March 20 will be a chance to see a twice-a-year phenomenon in Strasbourg Cathedral when, if the sun shines, a ray of green light will fall on the pulpit above the statue of Christ.
The light, shining through a stained glass window, is said to be part of an astronomical clock and marks the equinox and the start of spring.
First spotted by engineer Maurice Rosart in 1972, the ‘clock’ is separate from the cathedral’s well-known astronomical clock that has animated figures producing a display at 12.30 each day.
The green light appears in March and in September for the spring and autumn equinox when a ray shines through the foot of Juda, one of the sons of Jacob, and on to the pulpit.
It happens at 11.38 for the start of spring and at 12.24 for the start of autumn.
Mr Rosart said: “The green ray is without doubt the work of Gustave Klotz, the architect who oversaw the restoration of the cathedral in 1875, when the stained glass was fitted.
“It is fascinating. He has made use of a natural phenomenon that works whenever the sun shines, whereas the other astronomical clock is mechanical.”
Mr Rosart, 80, discovered the Strasbourg ray in 1972 shortly after restoration work replaced the old dim glass in the foot of Juda with a more transparent version