Article of the month January

Azimuth sundial 1

It is a simple matter to measure the direction, or azimuth, of the sun. The shadow of a vertical post shows it directly.
The figure below shows a horizontal scale, divided every ten degrees. It is a matter of agreement where zero is.

Divisions every 11 ¼ degrees make the scale a compass rose, measuring azimuth according to the points of the compass.

Azimuth scale in ten-degree divisions

We are interested in determining time from azimuth. This determination depends on date and latitude. Therefore, a date scale is constructed.
In this article, we use a circular date scale. The twelve months of the year are represented by twelve circular bands, but other choices are possible.

For each hour, the different azimuths over the course of the year are calculated for the latitude where the sundial is to be used. The azimuths are plotted and so form the hour lines.

The first example is for a latitude of 52 degrees and apparent solar time.
The date scale runs from the outside, for January, to the inside for December.

January outside, December inside
Apparent solar time

The second example shows the same sundial for the same location, but with the date scale running from July on the outside to June on the inside. The pattern looks completely different.

July outside, June inside
Apparent solar time

Now suppose we want the dial to read like a watch, showing civil time. That is possible by incorporating the longitude adjustment and the equation of time in the dial design.
The last example shows a sundial reading standard time. During summer, one hour should be added, or a second set of numerals could be added for summer time (daylight saving time).
The symmetry in the hour line pattern is now gone, and the equation of time has made the hour lines look quite capricious.

January outside, December inside
Standard time