The Grosse Cloche clock and its solar equation dial

The Grosse Cloche belfry is arguably one of the best-known landmarks in Bordeaux and is prominently displayed on the city’s coat of arms. Distinguishing features today include its two surviving towers, its gold-plated copper weather vane in the shape of a large feline (harking back to the period when Bordeaux was under English rule), the Great Bell itself… and its clock, the south face of which boasts an unusual semi-circular dial.

The belfry was added in the 15th century to the remains of the 13th-century Porte Saint-Éloi, one of the gates to the commercial and political heart of the city and a natural extension to the existing Saint-Éloi church. This entrance was also known as Porte Saint-James as it welcomed a steady stream of St. James' Way pilgrims passing through Bordeaux en route to Santiago de Compostela, in north-western Spain.

Positioned adjacent to the old city hall, of which no traces remain, the bell would toll to mark events such as the start of the grape-picking season, or to alert citizens whenever a fire broke out. When, in 1548, Bordeaux rebelled against the ruling Henri II, the King retaliated by confiscating the bell. It was returned in 1561 to much rejoicing. In the 18th century, the bell which remains to date was installed. Cast in 1775, it weighs 7,800 kilograms and is both two metres tall and wide. It now rings out only a handful of times a year, at 11AM on January 1st, May 8th, July 14th, August 28th (to mark the anniversary of the 1944 liberation of Bordeaux) and November 11th; its low-frequency vibrations apparently cause the windows in the neighbourhood to rattle!

Enough about the Great Bell though, how about the clock and its mysterious dial? The timepiece was originally built between 1756 and 1759 to the designs of the mathematician and astronomer Paul Larroque. It replaced a previous model, conceived by Raymond Sudre and installed in 1567. The mechanics were overhauled by local clockmaker Gaston Guignan in 1912 and the clock was recently fully restored.

The semi-circular dial does not feature on photographs taken in the early years of the 20th century, suggesting that it may have been added during the work carried out in 1912. The dial indicates the difference in minutes between the apparent solar time (as would be indicated by the shadow of a sun-dial) and the mean solar time (as displayed by our watches). Solar time is based on the idea that when the sun reaches its highest point, it is noon. The following day, when the sun again crosses the meridian, it is noon once again. But the time which has elapsed is sometimes more and sometimes less than our convenient 24-hour units of clock time.

Positive values on the solar equation dial therefore indicate that the apparent solar time is ahead of the mean solar time. This can be by as much as much as 16 minutes and 33 seconds around November 3rd. Negative values indicate that the apparent solar time is behind the mean solar time, “peaking” at 14 minutes 6 seconds around February 12th. The dial indicates zero four times a year: around April 16th, June 15th, September 1st and December 25th.

The clock face also details information about the month and the date, although this function is, at the time of writing, out of order.

Meanwhile, on the north flank of the belfry, the sister clock face displays the different phases of the moon during the lunar month, as well as sporting two reproductions of the city's coat of arms, one above and one below the clock face... permanent and "timely" reminders of the significance of the Grosse Cloche for the people of Bordeaux!

• Find it: Rue Saint-James, Bordeaux
• Information about the local clockmaker Gaston Guignan can be found here