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