Ottery St Mary Astronomical Clock

Astronomical clocks

On Monday 15th July, I went ringing at Ottery St Mary in Devon. They are a lovely 18½ cwt eight cast by Taylor’s in 1949.  Glancing around the tower (a ground floor ring with a long draught) I saw what looked like a birdcage clock mechanism in a gallery some height above the floor.

Fifteenth century clock mechanism
Fifteenth century clock mechanism

The ringers pointed out that I had missed the most important part – a fantastic astronomical clock dial, which is visible from the crossing. (Shown at the top of this post)

I was told by the ringers that the clock mechanism was found in parts by a horologist in 1907 who put it all back together ‘with the bits he could find’. You can see from the photo that much of the gearing is very badly worn, although this is hardly unexpected in a 600-year-old mechanism that has been running 24 hours a day.  The clock has ticked  about 19 billion seconds! The ‘anchor’-style escapement looks newish to me and is probably part of the 1907 renovation. The angel on top of the dial case is not an original part of the clock – it was taken from the pulpit canopy when that was removed some long time in the past.

Sadly, only the ‘going’ train is working and that is now electrically-wound, since the church could not find anyone who was prepared to wind it daily. The clock is thought to date from about 1400, but it has a pendulum escapement, which would have been added later, since these were not invented until the mid-1600’s.  Although called an astronomical clock, this one shows the time on a 24-hour dial, and the ‘age’ of the moon in days.  The moon is indicated by a sphere which is set into the dial plate, and is painted half white and half black. It turns on a radial shaft to show the phase of the moon.   However, to me the most interesting thing is that the gearing for the dials is all within the dial case – the clock mechanism simply turns a drive shaft at some steady rate – I didn’t have time to study it closely.

The dial mechanism
The dial mechanism

The ringer who showed it to me explained that unfortunately the moon-age pointer and the sun-time pointer are simply on opposite ends of the same rod, so in fact the moon-age pointer is incorrect – it only shows the clock time.  He doesn’t know why it’s like that, but I suspect the gearing for the 29.5 day moon rotation has been lost. Or perhaps it was disconnected because it added too much friction for the going-train to cope. I wasn’t able to look at this closely owing to it being quite dark and the fact that I needed to return to base.  Sadly, the clock was not fully working, showing neither the correct time nor the correct moon phase (just past new moon) as some component had been removed for overhaul by the current horologist.

This clock is quite simple by the standards of other astronomical clocks, but it is undoubtedly one of the oldest surviving examples of a clock with a dial.  The clock at Salisbury Cathedral, for example, does not have a dial, but sounds the hours on a bell – the word ‘clock’ being, of course, derived from the French ‘cloche’, meaning a bell.

Nevertheless, it involves some very intricate precision work.  It is made entirely from wrought iron, but how did those early blacksmiths form the circular wheels and cut the teeth?

Of course, the original verge-and-foliot escapements were not very accurate, as the swing of the verge depended on the amount of force applied, which would vary due to changes of stiffness in the gear trains – often gaining or losing half an hour a day.  But better than relying on the sun in such a cloudy country as England.

By this token, I can call my moon clock an astronomical clock, but I think I’d need to design a fancy dial first!



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