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    Re: Old style lunar
    From: Ken Muldrew
    Date: 2004 Dec 15, 15:12 -0700

    On 15 Dec 2004 at 16:28, Alexandre Eremenko wrote:
    
    > On Wed, 15 Dec 2004, Ken Muldrew wrote:
    >
    > > On 15 Dec 2004 at 12:39, Alexandre Eremenko wrote:
    > >
    > > > Why did not he measure altitudes?
    > >
    > > With a sextant and an artificial horizon one can
    > > only measure altitudes up
    > > to 60?. That is enough for this series,
    > > and probably many more, but
    > > Thompson (and his contemporaries) never measured altitudes.
    > > It just wasn't
    > > part of their procedure.
    >
    > I afraid do not understand this "procedure".
    > He took a time sight anyway.
    > Why did the procedure include a separate time sight
    > instead of just measuring the altitude of the star
    > involved in the lunar?
    > This is equivalent to a "time sight", is not it?
    
    If the star is rising in the East or setting in the West, then the time
    sight and altitude could be done at the same time (adjusting the altitude
    for the different time when the lunar distance was taken). Neither
    Thompson nor Philip Turnor nor Peter Fidler ever seem to have done that.
    Since all we have are the notes they left behind, it's hard to say why
    their "procedure" was the way it was.
    
    Even when Thompson uses the sun for a lunar, I think he still calculates
    the altitude even though he uses the sun for his time sight (I'm not sure
    that I've actually checked this very carefully...I may be mistaken here).
    
    > My second question is what exactly the procedure was
    > for computing the alitudes?
    >
    > To compute an altitude you have to know
    > the declination and right ascention of the body
    > (from the almanac) and LOCAL time at the place
    > of observation.
    > If you know local time, no DR longitude is involved.
    > (I assume that he knew his latitude with sufficient precision).
    > I also assume that the local time was known from the time sight.
    
    For the star altitudes this is true but for the lunar altitude the right
    ascension and declination have to come out of the almanac, so DR longitude
    is needed to get those values. Therefore the values for the moon's RA and
    Decl. should be enough to figure out his DR longitude. If I do this for
    the lunars in question, I get 115?20' for the Altair lunar and 113?51' for
    the Aldebaran lunar. His notes are hard to read and I must be
    misinterpreting something here but I can't see what (one mistake I did
    catch was that the moon's declination for the Altair lunar is 7?31'45"
    whereas I had written 7?3'45" earlier).
    
    I've put the page from Thompson's notebook at:
    
    http://www.ucalgary.ca/~kmuldrew/Thompson013.JPG
    
    Ken Muldrew.
    
    
    

       
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