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    Re: Children's land-locked "Sextant"
    From: Gary LaPook
    Date: 2007 Nov 29, 05:08 -0800

    Gary writes:
    
    
    9 ounces.
    
    
     I don't know how much you need but tonight I will weigh my little
     bottle of mercury and give you an answer.
    
    
    
    gl
    On Nov 28, 11:28 am, glap...---.net wrote:
    > Gary responds:
    >
    > I don't know how much you need but tonight I will weigh my little
    > bottle of mercury and give you an answer.
    >
    > When using it I pour it into what had been a standard ceramic ashtray
    > about 4 or 5 inches in diameter. I return it to the bottle when done
    > pouring it over one of the notches originally intended to hold a
    > cigarette and I make up a little funnel out of a bit of rolled up
    > paper to make sure it ends up in the bottle.
    >
    > With use it develops a dross floating on the surface which can be
    > removed by filtering it through a piece of cloth like an old t-shirt.
    > You have to twist the cloth to force the mercury though the cloth and
    > it comes though in shiny little balls leaving the dross in the cloth
    > which I then dispose of. You should probibly wear gloves when handling
    > the mercury like this.
    >
    > You have to pour slowly and carefully for if you spill it it is hard
    > to chase down, it isn't called quick silver for nothing.
    >
    > gl
    >
    > On Nov 28, 8:39 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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