Year 2005

Work of our members

Noon mark by
Eugène Roebroeck

Is a sundial difficult? Not at all.
A pin on the wall and a vertical strip already make a good sundial that can tell us much.
The example below is on the wall of a house in Haren, Groningen.

When the sun shines on this wall, the pin, called gnonon, casts a shadow line on it. The terminus of this shadow line is the point where the sundial should be read.
The photograph above shows the shadow line only vaguely. It ends at the vertical strip. Unfortunately the sun was hardly shining when the photograph was taken.

What does this sundial tell?
When the shadow point falls on the vertical line, it is true (solar) noon.
The number of hours and minutes that the sun has been up equal those until it sets.
The sun is in the south, on the highest point of its daily track.

Your wrist watch will not show twelve o’clock. In our country, on a longitude of about five degrees East, it will show approximately 12:40 standard time, or 13:40 if summer time.
Actually, the equation of time has still to be accounted for. More on this in the May 2003 ‘Article of the Month’ archive on this site..

On the photograph below it is just before true solar noon.

The length of the vertical strip is such that at the beginning of winter the shadow point falls exactly on the top end of it, and at the beginning of summer on the bottom.
At the beginning of spring and autumn the shadow point falls on the marker, which is where the colour of the wall changes at the soldier course.
It is clear to see then that the sun, in summer, is higher in the sky then in winter.

In former times, a noon mark such as this would be placed at a church or the town hall, in order to be able to regulate the mechanical clocks. That would be a task of the tower guard.

Since 1993, this simple but meaningful sundial embellishes the front of this house.

True noon at start winter 2004.