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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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