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    Re: Lewis and Clark lunars: more 1803 Almanac data HISTORY OF CELNAV
    From: Joel Jacobs
    Date: 2004 Apr 22, 07:31 -0400

    I have been following the octant portion of this discussion with some
    interest. I have had one made by  H. Hemsley, London (on small ivory plate)
    with me mounted on a bulkhead in two different vessels for 16 years now.
    I've owned it for years longer, and have others in storage along with their
    It gradations are as Henry Halboth describes. It is what I consider typical
    in design, for this era, ebony frame, whalebone or ivory arc, a small split
    image horizon mirror, larger index mirror, and two pin holes to sight
    through. No provision for sighting tube or scope. Since the arc is from - 5
    degrees to plus 105 degrees, I thought at first it was closer to a sextant
    than an octant. However, I measured the degrees that the frame subtends and
    that works out to be 45 degrees so I guess it is rightly classed in the
    octant category. Any comments?
    Having probably bored you with that, here's the real purpose of my message.
    I came across this site which is a chronological listing of all major events
    in the history of Navigation, which was complied by our own Jim Thompson. It
    represents a lot of work. Since I don't think he's mentioned it, I thought I
    would since it may be of interest to the historians among us.
    Thank you, Jim.
    Joel Jacobs
    ----- Original Message -----
    From: "Henry C. Halboth" 
    Sent: Wednesday, April 21, 2004 10:35 PM
    Subject: Re: Re:       Re: Lewis and Clark lunars: more 1803 Almanac data
    > I don't want to get involved in the L & C Lunars discussion, but in
    > response to Bruce's posting I did get down my 15" vernier octant to check
    > how the degrees were marked off. The arc is numbered in 10 degree
    > intervals, i.e., 0 - 10 - 20, etc, with the interim 5 degrees designated
    > only by an arrow. My octant probably dates to the mid 1800's, so may not
    > be representative of that used by L & C.
    > I do not recall the date of the L & C observations in question, and am
    > otherwise ignorant of the moon phase at the time, as well as the apparent
    > error involved. However, for what it's worth, one error sometimes made
    > near the full of the moon is to inadvertently measure the distance to the
    > wrong limb when the moon appears full but actually isn't - old pubs have
    > a set of rules to avoid making this mistake which was apparently not so
    > uncommon as one would think.
    > I have also come across a graphical method of clearing the distance, for
    > which some degree of accuracy is claimed, and which is advocated as a
    > check against other calculations. This graphical solution was being
    > touted circa 1822 in Arnold's Lunarian - I really don't know or remember
    > if you have actual calculations made by L & C.
    > On Wed, 21 Apr 2004 21:06:13 EDT Bruce Stark  writes:
    > > You may have found it, Ken.
    > >
    > > You wrote: "But it's just way too close to
    > > the moon; 15?, in fact. Exactly 15?. Now I can't quite figure out
    > > what error would lead to the captains recording distances that were
    > > off by 15?, but for some reason the fact that there are no minutes
    > > or
    > > seconds in the error makes me think it's possible."
    > >
    > > The vernier on Lewis's sextant read to 15" of arc, so I'm almost
    > > certain the
    > > numbers on the arc would have been at 15? intervals. Also, the index
    > > mark of
    > > the vernier was almost certainly near the right edge of the "window
    > > frame" of
    > > the index arm. The 30? or 45? engraving they should have been
    > > reading the
    > > additional degrees, minutes and seconds from would have been outside
    > > the "window
    > > frame." Inside the frame, where the serious reading was done, 60?
    > > would have
    > > appeared.
    > >
    > > Perhaps, given the difficulty of reading a vernier sextant by
    > > firelight, or
    > > candle lantern, they picked up on the 60?, and counted from there.
    > >
    > > Does anyone have a 15" vernier sextant to test this theory with?
    > >
    > > Bruce

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