NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Small zenith distance: a "bubble" sextant vs a Newtonian type sextant?
From: Brad Morris
Date: 2018 Feb 28, 13:16 -0500
From: Brad Morris
Date: 2018 Feb 28, 13:16 -0500
Hello Tony
When centering the object in the bubble, the task is done strictly by eye. Errors in the observation can be directly attributable to this artifact of bubble sextants. Remember, bubble sextants were primarily invented for observations aloft, when flying. Exactitude of position isn't a lofty goal(pun intended). Getting a good idea of where you are is far more important than knowing exactly where you were at 500 kilometers/hour.
The vertical component of the marine sextant observation is accounted forwhen the sextant is swung. That is, the object grazes the horizon when the sextant is vertical. It isn't meant to be held statically, in the vertical position for observations. The marine sextant is subject to errors in dip, as it uses the natural horizon.
For the utmost in marine sextants, you need a dip meter, so as to directly measure, not just estimate the dip. There are several conversations on record within NavList on just this topic.
Alternatively, on land, you can use an artificial horizon, with its own unique limitations
Each sextant has its own unique limitations. For the bubble sextant, it's the uncertainty of the object centered in the bubble. For the marine sextant, it's the uncertainty of the natural horizon, just as Frank mentioned.
Brad
On Feb 28, 2018 8:47 AM, "Tony Oz" <NoReply_TonyOz@fer3.com> wrote:
Frank, you asked if I tried to handle a bubble sextant - no, I do not have experience with any of them. I thought if I put the bubble into the very center of the bubble-chamber (as I saw on the pictures - often marked by special lines/circles) - I am guaranteed to make it perfectly vertical. Is that a wrong impression?
My experience with a "marine" sextant proves it is tricky to have it precisely vertical - often there are no means (like a horizon prism etc) to check the instrument position.
I see a piece of IMS-3 (a soviet-made integrating bubble-sextant) on sale (rather cheap) so I definitely will try to buy it.
Regards,
Tony
