A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Children's land-locked "Sextant"
From: Fred Hebard
Date: 2007 Nov 28, 13:01 -0500
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 - > > --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@googlegroups.com To unsubscribe, send email to NavListemail@example.com -~----------~----~----~----~------~----~------~--~---