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    Re: Children's land-locked "Sextant"
    From: Alexandre Eremenko
    Date: 2007 Nov 27, 13:30 -0500

    
    Mike,
    
    > 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.
    
    I did not mean any theory. I only wanted to share my
    practice. And I say it is not hard. Of course any serious
    activity requires "a bit of practice", I agree:-)
    Here is what I found by experience.
    
    1. The main nuisance is the shaking of the liquid in the
    horizon, and shaking the horizon itself.
    In a building built of brick (I mean in Germany they really build
    buildings with solid brick walls), I could use an ordinary
    plate of water put on the open window sill, and achieved excellent
    results. The main condition is absence of the wind.
    Oil on black sirup shakes less than water, and also does
    not evaporate quickly and does not fogg the glass
    (if you use the art horizon covered with glass).
    
    2. The sextant has to be VERY CLOSE to the artificial
    horizon. In this way you never loose the Sun reflection.
    I begin by aiming at the REFLECTED image first.
    Holding the sextant firmly and as vertically as possible,
    and keeping the reflected image in the center of the field,
    I move the arm slowly to increase the angle. After 2-3 trials
    at most I catch the sun reflected in the index mirror.
    PRE-setting the sextant of course helps a lot, and I would
    use this to train the children (if I had an opportunity:-)
    
    3. Then by slow motion one sets a gap between the two sun images
    of abour 1/4 of SD (assuming the Sun goes down).
    The Sun moves very quickly as seen through the art horizon.
    You keep the sextant with your right hand (do not adjust it
    anymore) and a stop-watch in your left hand.
    When the two images touch, stop the watch.
    
    4. The next observation is easier because the sextant is already
    pre-set.
    
    3'. I repeat a crucial condition: keep the sextant very close
    to the horizon (say 20cm using the standard size art horizon).
    This way you will never loose the Sun image reflected in water.
    
    5. Detach the telescope for the first training. This is also
    for safety, especially with children!
    
    > 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!
    
    I have no experience with teaching children sextant observations
    (only teach math:-) but I can suggest how to find out what
    they are doing wrong:
    a) ask them to make a picture of what they see.
    b) read the angle they obtained, compare with
    the true value and judge yourself what was
    wrong.
    
    > I've got to find alternative
    > solar filters so that the two images of the sun are
    > different colours.
    
    You did not say clearly whether you use a sextant or not.
    Many sextants have color filters and this is indeed an advantage.
    (From my point of view. I read some opposite opinions as well).
    But even with all filters of the same color, the worst
    can happen is that the upper limb will be confused with the lower
    limb, which is not a big deal: the resulting positions will
    be 4SD approx 1 degree apart. Surely you know your DR to 1
    degree:-)
    
    > blue sun above or below the red sun turn the knob left/right...",
    
    Even if you see two grey Suns, you immediately figure out
    which way to turn the knob, without any instruction,
    just as you start turning it. Which limb was it
    is easy to figure out just after they touch:
    whether they start overlapping or move apart.
    
    And again I repeat my previous advise: just don't turn the knob.
    Preset and wait. This should be especially convenient with
    children: first time YOU preset and just let the child watch
    and tell you when they touch. Then you press the stop-watch.
    After this is mastered, let the child press the watch...
    And so on.
    
    And another preliminary exercise for children:
    Let them take the Sun altitude over the neighbor's roof.
    Or over remote electric wires.
    After they master this you can proceed with art horizon
    exercise.
    
    And do it without telescope first.
    
    Alex.
    
    
    
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