Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Children's land-locked "Sextant"
    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 NavList-unsubscribe@fer3.com
    -~----------~----~----~----~------~----~------~--~---
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site