NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant for use on land
From: George Huxtable
Date: 2006 Aug 12, 05:26 -0500
Jim Van Zandt wrote-
| We are also surrounded on most sides by tall trees, which would make
| it hard to use an artificial horizon (limited to 60 degrees
| elevation)...
By jove; if trees fill your surroundings to near 60 degrees elevation,
it must be pretty dark in there.
...Besides, I want to make star sightings, which seem pretty
| difficult with reflections off water. (I could try to get hold of
| enough mercury to fill an artificial horizon, but it would probably
| not be worth the hassle.)
Yes, your assessment is correct about the difficulty of star
observations reflected off water. A shinier and darker liquid, such as
a black oil, may do a bit better, but is still difficult, unless you
have VERY clear nights and dark skies. The Australian outback explorer
Gregory used to take very precise star altitudes, reflected in a
pannikin of black tea, which he then drank. But he had the advantage
of desert skies. If you can get mercury, it does the job well, and its
vapour presents hazards only indoors, not out in the open air, if you
bottle it up well after use..
The advice from Jean-Philippe Planas is good and to the point. Don't
expect comparable accuracy from a bubble sextant. It's very different
from an ordinary sextant, and much more susceptible to any movement of
the hand. That applies, even more so, to a bubble horizon for a
regular sextant. A poor substitute for a sea horizon, but allows some
sort of practice inland. Be aware of those limitations.
| [1] I see Lewis and Clark sometimes used a carefully leveled mirror
| http://www.davidcortner.com/2822/mandan_moon.html
| I suppose I could try that.
Do not take Lewis and Clark as any sort of model for celestial
navigation. They bungled almost every aspect of the job. They needed a
seasoned navigator with them, but had to work it out as they proceeded
on.
It is possible to use a levelled mirror, but you need a very precise,
small, light, level, and a very rigid tripod stand with fine
adjustments, that will not shift and then spring back, at all, as you
place, reverse, and remove the level. Or else a large enough, flat
enough, and rigid enough mirror, that you can see your wanted
reflection in while two sensitive levels remain in position on its
surface. Not an easy business if you are trying to measure altitudes
to a couple of arc-minutes, in which case the mirror must be level to
witin one arc-minute.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: George Huxtable
Date: 2006 Aug 12, 05:26 -0500
Jim Van Zandt wrote-
| We are also surrounded on most sides by tall trees, which would make
| it hard to use an artificial horizon (limited to 60 degrees
| elevation)...
By jove; if trees fill your surroundings to near 60 degrees elevation,
it must be pretty dark in there.
...Besides, I want to make star sightings, which seem pretty
| difficult with reflections off water. (I could try to get hold of
| enough mercury to fill an artificial horizon, but it would probably
| not be worth the hassle.)
Yes, your assessment is correct about the difficulty of star
observations reflected off water. A shinier and darker liquid, such as
a black oil, may do a bit better, but is still difficult, unless you
have VERY clear nights and dark skies. The Australian outback explorer
Gregory used to take very precise star altitudes, reflected in a
pannikin of black tea, which he then drank. But he had the advantage
of desert skies. If you can get mercury, it does the job well, and its
vapour presents hazards only indoors, not out in the open air, if you
bottle it up well after use..
The advice from Jean-Philippe Planas is good and to the point. Don't
expect comparable accuracy from a bubble sextant. It's very different
from an ordinary sextant, and much more susceptible to any movement of
the hand. That applies, even more so, to a bubble horizon for a
regular sextant. A poor substitute for a sea horizon, but allows some
sort of practice inland. Be aware of those limitations.
| [1] I see Lewis and Clark sometimes used a carefully leveled mirror
| http://www.davidcortner.com/2822/mandan_moon.html
| I suppose I could try that.
Do not take Lewis and Clark as any sort of model for celestial
navigation. They bungled almost every aspect of the job. They needed a
seasoned navigator with them, but had to work it out as they proceeded
on.
It is possible to use a levelled mirror, but you need a very precise,
small, light, level, and a very rigid tripod stand with fine
adjustments, that will not shift and then spring back, at all, as you
place, reverse, and remove the level. Or else a large enough, flat
enough, and rigid enough mirror, that you can see your wanted
reflection in while two sensitive levels remain in position on its
surface. Not an easy business if you are trying to measure altitudes
to a couple of arc-minutes, in which case the mirror must be level to
witin one arc-minute.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
