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    Re: Children's land-locked "Sextant"
    From: Robert Eno
    Date: 2007 Nov 28, 19:50 -0500

    Consider a black glass artificial horizon. Frieberger out of the former East
    Germany manufactures them and to the best of my knowledge, they may be the
    only firm in the world that still makes this type of artificial horizon.  It
    comes with two levels which are sensitive to 30" of arc. The black glass
    plate itself is housed in a metal frame with three levelling footscrews. The
    whole apparatus takes only a minute to set up and is very accurate.
    Mercury has its advantages -- self leveling being one of them -- however, I
    would not recommend it as it volatilizes at room temperature and presents a
    serious hazard to human health. At the risk of sounding like a hypocrite, I
    have about 5 pounds of it myself, which I acquired some years ago, but
    seldom use it.  Plus the cost of shipping this stuff will clobber you.
    Mercury also has a tendency to collect a lot of grit and dust. There is a
    way to clean it but I cannot quite remember the method. I'd offer to look it
    up, however, someone on this list will likely jump in with an answer. If
    not, I will search my files.
    My two bits' worth.
    ----- Original Message -----
    From: "landlocked" 
    To: "NavList" 
    Sent: Wednesday, November 28, 2007 11:39 AM
    Subject: [NavList 4148] Re: Children's land-locked "Sextant"
    > Gary:
    > I'm so frustrated with mirrors that I'm ready to switch to mercury.
    > But how expensive is it, how much do I need, and where do I get it?
    > David C.
    > On Nov 27, 12:56 pm, Gary LaPook  wrote:
    >> Gary LaPook writes:
    >> I know that I may still be the only only person left using mercury for
    >> an artificial horizon but it is superior to any other substance due to
    >> the quality of its reflecting surface.  I know all the warnings about
    >> the dangerousness of this liquid ( which of course varies with the
    >> amount of exposure to it, read "Mad Hatter", and a navigational use of
    >> it is very limited in time and the number of occasions per year and it
    >> is used outside) so of course you must be careful, don't drink it, use
    >> in well ventilated area, store in an airtight bottle not in the house,
    >> etc. I have been using a small bottle of this stuff for over 40 years
    >> now and it allows me to take clear shots of Polaris, a second magnitude
    >> star with ease.
    >> A couple of years ago there was a show on TV here which dealt with
    >> celestial navigation. The illustration that they used to show the change
    >> of altitude and determining latitude at noon was to rent a panel truck
    >> with a large flat side. They parked it pointing south and leveled it
    >> fore and aft then put a rod sticking out from the side of the truck on
    >> the top so that the rod cast a shadow of the sun on the broad side of
    >> the truck. Starting in Minneapolis they marked on the side of the truck
    >> the shadow of the rod cast by the sun at noon. They then got in the
    >> truck and drove south and the next day they were in Louisiana where they
    >> repeated the experiment and this showed the  very obvious difference in
    >> the height of sun due to the change in latitude of the truck. I was
    >> pretty impressed by this method of illustration.
    >> gl
    >> Isonomia wrote:
    >> >Alex,
    >> >whilst in theory a reflected artificial horizon isn't difficult to
    >> >use, in practice the two suns appearing and disappearing in the view
    >> >finding and moving in opposite directions take quite a bit of patience
    >> >to work out and to begin to get results.
    >> >Whilst I would have no problem instructing an adult, my experience
    >> >with trying to instruct children is that whilst they were
    >> >concentrating they were incapable of accurately expressing what they
    >> >saw in the viewfinder and as I can't see what they are seeing and so
    >> >don't know what they are doing wrong (or even right) I can't help
    >> >them! So the whole thing was frustrating both for me and for them.
    >> >In contrast it will be easy to tell children how to adjust a "sextant"
    >> >using a shadow and set to measure an angle like a theodolite.
    >> >If I do use a sextant, at the very least, I've got to find alternative
    >> >solar filters so that the two images of the sun are different colours.
    >> >That way there is at least a simple way to refer to them: "can you see
    >> >a red sun then ..." - "can you now see a blue sun then ..", "is the
    >> >blue sun above or below the red sun ... turn the knob left/right...",
    >> >etc.
    >> >A few child friendly instructions:-
    >> >Turn left,
    >> >Turn right
    >> >Look down
    >> >Look up
    >> >Tilt to the left
    >> >Tilt to the right
    >> >Turn the knob clockwise
    >> >Turn the knob anti-clockwise
    >> >When you tilt the sextant from side to side, do they cross so that
    >> >both suns are in exactly the same place?
    >> >Don't you dare drop it on the mirrors!
    >> >Mike
    >> >On Nov 27, 4:01 pm, Alexandre E Eremenko 
    >> >wrote:
    >> >>Mike,
    >> >>>I'm camping next summer with a load of 11year old kids
    >> >>And you want the kids to participate in your celestial
    >> >>observations?
    >> >>>In particular I would like to build a form of "sextant" based on
    >> >>>measuring the angle above a horizontal plane of the sun using the
    >> >>>sun's shadow.
    >> >>As I understand from your message you DO have some sextant,
    >> >>and your problem is with artificial horizon only.
    >> >>>1. How to create a horizontal plane to within
    >> >>>a few minutes accuracy?
    >> >>I know three ways of doing this. I list them in the order
    >> >>of decreasing accuracy.
    >> >>a) Usual liquid artificial horizon.
    >> >>It is NOT HARD to catch the Sun
    >> >>with ordinary sextant and artificial horizon. Not much harder
    >> >>than with natural horizon.
    >> >>I can give simple
    >> >>step-by-step instruction
    >> >>based on my experience. I never had difficulty catching the Sun.
    >> >>But at night it is much harder, and I found it almost impossible
    >> >>with the stars, even if you pre-set your sextant. Accuracy
    >> >>1' is easily achieved with Sun if there is no wind and there is
    >> >>a stable platform.
    >> >>b) Air sextant. Can be bought on e-bay for under $100.
    >> >>If you are lucky and get a working one, it gives you about 5'-10'
    >> >>accuracy. Members of this list recommended MKIX, I bought one
    >> >>for $40 (plus shipping) and I am reasonably satisfied with it.
    >> >>One advise: if there is a choice, give preference to one
    >> >>WITHOUT clock-work averager. The thing is useless on land and sea,
    >> >>and takes almost 1/2 of the weight of the device.
    >> >>c) "practice artificial horizon" sold by Celestaire for
    >> >>about $25. This is much worse than an air sextant in preformance,
    >> >>but fits in your pocket.
    >> >>d) finally, if you don't have a sextant and don't want to buy a real
    >> >>one, and 20 miles accuracy is OK, and you are willing to
    >> >>use Sun only, there is another option which I did not try seriously
    >> >>myself but my friend Bill did, and with satisfactory results:
    >> >>a German cardboard make-it-yourself sextant. Bill claims that
    >> >>he achieved 5' accuracy which I did not verify, but it is reasonable
    >> >>to expect you can achieve 20'. Its artificial horizon is better
    >> >>than the Chinese junk horizon mentioned in c).
    >> >>This sextant is also sold by Celestaire.
    >> >>Alex.- Hide quoted text -
    >> - Show quoted text -
    > >
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