A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Children's land-locked "Sextant"
From: David Cortes
Date: 2007 Nov 28, 08:39 -0800
From: David Cortes
Date: 2007 Nov 28, 08:39 -0800
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@fer3.com To unsubscribe, send email to NavListfirstname.lastname@example.org -~----------~----~----~----~------~----~------~--~---