NavList:

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 <ereme...@math.purdue.edu>
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.
    

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