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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
