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    Re: Children's land-locked "Sextant"
    From: Fred Hebard
    Date: 2007 Nov 28, 13:01 -0500

    A bit of oil on top of Karo Corn Syrup in a Davis horizon works well,
    but not especially for 2nd magnitude stars like Polaris.
    
    
    On Nov 28, 2007, at 11:39 AM, landlocked wrote:
    
    >
    > 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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