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    Re: October Lunar
    From: George Huxtable
    Date: 2008 Oct 6, 18:20 +0100

    Jeremy wrote about his lunar on 2 Oct, and Frank replied in [6345].
    
    Jeremy was worried that its accuracy would be badly affected by the low 
    altitudes of the two bodies, and the shorter-than-usual lunar distance being 
    measured.
    
    Frank responded-
    
    "The low altitudes may be a problem for some lunars but probably not in this 
    case. ..
    You suggested:
    "Is it strange refraction due to low altitudes?"
    Because the two bodies were at nearly equal altitudes, we can safely rule 
    this out. First of all, if there were any unusual refraction, it would 
    probably affect both objects by nearly the same amount. More significantly, 
    since the lunar distance was nearly horizontal, the corrections to that arc 
    due to the altitude corrections (which are entirely vertical) is much 
    reduced."
    
    That calls for a bit of qualification. We can't rule out refraction as a 
    problem "because the two bodies were at nearly equal altitudes", in the 
    general case. It's only negligible because they were at nearly equal 
    altitudes, and in this case were ALSO rather close in azimuth, because the 
    lunar distance was, unusually, so short. Only then does the lunar distance 
    become nearly a horizontal line, and the refraction correction much reduced.
    
    Frank added-
    "The short distance is not an issue (two reasons: the altitudes are 
    calculated so their accuracy is not a problem, and also we don't have to 
    interpolate between geocentric distances three hours apart, which was an 
    issue historically but not today)."
    
    I don't understand the relevance of "the altitudes are calculated so their 
    accuracy is not a problem" to the acceptability of a short lunar distance. 
    Anyway, even if the altitudes had been measured rather than calculated, that 
    measurement can be made to quite sufficient accuracy.
    
    Frank is right about the interpolation question, but that left unaddressed 
    the other problem about short-distance lunars; that a planet, even though 
    never far from  the Moon's orbital plane, can get way out-of-line with the 
    Moon's direction of travel on near approach. This is starting to show up in 
    Jeremy's example, where at a guess (estimation, not calculation), looking at 
    a sky map of that day, Venus is around 40� off from the line of the Moon's 
    path. That reduces the sensitivity of the lunar distance as a means of 
    assessing Greenwich time, to something like 75% of its nominal value. If 
    measurement of lunar distance is treated as an end-in-itself, then that is 
    unaffected. As we have discussed before, the estimated longitude error, 
    stated in Frank's reduction program, doesn't allow for that angular offset, 
    so paints an over-optimistic picture in such a situation. He is considering 
    a revision, which would be useful.
    
    contact George Huxtable, now at george{at}hux.me.uk
    (switched from 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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