NavList:

Frank, you raised two interesting points about time balls in your last post :

1. Duration of fall: I attended a short presentation by a docent of the Royal Greenwich Observatory on the day I spent there.  He mentioned that the time signal was indicated by the moment the ball was released from the top, and not by the moment it arrived at the bottom.  If this is the case, then the speed of the ball or the amount of time it took to fall isn’t relevant.  The same explanation was mentioned in the lecture that Anna Rolls (formerly a curator in the Greenwich Observatory) gave to the Horological Society of New York, seen in the YouTube video recently linked by Adrian F (thanks for finding this, Adrian).  She also explains how the fall was, and still is, dampened to prevent the ball from crashing down. Using the onset of motion as a signal makes sense considering the physiology of vision.  Among other features of a visual stimulus, the visual system of humans and other animals has evolved to perceive motion, so detecting onset of motion is a sensory task that may be accomplished accurately.

2.  Timing the ball’s drop at 13:00 rather than at 12:00: The explanation I heard in the docent’s talk for not dropping the ball at 12:00 (that the observatory staff  were observing the Sun’s meridian transit at 12:00 + equation of time), was also put forward by Anna Rolls in her lecture. But the explanation you have heard also has the ring of truth.  The moments during the change from the forenoon watch to the afternoon watch seem particularly unsuitable for training a spyglass on a time ball. Perhaps 13:00 was a convenient time for both parties (ball droppers and ball watchers), and became the traditional time .

As an aside, a general caveat when considering past traditions is that the reasons for them were considered so self-evident in their time that no one bothered to write them down.  After these traditions become quaint relics of the past, later generations propose reasons which may or may not be the original ones, and that get passed on as “docent lore”.  In this specific case, no one is alive today from the days in which the Greenwich time ball was a useful timekeeping signal rather than a charming attraction for sightseers.  The docent I heard in Greenwich seemed to be very well informed, though.  For example, he mentioned that the 1707 Scilly Isles naval disaster wasn’t the most important reason for passing the 1714 Longitude Act, and that this disaster was caused by errors in latitude estimation as much, if not more, than by errors in longitude estimation.

Rafael