Article of the month January

Bi-gnomon sundial 2

Sundial for clock time

Sundial for clock time
An idea of Hendrik Hollander

Last month, December 2005, we showed a horizontal sundial incorporating, in a special way, both longitude correction and the equation of time.
That sundial used two gnomons, one for each half year, but the hour line pattern was the same for both periods.
We will now discuss a further development of this sundial.

Latitude 52 N, longitude correction 40 minutes, summer or standard time.

Instead of two gnomons, one centrally placed vertical cone is used. The circle represents the base of the cone, the cross its top.
On lengthening days we use the shadow of the right-hand side of the cone, and on shortening days that of the left-hand side . (looking north with the sun in the back.)

The principle of the sundial readout remains the same. Again, one should know the date.
The time is read at the intersection of the appropriate date line and cone shadow.
In this example, the hour lines are numbered both for summer and for standard time.

Sometimes, the cone shadow does not quite reach the appropriate date line.
In the previous model, the gnomon could be lengthened; no such solution is possible here. We can use a ruler to extend the shadow line.

An advantage of this implementation of the bi-gnomon sundial is that some early morning and late evening hours may be read.

There is more that you can do with the cone. You could use it with an elliptical sundial face, or have it lean over. We will not discuss these possibilities now.