Article of the month November

Foster-Lambert sundial, part 2

The previous month, we noted that the horizontal Foster-Lambert dial may be used not only with the style between pole style P and vertical gnomon Z, but also with one at right angles to that style.

Thus, there arises a double sundial with two patterns of hour points running in opposite directions.
The picture shows the patterns for 52 degrees latitude.
Note that – in general – for the same date scale, the radii of the hour scales are different.

If such a double horizontal sundial is made for 0 degrees latitude, the hour scales have equal radii in relation to the date scale.
Below is a study model from 1982. It is inclined for use at a different latitude.

The double Foster-Lambert sundial is self-orienting. One does not need a compass to place it correctly.
When both hour scales indicate the same time, the sundial is oriented correctly.

It is instructive to observe the shadows of the two styles moving across the sundial face in contrary motions.