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Subject: Re: Sumner lines
From: George Huxtable (george@XXX.XXX)
Date: Sun Feb 18 2001 - 07:32:18 EST
I wish to comment on the recent useful contribution by Nigel Gardner, in
which he says-
>It might be well to remember the ethos of the age, as a previous
>contributor has commented, one has to get into the mind of the 18/19C
>mariner, "getting a fix" was not the be-all and end-all of things. A
>longitude by the Prime Vertical at breakfast and latitude by mer-pass at
>lunchtime served most of their needs.
>
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My disagreement is with his reference to the Prime Vertical, the East-West
plane passing through the observer. Because he mentions "breakfast", then
presumably he is considering altitudes of the Sun only.
The Sun is only visible in the East-West direction for half the year.
During the Winter months, the Sun, when it would be in the East or West
direction, is below the horizon. And even when it's above the horizon, it
has to be several degrees above it to prevent altitude measurements being
bedevilled by near-horizon refraction. So, although it's true that an East
or West sighting at breakfast or supper time could give the best longitude,
in practice for much of the year the navigator has to make do with a Sun
altitude measured away from the prime vertical. Unless he restricts his
travels to the Summer months, that is.
George Huxtable.
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george@XXX.XXX
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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