There are very few intended meridian lines in the world, one of the best being in Bologna Cathedral where a tiny opening very high in the wall results in a disc of sunlight falling on the floor. At noon, the disc crosses a meridian line (the North-South line at that point on the Earth) marked on the floor.

The Serres meridian line is not quite so conventional. Firstly, the disc is the centre of a larger stained glass window which throws coloured light into the room. Secondly, as it is in a house and not a cathedral, the height was not available and the wall too thick, for the disc of light to reach the floor in the summer. Therefore, it was decided to make a meridian line for only the six months from the autumn equinox until the spring equinox.

Finally, there are two meridian lines: as well as the true meridian, there is a line for the moment that it is noon at Greenwich (the starting point for global navigation) which is six minutes later.