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    Re: sight reduction tables
    From: Gary LaPook
    Date: 2007 Sep 29, 12:18 -0700

    Gary writes:
    
    H.O. 249, volumes two and three are arranged exactly like H.O. 214 and 
    are used in exactly the same way. Because declinations are only 
    tabulated for whole degrees ( not every 30 minutes as in H.O. 214)  the 
    entries for one latitude occupy only six printed pages, printed front 
    and back. Once you have opened the book to your latitude you never have 
    to flip more than three pages to work all of the sights and usually not 
    that many.
    
    The declination range covered is only 30� but this covers the sun, the 
    moon, the planets and 31 of the 57 stars listed in the Nautical Almanac. 
    H.O. 214 has continuos coverage of this range of declinations but needs 
    15 pages to do so compared to the 6 pages in H.O.249. In addition, H.O. 
    214 uses another 9 pages of discontinuous coverage to handle the stars 
    with declinations exceeding 30�. In total, H.O. 214 has 24 pages per 
    latitude versus the 6 pages in H.O. 249.
    
    So, how much of a limitation is the 30� declination restriction.? Of the 
    57 stars listed in the Nautical Almanac only 9  have declinations 
    exceeding 30� north so you lose the use of these stars such as Vega and 
    the stars in the Big Dipper (the Plow for our English friends.)  
    Seventeen of the stars listed in the Nautical Almanac have declinations 
    exceeding 30� south  meaning they will never be more than 10� above the 
    horizon in England (approximately 50� north) meaning that you would not 
    choose to use them anyway. In fact, only one of these stars, Kaus 
    Australils at 34� south, would ever be above 5�, the minimum altitude 
    tabulated in H.O 214. So for navigators near England using H.O. 249 
    versus H.O 214 there is no real loss from not being able to use these 17 
    southern stars. Navigators further south would loose the use of fewer of 
    the southern stars but but since people manage to navigate around 
    England without them navigators further south can probably get by 
    without them also. Since volume two covers 0� to 39� of latitude and 
    volume three covers all higher latitudes, most navigators can get by 
    with carrying only volume two or volume three of H.O. 249 and at the 
    worst, both volumes.
    
    Turning now to volume one. Volume one is tabulated by latitude and 
    covers the entire planet in one volume. Printed on two facing pages are 
    the azimuths and altitudes of selected stars computed for each one 
    degree interval and tabulated based on the LHA of Aries. This makes 
    finding the data for a round of sights much easier. You only need to 
    look at the Almanac daily pages once and compute the LHA for Aries not 
    for each star. If you complete the round of stars in four minutes the 
    LHA of Aries will be the same for all shots. You then figure out the 
    assumed longitude by moving it westward based on the increments pages in 
    the almanac for the differences between the times of the sights and 
    these should all be found on two facing pages in the almanac. So you 
    just look in H.O. 249 for your latitude and take out, without any 
    interpolation, the altitudes and azimuths of the stars you are going to 
    shoot from one line (LHA Aries) of the tables.
    
    Now one difference in aviation navigation versus surface navigation is 
    that the flight navigator pre computes his data so that he can plot his 
    fix rapidly after taking the shots which is important in a fast moving 
    airplane. Since his has to pre plan his shots it is natural for him to 
    look at volume one and select which of the 7 listed stars he will shoot 
    and then do his pre computation. If a surface navigator wants to use 
    H.O. 249 volume one then he must also pre plan which stars he will be 
    shooting. He can't just go one deck and pick stars at random and then go 
    below because he will find he did not pick one of the listed stars. 
    However, after the first day off shore, he will know which stars are 
    listed for the time of twilight as his position on a sailboat changes 
    slowly and he will be using the same stars every evening for weeks at a 
    time.
    
    The data listed in volume one are based on the position of the stars at 
    the beginning of the epoch, each 5 years. If you look in the Almanac you 
    find that the positions of the stars change during the year so the 
    values in volume one are only accurate on the date of the epoch. Since 
    the table is to be used for a 5 year period a table is provided to make 
    corrections to the position for other dates. For example, the current 
    volume one is for Epoch 2005 and provides a correction table so that it 
    can be used from 2001 though 2009. To the precision of the tables, no 
    correction is needed during 2004 and 2005.
    
    Most of the change of coordinates of  the stars published in the almanac 
    are not due to the stars' proper motions but are due to a change in the 
    coordinate system used to define the stars' positions, a coordinate 
    system that changes with the earth's movement. The zero point of the 
    stellar coordinate system is the vernal equinox. Since the earth is 
    precessing and wobbling on its axis this zero point moves along with it 
    making it appear that the stars have moved. Since the earth completes on 
    cycle of precession every 26,000 years, the vernal equinox moves eight 
    tenths of a minute of arc every year and this accounts for most of the 
    changes seen in the stars' coordinates.
    
    H.O.249 has a correction table for "Precession and Nutation" which 
    allows for the movement of the earth's axis and the accompaning 
    coordinate system. Corrections are listed for each year and they are 
    entered with latitude and LHA Aries. The correction tells you how much 
    and in which direction to move the plotted FIX (not each LOP) to correct 
    for the changes in the apparent positions of the stars. The largest 
    correction tabulated for 2001 is 3 nautical miles and in 2009 is 4 
    nautical miles. For a sailboat, the precession and nutation correction 
    will not change for a long time and you can use the same values for 
    weeks at a time.
    
    In summary, a navigator can get along just fine without using volume one 
    and can do his star sights using just volumes two or three exactly like 
    using H.O 214 with little loss of useful stars. If he wants to avail 
    himself of the convenience of using volume one he needs to pre plan his 
    twilight sights, at least for the first night off soundings, and after 
    that these sights are extremely easy to work.
    
    
    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---.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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