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I do not accept Frank Reed's chidings when he writes, in [5591]-

Gee, George, give the guy a break. OF COURSE, it was simply a slip in his
email text. He meant "tan(zenith distance)". That's obvious.

Well, that's what I thought, too, which was exactly why I said so. There was 
no need for Frank to say so a second time. And whatever the reason behind it 
it, it's still an error. And as an error, it still needs to be corrected, 
and I make no apology for doing just that.

Indeed, when I get something wrong myself (which happens more often than I 
would wish), I hope, and expect, that some alert Navlist member will 
promptly pick it up and put it right. What's the alternative? Should we let 
mis-statements stand, in order to "give the guy a break"? Nonsense.

As for the constant value in that formula, as Kent was providing it to two 
decimal places, he might as well use a modern value adopted by astronomers, 
rather than another one that isn't, wherever that came from. The practical 
difference, in navigation, is indeed negligible; though greater that the 
"single arcsecond" that Frank allows, because Tan (zenith distance) can much 
exceed 1.

I wrote that the formula breaks down, and badly, at lower
altitudes (than 20�), and then a more complicated expression is required."

To which Frank responded-

"This is well-known and presumably OBVIOUS for anyone who has looked at the
matter for more than two minutes."

Not obvious to Kent, it appeared, who wrongly ascribed it to the need for 
changed corrections for temperature and barometer. Well-known and presumably 
obvious to Frank, perhaps, but because something is well-known and 
presumably obvious, does that mean that it shouldn't be said? That would 
rule out much Navlist correspondence.

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

I had described the complex procedure, invoked by Kent for obtaining 
longitude, as "roundabout", to which Frank responded-

"The reason his version is so "roundabout" is because he's describing the 
procedure for getting local time from observations of stars (or other bodies 
besides the Sun)."

No, it isn't. Think again, Frank. You can use the simple procedure I 
described for the Sun, exactly the same for any other body, by taking its 
GHA directly from the Nautical Almanac, just as was done for the Sun, and 
adding or subtracting the Local Hour Angle, just as was done for the Sun. 
Provided, that is, that you use an Almanac from the last half-century, which 
provides GHA of bodies, rather than their Right Ascension. For a star, GHA 
calls for GHA Aries and SHA to be added, just as you always have to do for a 
star, but nothing more complex than that.

The archaic procedures invoked by Kent, involving RA and sidereal time, may 
well have been needed when working with almanacs from a bygone era, so need 
to be understood by anyone doing historical reconstruction using those 
almanacs; but not by the rest of us.

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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