Today, he consolidates the ideas in a more structured article. The concept is important enough to warrant careful consideration. The ultimate goal of a sundial is to measure the Sun's hour angle. We can do this directly, with a pole style dial or a point dial. A point dial measures declination as well, giving us the date.
Many dials measure the hour angle indirectly, from other measured data.
They were developed more or less by accident and not through any systematic investigation into the principles concerned. From these five variables, we can solve for two if three others are known. This leads to ten combinations, or gnomonic modes . Then follow the four Az-modes, solving for azimuth. A mode is not a sundial, but may be used to characterise a group of sundials.