NavList:

   

Reply

Renee Mattie is applying a bit of discipline to the Wikipedia entry on Lunar
Distance (Navigation), and a good thing too.

But it's still not right, as I see it, and a bit more tinkering may be
called for.

It now reads-

"Theory
The local time at Greenwich is always one hour later than the local time 15
degrees West of Greenwich. Suppose one person observes the moon at Greenwich
at local apparent noon (when the sun crosses the observer's meridian). Then
suppose a second person observes the moon 15 degrees West of Greenwich, also
at local apparent noon. The two observations take place one hour apart. The
second observer will see the moon in a different position than the first
observer did because, during that hour, the moon would have moved
approximately its own diameter across the sky."

But in that hour, the Moon, just like any other object near the ecliptic,
moves through something like 15 degrees across the sky; far more than its
diameter.

What the observers have to measure is the position of the Moon with respect
to the star background, or the Sun, which is where the lunar distance comes
in. It's that angle that changes by a Moon diameter in an hour, roughly
speaking. And of course, there's no call for that mythical observer at
Greenwich anyway; what he would measure has been predicted in the almanac.

What I'm suggesting is that the whole paragraph could do with a wholesale
revision, rather than tinkering with one clause at a time. What's the best
way for that to be done, collectively, by this list? We're as clueful a body
as any to apply our wisdom to the job. It seems counterproductive for Renee
to make a change, then another of us to follow it up by another piecemeal
edit process, and so on. Wouldn't it be better for us, on this list, to
thrash out a text we all can agree on, if that's possible, and then Renee,
who has shown herself to be so adept, can act as our ambassador and make the
change on the list's behalf. Does that make sense? Would anyone like to set
the ball rolling?

And it's not just that paragraph. The introductory paragraph to the whole
article reads-

"In celestial navigation, lunar distance is the angle of the Moon's centre
from the Sun or a bright star as measured using a sextant. Given a lunar
distance and a nautical almanac, it is possible to calculate the difference
between local current time - obtained by observing the height of the moon
and the second celestial object - and current time at the meridian of the
nautical almanac - usually Greenwich - which gives the difference in
longitude between the two places."

Indeed, taking a lunar, it is necessary to observe "the height of the
Moon and the second celestial object", but that's mainly in order to get
information for "clearing" the lunar distance: correcting it for the effects
of parallax and refraction. It's quite true that under certain
circumstances, either the Moon altitude or the star altitude (no need for
both), might be used
to determine local timebut usually local time will be
found separately, from a different observation. The text gives quite a
misleading impression of why those altitudes are needed.

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

If we took a rigorous look at the whole piece, it's likely we would uncover
more deficiencies. All I have done, so far, is to quickly scan it over.

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.


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply