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Renee wrote-

| I have recreated the simple summary in METHOD, and put Jim's practical
| details in a sub-section called "in practice".  Rather than describe
| how two sights and a time can be used to find position, I link the
| interested reader to the article on celestial navigation.  This, I
| think, removes George's objection that the reader could be confused
| about how time, longitude, and star sights were actually used in the
| age of lunars, as opposed to how they might be used by lunarians
| today.
|
| In Errors, I again simplified and reorganized the discussion, moving
| the discussion of longitude prize into the History section.  I find it
| easier to follow in its new form.  I did mention the wreck of
| Shovell's squadron.  I noticed that, though the section says clearing
| involves atmospheric corrections, it does not say where, when, or how
| the corrections are made.  Are the corrected altitudes plugged into
| the parallax formula?
|
| I moved the mention of the longitude prize to "history", and could
| have moved the wreck of Shovell's squadron there as well, but I
| thought the wreck of the squadron gave context to the subject of
| Error.

=======================

Renee is giving this topic plenty of necessary attention, but in some
respects it seems that the latest changes may be a step back, rather than an
improvement. Also, perhaps some earlier statements could do with some
revision. I will get out my nit-picking tool, and wield it about.

But please, anyone, argue back if you disagree.



Figure caption-

"The altitudes of the two

bodies are used to make corrections and determine the time."







"In Celestial navigation, precise knowledge of the time at Greenwich and the

positions of one or more celestial objects are combined with careful

observations to calculate latitude and longitude."







"But reliable marine

chronometers were unavailable or unaffordable until well into the 19th
century."



What about inserting "to many mariners" after "unaffordable"?



The navigator then consults a prepared table of lunar distances and the

< insert "Greenwich"?> times at which they will occur



Having measured the lunar distance and the heights of the two bodies, the

navigator can find Greenwich time in three steps.

  Step One -- Lunar Semidiameter

  Almanac tables predict lunar distances between the centre of the Moon and
the

  other body.[citation needed] However, the observer cannot accurately find
the

  centre of the Moon. Instead, lunar distances are always measured to the

  sharply lit, outer edge ("limb") of the Moon. The first correction to the

  lunar distance is the distance between the limb of the Moon and its
center.

  Since the Moon's apparant size varies with its varying distance from the

  Earth, almanacs give the Moon's semidiameter for each day.[citation
needed]







Because the Moon is so much closer to the Earth than the

  stars are, the position of the observer on the surface of the Earth shifts
the

  relative position of the Moon by up to an entire degree[citation needed].







 The

  parallax correction is a complicated function of the observed lunar
distance

  and the altitudes of the two bodies[citation needed].









  Step Three -- Finding the Time

  The navigator, having cleared the lunar distance, now consults a prepared

  table of lunar distances and the  times at which they will
occur in order to

  determine the Greenwich time of the observation.[1][5]

Having found the  time, and having the altitude of a star as
well as the Moon,

the navigator can now use the techniques of Celestial Navigation to find the

vessel's position at the time of the observation.[1]







 Errors

  Effect of Lunar Distance  Errors on calculated Longitude

  A lunar distance changes with time at a rate of roughly half a degree, or
30

  arc-minutes, in an hour[citation needed]. Therefore, an error of half an

  arc-minute will give rise to an error of about 1 minute in Greenwich Time,

  which (owing to the Earth rotating at 15 degrees per hour) is the same as
one

  quarter degree in longitude (about 15 nautical miles at the equator).





  Almanac error

  In the early days of lunars, predictions of the Moon's position were good
to

  approximately half an arc-minute[citation needed], a source of error of up
to

  approximately 1 minute in Greenwich time, or one quarter degree of
longitude.







 "Knowledge of latitude to within 30

  nautical miles would have prevented the wreck of Admiral Cloudesley
Shovell's

  squadron in 1707.[citation needed]"



< Here, what is referred to is knowledge of latitude, not of longitude. And
it's quite true that the Shovell disaster was related to a poor acount of
latitude being kept. Made worse by a 10-mile error in the positioning of the
Scillies, 10 miles too far North on the chart. Even so, Shovell had no
business to be so far North. Although Sobell makes much of the Shovell
disater, it had little or nothing to do with longitude, and it's quite
misleading to refer to it at all, in the context of longtitude. It could,
and should, be scrubbed.>



George.



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.


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