 1999 1245

17 Jan 99 1250 - Comparison with magnetic compass

17 Jan 99 1300. Case of magnetic compass is adjusted for local variation

17 Jan 99 1520 - New location later in day

The sundial face was obtained compliments of Great Circle Studio, Pacific Grove, California, USA via their website at:

The black curves (hyperbolas) represent the months of the Zodiac. The lowermost curve in the first image represents the locus of the gnomon's shadow for 22 December. The hyperbola for 21 March or 23 September is a straight line (forth from either end). The uppermost hyperbola is for 21 June. (Note that the five inner curves each represent two months in opposite seasons.)

The 4-color curves (blue, green red, orange) representing the four seasons (Winter, Spring, Summer, Fall, respectively) are the locus of the shadow for the same civil hour each day over the year.

Note: The gnomon used above is an aluminum roofing nail cut to the appropriate length (the 'Side A' length from the Great Circle Studio pattern). The cut nail was sharpened to decrease the size of the tip's shadow, and then was inserted through a hole from the back of the light cardboard face. This gnomon has proven to be quite a bit sturdier and stable than the recommended version and is much easier to attach.


JCF - 011799