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Bruce Stark wrote:

> For the last hundred years or so, navigation authorities have been
> trying to explain the old nautical astronomy in twentieth-century
> terms. It hasn't worked very well. The logic of the old system is
> simple, but doesn't fit the present way of thinking. The world has
> changed, and the facts a navigator deals with have changed
> accordingly. Here are some of the facts that, for navigators from
> James Cook to Joshua Slocum, were too obvious to mention:
>
> (1) A lunar distance didn't give you the time. It couldn't. What it
> did was give you the longitude, so you could correct the dead
> reckoning.

This is either a play on words or reflects a  misunderstanding of what a
lunar distance does. I would guess, it is the former. A lunar distance
gives 'time' as we understand it. It gave LAT in Greenwich. Soon after,
it also gave LAT in Paris. Then it gave GMT. Now it gives UT. With an
adequate theory of the sun (which is a prerequisite for lunar theory) it
always gave sidereal time in both reference meridians and all the other
ones that were to follow. In short: A lunar distance tells you about the
momentary status of the earth rotation with respect to the celestial
sphere. This is what astronomers call time. It is true that a word can
have different meanings in different contexts. In a standard navigation
text of the 19th century 'time' will refer to Local Apparent Time, in
other words to the LHA of the sun.

It goes without saying that the historian has to familiarize himself
with the terminology of a historical text if he wants to understand it.
If Cook and Slocum did not address this issue, it's not because it was
too obvious, but because they had no need for it.

> (2) A chronometer, set to GMT, didn't give you the time either. Not
> unless it had been "regulated" with a time sight. Like a lunar, it
> gave you the longitude.

A chronometer set to GMT gave time: It gave GMT. To regulate it, you had
to HAVE longitude. Once you left land, you could no longer regulate the
chronometer with time sights. That's what the lunar observation was for,
exactly for the reason that it does indeed give GMT. The chronometer
does not give longitude. Chronometer and time sight together give
longitude. Just like today.


> (3) You found time by time sight. The hour angle between you and the
> sun was THE time. That's what you based your calculations on. NOT
> Greenwich time.

Yes.

> (4) You had no need of accurate Greenwich time when taking out data
> for working observations. A crude estimate did the job.

Oh yes, navigators had the need of accurate Greenwich time. This is why
George III paid so much money for it. It's just that before they had
Greenwich time, they had to confine themselves to whatever they could do
without it. True: They didn't need Greenwich time for working problems
that could be done without it. They certainly could not find longitude
without it.

> (5) As a navigator (as opposed to an astronomer) you needed very
> little from the Nautical Almanac beyond the predicted lunar distances.
> If you'd been able to rely on a chronometer you probably wouldn't have
> bothered with an Almanac. Your navigation manual had all the
> astronomical data you were apt to use.

In theory you could navigate with Ptolemy's Handy Tables. The almanac
merely solves you the labour of resolving a few triangles.

> I'm hoping it will stir up a controversy.

You bet.

Herbert Prinz

   

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