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This discussion started with criticism of the entry about Lunar Distance
(Navigation) in Wikipedia, which went as follows-

"[edit] Theory
If there are two people, one at Greenwich and one 15 degrees West, the time
by the sun will be one hour later at 15 degrees West. So, if the person
observes the position of the moon at Greenwich at noon and another person
observes the moon 15 degrees west of Greenwich at their locally determined
noon, then due to the one hour difference, although the sun is at its
zenith, the moon would have moved approximately its own diameter across the
sky."
================

In respect of quibbles about the usage of "zenith", Fred wrote-

| Another meaning for zenith given by the Oxford English Dictionary
| (http://www.askoxford.com/concise_oed/zenith?view=uk) is "the highest
| point in the sky reached by a given celestial object."  This is an
| additional meaning to the point directly overhead.  However, both the
| Wikipedia and Encyclopedia Britannica mention only the point directly
| overhead.  It would appear the scientific usage is restricted to the
| point directly overhead.

Thanks for that information. I was unaware of that usage of "zenith", but
prompted by Fred, I have since found corresponding entries in other
dictionaries.

And Wikipedia did use the phrase "the Sun is at its zenith", so clearly it
was referring to the Sun's zenith, whatever that might be, and not the
observer's zenith.

Perhaps, then, we should follow Fred and stop quibbling about that usage,
especially as it's obvious what was intended there.

But I think the other quibbles about the Wikipedia entry stand.

Fred Hebard wrote-

"I read that to mean they are determining the time of local apparent noon,
and that it will be one hour later 15 degrees west."

And I replied-

"Well, the time of local apparent noon is always noon."

Fred has come back by saying-

"The GMT time, or even some other mean time."

====================
Here's a diversion, about mean and apparent time.

I suggest that Fred has just added an unnecessary layer of complication, and
done us a disservice by involving the difference between apparent time and
mean time (the Equation of Time), which here is only serving to muddy the
waters.

Longitude (from Greenwich) is derived simply from the difference between
local time and Greenwich Time, at 15 degrees per hour. Either the difference
between the apparent times, or the difference between the mean times; those
differences being (almost) the same. If you stick to the same system, mean
or apparent, there's no reason to be bothered with the equation of time.

Before they had chronometers, mariners had no interest in mean time. Noon by
the Sun was noon apparent time, and up to 1834 , throughout the heyday of
lunar distances, the Nautical Almanac worked almost entirely in terms of
apparent time. Lunar distances were tabulated every 3 hours, starting at
noon, but those times, and that noon, were apparent times, and apparent
noon. Indeed that label, "apparent" that we give to such moments now was
seldom bothered with then, because apparent time was simply "the time", just
as now, mean time is "the time".

End of diversion.

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

Fred wrote-

"I read that to mean they are determining the time of local apparent noon,
and that it will be one hour later 15 degrees west."

If we stick to the same sytem, apparent time, will Fred allow us to expand
his statement to read-

"I take that to mean they are determining the apparent Greenwich time of
local apparent noon, and that it will be one hour later 15 degrees west".

And, of course, that is perfectly true. At noon, in 15 degrees West, the
Greenwich time would be 1 pm. If the Wikipedia entry had expressed it in
those words there would be no problem.

From a time-sight of the Sun or a star, local apparent time is deduced. From
a lunar, Greenwich apparent time is deduced. From the difference, longitude
is deduced. Simple as that.

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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