Article of the month September

Seesaw Sundial

In a senior citizens complex in the German health resort Hartha, some 15 km from Dresden city centre, is located since August 2009 the instrument shown below.
Originally, this was a seesaw, made in 1988 for a children’s holiday park in Grillenburg, not far from Hartha.
The seesaw can of course seesaw, but it can also turn about its vertical axis.
The part where the children would normally sit is now inaccessible.

The main part of the seesaw is a vertical sheet.
By rotating it until its shadow is just a line, one aims this sheet at the Sun, and, in principle, it shows the sun’s azimuth.
However, there is no azimuth scale or compass rose in the instrument, although the four cardinal points are indicated on the ground of the new location.

On the fixed part of the cylinder pedestal is an azimuth sundial.
This sundial has a date scale with parallel lines used twice; for the lengthening, and for the shortening days.
There are curved hour lines that read local apparent time.
A zero-mark on the rotating part of the cylinder, where the city name Grillenburg is shown (not visible in the photograph), indicates where to read the sundial.

On this sundial are several other names of places all over the world.
If one first determines the time of day in Grillenburg, and then turns the top part until a particular city is above the readout point, the turned Grillenburg mark will indicate apparent time in that locality.

The azimuth world sundial

In the centre, on the rotating part, there is a horizontal semi-cylinder with scales, as shown in the photograph below.br> To the right is an altitude scale ranging from 16 to 62 degrees.
Two circles are fixed to the vertical sheet, enabling measurement of the sun’s altitude at any time.
One first rotates the seesaw to the correct azimuth, and then elevates it until the shadow on the horizontal cylinder of one of the circles shrinks into a line.

When performed at true noon, this altitude measurement will show the actual zodiacal date on the central scale, while the solar declination may be read on the left.

Note: the relocation from Grillenburg to Hartha is only minor, and the subsequent readout error is negligible.