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contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Paul Hirose" 
To: 
Sent: Friday, July 27, 2007 9:48 PM
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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