NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: chronometer dials: 12 or 24 hours?
From: Frank Reed
Date: 2009 Jan 18, 20:03 -0800
From: Frank Reed
Date: 2009 Jan 18, 20:03 -0800
Paul you wrote: "Why not? Chauvenet's point seems logical to me." It is logical. Custom is swayed by logic only when it improves the practical result. Chauvenet was a mathematician with some limited sea experience, but he was not a practical navigator. I believe that the concern he has would only rarely arise in practice. An ocean voyage is a continuous activity, and in the 19th century, a slow one. Each day at noon, you shoot your LAN sight and simultaneously record your DR longitude. That DR longitude immediately tells you the approximate Greenwich time, so long as you don't foul up the rules about east and west longitude. So as I sail west across the Atlantic and my longitude climbs to 15, then 30, 45, 60 degrees west, I know on each of those days at local apparent noon that the local apparent time back at Greenwich is 1pm, then 2pm, 3pm, 4pm respectively. And that will be the time on my chronometer at noon near enough (even closer if I throw in the equation of time). This estimation of GAT/GMT was a standard part of the noon sight in many navigation methods including the ones recommended by Bowditch. Of course, to calculate the noon sight correctly, you need the Sun's declination and this depends on the approximate Greenwich Time. So one of the first steps in working the sight is to take your longitude by account and convert it to Greemwich Time. They did this every day. Assuming that the navigator who does the noon Sun sight is the same as the one who works other sights (morning or afternoon time sights specifically), it's hard for me to imagine a case where they would look at the chronometer and get the hours wrong by twelve. Additionally, if time sights are taken about the same time of day, e.g. 9am local time, the corresponding Greenwich Time on the chronometer will only change by a few minutes each day. There would never be an opportunity to skip 12 hours. But maybe I'm just not visualizing the right sort of case... Losing the date, through an accounting error, strikes me as a bigger problem than losing twelve hours. For a vessel that visited a lot of ports, switching from "Sea days" to "Civil days" and back, maybe crossing the dateline for good measure, you could end up with the wrong day of the month. It would be easy to miss this until you compared notes with another vessel, as they often did ("speak" other ships to compare position and time information, that is). -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---