Hi Murray,
Actually, the ball at Greenwich drops at 1:00 pm every day. For
everyone who can get to London, the observatory is well worth at least
a half day visit, they have several working Harrison clocks and yes
you can set your wristwatch at 1:00 when the ball drops :) there is
also a wonderful transit.
Cheers,
Bob
On Nov 4, 2010, at 4:30 PM, Murray Greenman wrote:
Navigators used chronometers to determine their longitude. If they
were
stopped in one place long enough, they could work out longitude by a
complicated process of star and lunar observations; however, when they
left an established port, they usually took with them a time standard
based on local measurements of the sun and the known location of the
port.
These measurements used a device called a 'Transit', which was a
simple
telescope mounted so that it pivoted in elevation, but was fixed N-S
in
azimuth. Midday was marked by the time at which the sun transited the
telescope. It thus had higher resolution than a sundial. Getting N-S
axis correct involved determining by iteration and surveying the axis
that gave maximum elevation at time of transit.
Once the transit was observed, a large ball on top of the building was
dropped, indicating midday, and in some locations a cannon was also
fired. Ships in port could observe the ball drop and hear the
cannon. To
this day the ball drops at midday at Greenwich.
73,
Murray ZL1BPU
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