Article of the month September

Bi-Gnomon Sundial, part 3

Sundial for Mean Time
An idea by Hendrik Hollander

The two-gnomon sundial game never ceases to surprise.
Earlier publications on this theme are in “Article of the month” for december 2005 and januari 2006.

Here we present a new idea, using a cone gnomon
with the axis of the cone parallel to that of the earth.

The sundial has straight hour lines, but the equation-of-time is incorporated nevertheless.
The date lines are adjusted, but the finished dial looks almost like an ordinary sundial.

And yet, the sundial reads mean time.

For this novel concept, Hendrik Hollander received the

2006 Sawyer Dialing Prize.

Horizontal sundial for a latitude of 55 degrees north .....

..... on a slant for use on a different latitude.

Earlier published examples suffered from the drawback that for some dates, the shadow of the gnomon or cone had to be extended to the appropriate date line in order to read it.
This drawback has been completely removed by switching the cone reading edge in a different way.
Dates by the foot of the cone indicate when to use which side of the cone.

The sundial has date lines for every tenth day.
Observe where the shadow of the proper side of the cone intersects today’s date line, and read the time there.
This particular dial was made for local mean time, without longitude adjustment.