Article of the month September
Equator projection sundials 2
The horizontal Foster-Lambert sundial, which we described previous month, has a gnomon intersecting the plane at an angle of 45 + ½ phi degrees.
However, it is also possible to use a gnomon at right angles to that. It meets the plane at an angle of 45 - ½ phi degrees and is oppositely directed.
The figure shows both gnomons in red.
Notice how one gnomon points north, the other south.
The next two figures show the projections for both gnomons.
Last month we gave the first projection; the second is for the other gnomon.
In both cases, the hour scales have the same diameter.
Notice how the hour points in the second sundial run in the opposite direction.
Furthermore, the date scale is rather longer and even extends outside the hour circle.
We can combine both sundials.
To this end, we scale the second sundial so that its date scale has the same length as that of the first.
It is particularly fascinating to observe the contrary motions of the two shadow lines on the combined dial.
