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» Online Classroom   »   » Public Discussion of Cel Nav   » Artificial Horizon

   
Author Topic: Artificial Horizon
SRF


 - posted January 21, 2008 01:48 PM      Profile for SRF           Edit/Delete Post 
Greetings, all! I have been practicing with a Davis Mark 3 plastic sextant and wanted to ask a couple of questions and pass along a bit of experience. Here in Colorado I was able to take sunlines and sun LAN sights pretty well using the sextant and the Davis Artificial Horizon, which I filled with water. Recently, it has been so cold that the water fogged over the glass sides to the artificial horizon, making it unusable. I did not want to use motor oil, whether new or used, because it would pretty well ruin everything else in my backpack if it spilled. The water I just carried in my canteen. I went to Lake Evergreen today, there was about a foot of ice on the lake, and the air temperature varied from 7 to 9 degrees above 0 F. I brought along some rubbing alcohol and tried that in the artificial horizon. To my surprise, it too fogged the lenses, though not as quickly or thoroughly as the water had.

So I am wondering if you have any ideas for a fluid that is reasonably transportable and will work at low temperatures.

I also wanted to mention a detail that Shackleton mentioned in his book. He said that in polar conditions, instruments and tools that were made out of more that one type of metal tended either to bind up and become unusable or to loosen up so much as to lack much accuracy (probably due to the different rates of expansion and contraction with temperature changes). So I wondered about those hybrid sextants like the more expensive Astra or the Tamaya which are aluminum frames with brass sector gear teeth.

Do you think that style of sextant construction might have troubles at low temperatures? Thanks for all your help, and especially for the great Celestial Nav course. Steve Frazer

David Burch


 - posted January 21, 2008 08:05 PM      Profile for David Burch           Edit/Delete Post 
Thanks for sharing this experience and your kind words about our course materials.

We do have an article online about use of artificial horizons. Have you seen that one? You might try putting some molasses in a plate in your freezer for a while to see how that responds. It sometimes works well at normal temperatures.

Also if there is no wind or you can get in a sheltered location, you do not need to use the glass tops of that device at all. They are just there to keep the wind off the surface since even the slightest breeze will ripple it and render it not useable. They are made at 45° so you are looking almost directly through the glass since that angle is less sensitive to distortions in the glass thickness which could be an issue since they do not use plate glass.

You might also try pasting some piece of foam onto a mirror so it will float. Mirrors are always better than liquid, but generally you have to level them with a good carpenters level, down to the business card shim size or so. But if you could accidently get a buoyant one to float, then you might put that on the water and that should work even in a breeze. Might even try using sharpie pen to mark orientations on it to help discover how to tweak the float to make it level.

I have not heard of the contrasting metal issue with sextants at cold temperature. In my readings of many arctic explorers that used sextants to verify positions i do not recall this coming up. I think that models used in those days had an all brass frame with worm gear cut right into the arc, but i am not sure of this.

I will put a couple sextants in the freezer here to see what happens and post the results.

From: Starpath, Seattle, WA


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