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-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf
Of Paul Hirose cfuhb-acdgw-at-earthlink.net |Renee Mattie on NavList|
Sent: Friday, July 27, 2007 4:49 PM
To: .....................
Subject: [NavList 3047] Re: Lunar Distance in Wikipedia


George Huxtable wrote:
> Trouble, is, the various sections on that page all interrelate, so
> it's hard to tinker with one without altering others. Nor do I think
> the way the topic is divided under different headings is entirely
sensible.

I think the article has too many headings. One for each paragraph is
excessive. The main part of the article (if you omit the Theory
paragraph) actually reads more smoothly if you pretend the headings aren't
there.


> I wonder if there are specific rules about the length of such
> contributions.

There is a Wikipedia style guide, and even an article about article size:
http://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style
http://en.wikipedia.org/wiki/Wikipedia:Article_size

The amount of stylistic guidance is somewhat intimidating. It's a good thing
you can't get fired for breaking a rule.


> "In celestial navigation, lunar distance is the angle between the
> Moon's centre and the Sun or a bright star, slanting across the sky,
> as measured using a sextant. Such an observation, usually abbreviated
> to just "a lunar", can be made by a mariner, anywhere in the World, if
> the Moon is visible, together with the Sun or a special star. Without
> needing a chronometer, it allowed him to calculate what the time was
> at some reference longitude (usually Greenwich) at the moment of that
> observation, using data which used to be published in a nautical
> almanac. That was an important step in finding his own longitude, from
Greenwich."

All of that is correct as far as I can tell. However, I believe less detail
and precision are appropriate for a lead paragraph. E.g., "In celestial
navigation, lunar distance is the angle, observed with a sextant, between
the Moon and the Sun or a star." (Were planets used too?)

It should be obvious that both bodies must be visible, so to continue, I'd
just say, "From such an observation, usually called 'a lunar', a navigator
can obtain Greenwich time. That enables the determination of longitude
without a chronometer."


> In Celestial navigation, precise knowledge of the time at a reference
> point and the positions of several celestial objects are combined with
> careful observations to calculate latitude and longitude. But reliable
> marine chronometers were not invented until 1761, and were not
> generally available for many decades afterwards. For nearly one
> hundred years (from about 1767 until 1850), the method of lunar
> distances was used to determine Greenwich time, in order to deduce the
longitude at the time of the lunar observation.
> Such time information could also be used to check chronometer error.

In the first sentence, rather than "a reference point", I would use
"Greenwich". While the former term is more strictly correct, it's also more
abstract. I think most readers will be best served by simply using Greenwich
as the basis for the almanac, time, and longitude.

The paragraph puts the era of lunars *after* the invention of the
chronometer. I don't question the historical accuracy, but it reads oddly.
Perhaps the second sentence could simply say, "But accurate Greenwich time
from chronometers was not generally available at sea until well into the
19th century." I believe that would make a smoother transition between the
first and third sentences.


> "This method relies on the relatively quick movement of the Moon
> across the background of the stars. Although the Moon, with every
> other body, appears to circle round the sky in about 1 day, with
> respect to the star background it completes a circuit in 27.3 days,
> and with respect to the Sun in 29.5 days. This implies that with
> respect to the Sun and to stars that lie near its path in the sky, it
> is moving by approximately its own diameter, about half a degree,
> every hour. So lunar distances to those bodies are generally changing
> at about that rate, some increasing, others decreasing. That motion of
> the Moon is by far the fastest such change that can be seen in the
> sky, and because it is predictable in advance, it can be used as a
> measure of time. Wherever on Earth the Moon is seen from, at that
> moment, observers will agree about that time. If the predictions they use
are based on Greenwich, that time will be Greenwich Time.

That's better, though I believe the apparent daily rotation of the heavens
is familiar enough to the general reader that it need not be mentioned. So
for the second sentence I'd say, "It completes a circuit of 360 degrees in
about 30 days, equivalent to 12 degrees per day. This implies..." This
combines less mathematical precision with a bit more filling-in on how the
figure for angular rate is derived.

Is this a good place to mention the almanac? Perhaps the last two sentences
could be replaced with something like, "If lunar distances to selected stars
are tabulated at intervals of Greenwich time in an almanac..."


> As for the bit that caused the trouble to start with; the section
> headed "Theory", I think it's awful, in so many ways, that just
> tinkering with it will not do. For example, why invoke two observers,
> when in reality there's only one? It needs a complete rewrite, and I will
think about that.

Amen. Maybe delete the current Theory paragraph and use the one under the
Method heading instead.

A "obituary" paragraph at the end would give a nice sense of completeness to
the article. E.g., "Lunars went out of fashion when..."

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