Engineer Maurice Rosart first spotted the light, which marks the equinox and the start of spring.
If the sun shines, the light will be visible at just after 11.35 for the next few days, falling lower on the statue as the sun rises each day.
In autumn, for the autumn equinox, visitors can see the green light (again, if the sun shines) in the six days preceding the equinox, which happens at 12.24 on September 23.
Find out more in this article from the March issue of Connexion.