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05 Dec 2009 [07:45]
another glitch [link] . BUT we now know what causes the glitches: Mr Down (a College Porter) saw a pigeon sitting on the minute hand. Indeed, the glitches always occur around hh:40 to hh:50 (when the minute hand is horizontal and rising) , and always in daylight hours. The pigeons are present on the dial in large numbers especially when the sun is shining - a warm south-facing place to sit [link] . A quick calculation: suppose two pigeon each weighing 1kg are sitting on the minute hand at a distance of 0.5m from the pivot, and so rising at about 1mm/sec. They requires 10mW of power to lift them (force*velocity). Then the 80kg _going_ weight falling 9m in 7 days generates 12mW of power (again force*velocity). So a power balance suggests that a pigeon or two can easily stop the clock.

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