NavList:

   

Reply

Bill and George,

you are touching my sore tooth by your tongue. I remember very well, what you 
are referring to. George Huxtable had stated in his paper on lunars and in 
other postings to the list that the effect of the daily parallax changing 
with altitude alters the rate of the momentary change of a lunar distance 
considerably and that this effect is the more sensible, the nearer the Moon 
approaches the local zenith in her daily path through the sky. As George 
said, no mention about this effect was made in the rich literature on lunars 
in the past, with one exception of the short passage in an old German 
handbook.

I corresponded about this matter with George off list - I supposed that such 
effect (which is undeniable, owing to George's observed values of lunars) is 
present only in uncleared, "apparent" distances and removed by clearing them 
from the influence of the daily parallax. In my opinion, it would be very 
awkward if this effect were observed in true distances and unnoticed in all 
the literature on lunars written since 1755.

I planned to compute a series of true lunars from ephemerides values for a 
geographical position where the Moon crosses the sky near the zenith, to 
unclear them to the apparent lunars and to compare both - would be the 
retardating effect of the parallax changing with altitude observable only in 
apparent distances (as I suppose) or in true ones, too? But I didn't fulfill 
this my plan yet. Therefore, I didn't discuss this matter in the list. Only 
now, when you mention it, Bill, I should respond to you.

And I can only guess - as all values of lunars and their angular errors cited 
in this thread were the true lunars and the true angular errors, their effect 
on the instantaneous time errors, be it such or such, depends only on the 
velocity of the Moon in her path, i.e. in the right ascension above all (and 
on the angular distance of the distance body from her path secondarily, of 
course).


Jan Kalivoda



----- Original Message -----
From: "Noyce, Bill" 
To: 
Sent: Tuesday, January 06, 2004 5:15 PM
Subject: Re: Real accuracy of the method of lunar distances


> I've been following the discussion of statistical tests with interest (and not a lot of
> understanding), but one statement of Jan Kalivoda's stood out:
>
> > For lunars, PE of 20" times 4.5 gives 90" = approximately 180 seconds of time = approximately
> > 45 minutes of longitude (the exact value depends on the actual velocity of the Moon in R.A.).
>
> In fact, the actual value depends on the velocity of the apparent moon and comparing
> body in topocentric coordinates, not RA.  As George Huxtable has pointed out, the rate of
> change of an observed lunar distance can be surprisingly slow, due mostly to refraction and
> parallax.  My recollection is that it can be slow enough that a 90" difference in observed
> distance could correspond to over 300 seconds of time -- is that right, George?
>
>         -- Bill
>
>
> --
> Incoming mail is certified Virus Free.
> Checked by AVG Anti-Virus (http://www.grisoft.com).
> Version: 7.0.209 / Virus Database: 261.5.6 - Release Date: 2.1.2004
>

   

Reply