Pages from a Dialist's Notebook - The Laser Trigon

This page introduces a unique instrument which uses a laser beam to simulate rays of sunlight. The purpose of the instrument is to lay out sundials on large or complicated surfaces.

The device was made out of a small transit (or theodolite). A transit has two axes of rotation which are aligned at right angles to each other. The Laser Trigon uses these two axes to control the direction of a laser beam so it can project sundial hour lines, as well as the lines and curves of the sun's declination, onto any surface regardless of orientation or complexity.

The laser used is an ordinary laser pointer mounted at the center of both axes. In use, it projects a point of light onto the surface on which the dial is to be drawn. It should be pointed out that the laser only makes a point of light. It does not "burn" or have any other effect on the receiving surface.

Drawing of the Laser Trigon

When properly in position, the Laser Trigon becomes a model of the celestial sphere as it relates to the position of the sun and the local site. The instrument is mounted so the 96 tooth index gear lies in the plane of the equator where it becomes an "auxiliary equatorial dial". The 96 teeth provide the hour angles for 24 hours, although only 12 or so will be used. 94 teeth provide that each hour can be sub-divided into quarters. The laser beam is swept along the hour lines by sweeping the unlocked vertical motion up and down.

The declination index (the vertical motion of the original transit) is at right angles to the equatorial index, thus in position to control the laser for the lines and curves of declination. This index has three positions corresponding to the solstices and the equinoxes. When used to locate a declination, the equatorial motion is free to sweep from side to side so the point of laser light indicates the point(s) where the indications are desired.

Photo of the Laser Trigon This machine can project
a sundial on any surface
that sits still.

Below are three sketches of the Laser Trigon in position to draw declination indications on a dial at latitude 40° north. From left to right the declination index is shown in position to project the equinoxes, the winter solstice, and the summer solstice.

Three views of the Laser Trigon

A more complete discussion of the Laser Trigon, including a description of its origins in traditional dialing tools can be found in Compendium 3-2 of the North American Sundial Society (June 1996).

I recently built a large experimental sundial by using the Laser Trigon to project a dial on a rather unusual object. Visit the Shadow Garden,