Astronomical Clock

The main dial of the astronomical clock is a remarkable working model of the solar system as understood early in the 15th century, when Earth was believed to lay at the centre, circled by the moon and then the sun, all set against a heavenly blue.

The sun is a black disk decorated with a fleur-de-lys. It points to the hours on the outermost ring where Roman numerals I to XII are repeated, providing a 24-hour clock with 12 midday at the top and 12 midnight at the bottom. The minutes past the hour are displayed on the separate upper dial added in 1759/60.

The moon is attached between 29 and 30 on the inner ring. It is a sphere, with half its surface silver and half black, which rotates to show its phases. Day 1 is a new moon when the black half of the moon’s spherical surface is displayed. As the sphere rotates slightly each day, the amount of silver visible increases until half way through the cycle (day 14/15) when the entire silver surface depicts full moon. The amount of silver visible then decreases to show the waning of the lunar cycle.

The fleur-de-lys tail points to the day in the lunar month on the inner ring indicated by the Arabic numerals 1 to 30. Numbers 29 and 30 are close together as the lunar month is, on average, 29 days 12 hours and 44 minutes.

The positions of the sun and the moon within the dial also indicate where they are in the sky. The top of the dial represents south and the bottom is north.

PEREUNT ET IMPUTANTUR below the main dial can be translated as ‘The hours pass and are reckoned to our account.’ The phrase originates in late 1st-century writings by the Roman poet Martial and is used on many clocks and sundials.

These details supplement information provided in the cathedral on the stand Clock of the Sky.