Introduction: The vernier system and longitude adjustment.
Aten shown, others similar.
Here is a diagram of the vernier system used on our Aten and Aten II heliochronometers. As you can see, the system consists of an hour dial engraved every 15 degrees with an hour line and numerals, much like the face of conventional clocks. In addition to the hour lines, there are smaller lines; these are used to line up with the vernier scale for more precise measurements.
The vernier scale attaches to the pivot that carries the analemma and the sight. It is adjustable for longitude. Major lines of longitude are spaced every 15 degrees around the earth to coincide with 1 hour increments, but they are not always at exactly 15 degrees due to the inconvenience that a time shift would entail if within one city. In the diagram, the longitude adjustment lines are shown with an arrow, they are in 1 degree increments, this equates to 4 mins. per line. To set the instrument to your longitude, first determine where you are in relation to the major lines of longitude and your time zone. We are in Tucson, Az. at 111.1 W longitude, so our heliochronometers must be adjusted 6 degrees to compensate for the prevailing time zone at 105 degrees W. The vernier is adjusted by means of a curved slot and screws/tapped holes on the bottom of the vernier which attach it to the pivot, there is a series of tapped holes in the vernier, so many adjustments are possible by repositioning the screw(s).
Below is a photo of the area and vernier, note the red line on the vernier has been moved a tad over 6 lines (degrees) to the left of the center line of the pivot. This is to accommodate the 111.1 W. longitude here. Any line on the vernier can be used as a reference to the Long. lines on the pivot, or, during setup, you can "pencil in" your own reference mark.
