A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2013 Nov 6, 16:59 -0800
Lu, you wrote:
"When you say "longitude by chronometer," how? Prime Vertical sights, trying to average across a Meridian Transit sight, or what? "
A logbook would say "lon by chro" to distinguish from "lon by acct" (DR) or "lon by lunar". But whether they get the GMT by "chro" or by "lunar", the local time came from a plain old time sight as I described previously. I should add that they didn't call it a "time sight" back then. That name is a later invention. Instead it was called finding the "true time". True time was the original navigator's name for "Local Apparent Time". While there were other procedures that could be used, standard practice was to shoot the Sun in the morning or the afternoon and then work the short logarithmic calculation as I described previously. EVERYBODY did it this way. It was easy. There are very few situations in history where you can say "everybody" and have it mean something like 99%, but this is one. You mentioned "Prime Vertical" sights. You may be under the impression that time sights had to be taken when the Sun was exactly east or west (and thus on the prime vertical). That's not the case. Roughly east or west (within 45°) is fine, and exactly east or west is only required when the latitude is very uncertain.