Article of the month February

Sundials topsy-turvy

Nearly all point sundials project a shadow point on a surface marked with a pattern of hour and date lines.
The shadow of the point is where we read the time and date in the sundial pattern.

It is possible to do it the other way around: the sun projects a pattern of hour and date lines onto a surface marked with just the readout point.
We read time and date in the projected sundial pattern at the readout point.

The sundial pattern can be, for example, on a window. The readout point can lie on the windowsill, or perhaps even on the floor.

Southeast and southwest dial for 40 degrees north latitude.

This model applies the principle to a combination of a southeast and southwest dial.
The readout point is on the horizontal surface. It is almost half past two, and the date is probably near the end of August, or the middle of April.
Our member Thibaud Taudin Chabot first presented this type of sundial within the Zonnewijzerkring in 1984.
A plan of the sundial is shown below. It was published in the NASS journal Compendium, 1994, Vol. 1 nr. 3.

Southeast and southwest dial for 40 degrees north latitude.