[time-nuts] Setting clocks 100 years ago

[Murray Greenman]

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