NavList:

Gary LaPook writes:

I was re-reading some old postings and realized that I had not responded to one of the questions posed by George.

"But I wonder whether HO 249 is as quite as convenient as has been made out.
True, it seems to me, if you're just doing a round of star-sights with the
choice of seven stars as listed in vol 1, they are all to be found together.
But then, if you want to add a planet, or the Moon, with HO 249 you seem to
have to switch, not just to another page, but to quite another volume, 2 or
3 depending on your latitude, and then these different bodies (or a
low-declination star) may require a scan of different, but closely spaced,
pages. Have I got that right?"

It is true that if you wanted to work in a moon or planet sight with your star sights you would need to switch volumes and methods. But, why would you want to add a planet or the moon to your perfectly good star sight fix? The moon's main utility is that it can be used for a two body fix with the sun during the day when the stars are not visible and there is no reason to use it at night when the stars are available. There is no reason to shoot a planet at night either when there are many more stars than you can possibly need up in the firmament. I have heard that in some cases a planet may be visible during the day for a fix with the sun but I myself have never been able to find a planet during the day. I suppose the moon and the really bright planets might be useful at night to a navigator who has not learned to identify the stars (it is pretty  easy to identify the moon) and such a navigator might also be able to pick out the planets.


gl

George Huxtable wrote:

John Karl asked-

"Which brings me to the one question I asked in the book that I can't figure
out myself:  Why are the tables in H.O. 249 and H.O. 229 ordered
differently?  I much prefer H.O. 249's order because latitude changes slowly
at sea, while we're always skipping around in LHA."

And Gary LaPook added-

"That is the same question for which I have never gotten an answer. The
arrangement of tables in H.O.249 and H.O. 214 is much more convient, often
allowing you to do the entire round of sights with only one book opening.
So who's bright idea was it to use LHA instead of latitude in H.O. 229?"

Comment from George-

I don't know anything about HO 229 and HO 249 except what I have read about
them in Dutton's, and that ignorance may become apparent here. All I have
been familiar with is their predecessor, HO 214, in its British vesion.

But I wonder whether HO 249 is as quite as convenient as has been made out.
True, it seems to me, if you're just doing a round of star-sights with the
choice of seven stars as listed in vol 1, they are all to be found together.
But then, if you want to add a planet, or the Moon, with HO 249 you seem to
have to switch, not just to another page, but to quite another volume, 2 or
3 depending on your latitude, and then these different bodies (or a
low-declination star) may require a scan of different, but closely spaced,
pages. Have I got that right?

Of course, HO 249 achieves its condensation by sacrificing precision,
working only to 1 minute instead of 0.1 minutes. And that allows the chosen
stars to have fixed positions, whereas aberration alone can shift some stars
by up to 0.3' each way, over a year. But it leaves another question in my
mind. How does HO allow, for its named stars, for the slow drift in star
position caused by precession? Is there some way provided of correcting for
this drift, year on year? Or is Vol 1 of HO249 reissued after a period, when
precession starts to put star positions unacceptably far out?

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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