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    Re: Celestial Navigation
    From: George Huxtable
    Date: 2002 Jul 1, 21:39 +0100

    Vic Fraenckel  wrote-
    >I joined this Mail List back in March. I have not received any mail from the
    >list in the past month or two. Is anyone out there any more?
    There has been a reduction in traffic on this list in the last 6 weeks or
    so but if you have received nothing at all, something has gone wrong. But
    in that case, you won't receive this message either, so how do we tell you
    so? I will send this message to the Nav-L list and also copy it to your own
    email address.
    >I have wanted to do tableless celestrial navigation on my computer for quite
    >some time and have been searching for a way to do this. My trusty Google
    >engine finally hauled me to Paul J. Heafner's "Funamental Ephemeris
    >Computations" and the "JPL Planetary and Lunar Ephemerides" CDROM.
    I am unaware of the Heafner work. But I wonder if you are taking the hard
    road to your goal.
    It is quite possible, and very satisfying, to write your own substitute for
    the almanac, especially if you are a software professional. I am only a
    dabbler, but have used for many years routines I've written in Bastard
    Basic to run on a Casio programmable calculator. It will be much easier on
    a proper computer.
    Put it together a bit at a time, starting with the Sun, which is the most
    useful body. You will need this Sun prediction routine when you come to
    compute planet positions (a much more complex matter). Stars are relatively
    simple but (depending on how wide a choice of stars you intend to offer)
    involve entering a lot of data precisely.
    I know, and thoroughly recommend, Jean Meeus' "Astronomical Algorithms"
    (2nd ed. was 1998) and its paperback predecessor, "Astronomical Formulae
    for Calculators (2nd ed.was 1982), from which the later work was enlarged.
    For the job you are tackling, you should certainly have one of these at
    Be aware that astronomers and navigators have some rather different
    concepts. Where we use altitude and azimuth, they prefer to have a position
    in the sky defined in terms of declination and right-ascension, which are
    marked on the graduated circles of their polar-mounting telescopes, and in
    which a star's position is very-nearly constant. When an astronomer
    condescends to use azimuth, it is often measured from the South point, not
    clockwise from the North. Time is often measured by astronomers in terms of
    sidereal time rather than in UT (GMT).
    Navigators usually think in terms of the convenient fiction of the Sun
    going round the Earth, rather than vice versa. So to obtain the direction
    of the Sun from the Earth, in, say, Meeus, you have to use the formulae for
    the direction of the Earth from the Sun, and then take the exact opposite.
    I'm not familiar with the JPL Ephemerides, but understand that they take
    predictions to a much higher level of accuracy than a navigator would ever
    need. Meeus, too, gives many more terms in his expansions in terms of angle
    than a navigator, aiming at an overall accuracy of 0.1 minutes of arc, ever
    requires. It calls for a bit of common sense to truncate the operation at a
    sensible level. Meeus provides guidance about this. Even so, predictions
    for the most difficult cases, Jupiter, Saturn, or the Moon, may call for
    nearly 100 such terms.
    This will allow you to compute, for the next 100 years or so, the position
    of any navigational body as seen from the centre of a transparent Earth
    (just what the almanac does, for one year at a time). Given an assumed
    position, it's then easy to compute the necessary corrections for
    height-of-eye, dip, and refraction, to a sextant observation, and derive a
    position line. Meeus has the formulae for doing this.
    Make no mistake, the operation you propose will involve a lot of
    head-scratching and puzzling, but it's worth it in the end. And it's far
    more satisfying to use routines you have written yourself, so you can
    tinker and adapt them, rather than copy in someone else's routines that you
    may not fully understand.
    If you still find difficulties, the Nav-L list will, I'm sure, do its best
    to help.
    Enjoy the challenge!
    George Huxtable.
     I have
    >been studying the book to try and get an understanding of what's going on in
    >there. In the past couple of months I have progressed about 1/3 of the way
    >thru the book and have tried out many of the routines found there. Now that
    >I have invested a goodly amount of time in this project I have come to the
    >point of wondering if I am barking up the wrong tree. The focus of the book
    >is Astronomy. As I am coming into this as a beginner as far as the Astronomy
    >is concerned, I would like to know or understand that this research is going
    >to provide me with what my goal is - tableless celestrial navigation.
    >Programming is NO mystery to me as a retired Software Engineer. I am
    >assuming that given the understanding of extracting data from the JPL
    >Ephemerides and massaging it with software will yield me the figures that
    >are equilvalent to those that we all can find in the Nautical Almanac and
    >that I will end up with the proper algorithms to go from sextant to a
    >position that is reliable.
    >ANY enlightenment would be greatly appreciated.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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