A small hole in the roof lets through a ray of sunlight, shining on the wall or on the floor. The actual time and date can be read from the intersections of the horizontal blue lines (date) and vertical red or green lines (time), by matching the same colours as for the labels. The green lines are used in spring, the red in autumn. They differ because of the inconstant speed of the sun over the year, known as the equation of time.
Remember that due to the occurence of leap years the sun might be off by half a day from its designated blue date line. Also the solar declinations of, for example 15 February and 27 October, although drawn with the same line, are not exactly the same. In fact if one looks carefully then over the course of the day it can be seen that the sun will move up in autumn (February) and down in spring (October) away from the given date line.
Construction started in March 2005 by marking the places where the sun shone on particular days and hours; every week, every 15 minutes. By the end of 2005 enough points were obtained to measure their coordinates and put them in a theoretical model. Using the method of the least squares the optimal parameters could be found and the position of the wanted points calculated. Then the first half of 2006 was used to drill holes on these positions, put wires in between, and label them.
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