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    Re: Lunar longitudes, not by lunar distance.
    From: George Huxtable
    Date: 2009 Aug 9, 10:07 +0100

    Hanno has come round to accepting the theodolite as a suitable instrument 
    for his project. That seems wise. It's quite good enough for his purposes. 
    The astronomer's transit instrument was designed to achieve the ultimate in 
    precision.
    
    All he needs to do, once it's been placed firmly on a well-set tripod, is to 
    set up his instrument so that its vertical axis is truly vertical, for which 
    the built-in levels should be quite adequate. Then to set it so that its 
    telescope swings in a near-meridian plane. One way to do this is to 
    establish North (in the Northern hemisphere), by bisecting the most-Eastern 
    and most-Western azimuth swings of Polaris. However, that's only possible at 
    certain times of year; at others, two different stars may have to be used, 
    but this presents no problem now that stars have been catalogued so 
    precisely. Then swing by 180� to look Southwards.
    
    The time of the Moon's centre passing the central hairline is needed, but as 
    it's only the outer-limb that can be observed, corrections for semidiameter 
    are needed. The time of a nearby well-known star crossing the hairline is 
    also needed, so a timepiece with known rate is required. The closer Moon and 
    star are together, the more relaxed can be the requirements for timing and 
    for theodolite alignment. From that time difference, longitude and GMT can 
    be deduced. If star altitude is measured and corrected for refraction, 
    latitude will become accurately known.
    
    Chauvenet, in his vol.2, describes such observations, and the use of the 
    transit, in great detail, with engravings of instruments, but as he is 
    writing for astronomers who are after the ultimate in precision, he goes 
    into unnecessary detail (for us). It's easy to get distracted by his 
    accounting for the most minute corrections.
    
    The advantage of timing such Moon culminations is that there's no effect of 
    parallax, refraction, or dip to bother about. It's all very simple. Pity it 
    can't be done at sea. Such a technique will benefit from more accurate 
    predictions than those of the Nautical Almanac, as are to be found in an 
    astronomical almanac or on a computer-predicted website.
    
    Please note that I've never actually made the type of observation described 
    here, nor even do I possess a theodolite, so others that know better may be 
    able to make corrections to those words.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 
    
    
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