The clock is thought to date from around 1484.
The outermost disc, decorated with a fleur-de-lis, represents the sun, orbiting the dial once every 24 hours. The tail of the fleur-de-lis points to the day in the lunar month on the inner ring. The half black, half silver moon inside the lunar month ring rotates on its axis to show the correct phase of the moon. The earth is represented as a fixed golden ball in the centre of the dial. The inscription "Pereunt et imputantur" below the main dial translates as "The hours pass and are reckoned to our account".
A small bell located behind the clock dial chimes the quarter-hours. On the hour, this is followed by the striking of the Peter Bell in the tower above.
In 1759 the small upper dial was added with a single hand to indicate the minutes.
The clock-room is behind the dial on the north wall of the transept, and still houses the clock mechanism. Legend suggests that the hole cut in the door was for the cathedral cat to get access to keep the clock clear of mice and rats. The modern mechanism was installed in 1885 by Gillett & Bland of Croydon.
The clock is also reputed to be the source of the nursery rhyme Hickory Dickory Dock.