# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: chronometer dials: 12 or 24 hours?
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

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