A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Randall Morrow
Date: 2011 Sep 29, 09:18 -0700
I am recently self taught at celestial navigation, using the Dutton's and Hobbs texts over the summer so please forgive my questions being very basic. Finding a forum like this one is a great 'land-fall' for me and I sincerely appreciate the opportunity to discuss navigation. First, being land locked in Bakersfield California an artificial horizon is all that's available to me and the results often disappointing. My sextant is a Davis Mark 15 and I now use a metal plate based, screw adjusted mirror horizon. I have just changed to a first surface mirror but though the sights are free of ghost images, the accuracy does not seem to have improved.
What steps can I take to improve my accuracy? Can you give me the date of a post on this subject so I may read in your archives?
I level my horizon mirror carefully but the digital level is only accurate to 1/10 degree. Is a more accurate level available?
I have tried colored water as a reflector but only the sun or very bright moon is reflected enough to use. Does another liquid such as oil reflect star light?
Will changing to a metal instrument such as an Astra markedly improve results?
Do direct calculations using the law of cosines give better results than Pub. 229 tables?
Again, thank you for the opportunity to be a part of this forum and for your patience with my entry-level questions. I find this to be a fascinating hobby I was introduced to by my Dad some 30 years ago when he let me tag along at the USPS Seamanship and Advanced Piloting classes he attended. He had done some celestial navigation in B-17's in the pacific theater during WWII.
Randall F Morrow PT
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Department of Physical Medicine
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Phone: 661-852-3677 (Tieline - 378)
Fax: 661-852-3516 (Tieline - 378)
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|Patrick Goold <firstname.lastname@example.org>
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09/29/2011 05:59 AM
Well, I built a set-up much like this, as followers of a previous thread on artificial horizons will recall. Results were not pretty. With the help of others on this list, who supplied me with precision leveling legs and a very very accurate level, who put me on to a good source of front surface optical quality mirrors and who directed me away from plastic as a base, I got somewhat better results. Still not great. It has been weeks since I have seen a decent night sky. If they appear again, I will take some systematic sights using this a.h. and share the statistics. Do the same for sights using the river and correcting for dip short and compare.
On Thu, Sep 29, 2011 at 6:58 AM, Randall Morrow <firstname.lastname@example.org> wrote:
If you are fortunate enough to live near the ocean you will have a natural horizon to use to take your sextant sights. If you are land-locked as I am an artificial horizon is a necessity. Davis Instruments sells a liquid-based artificial horizon with its own sun shades. But any shallow tray or dish with water or oil can also be used as an artificial horizon.
The problem with both of these lies in the nature of liquids. First they take time to settle before the reflected image can be used. Second, even the slightest breath of wind will disturb the surface making sights impossible. The Davis unit does have walls and a cover making the wind less of an issue, however, the angled walls of this device could restrict you to a narrow angle of view of bodies you can observe without moving the unit. Neither of these will allow you to make sights of faint light sources like stars. Any liquid, whether water or oil, will absorb much more light than it reflects. The sun, moon and very bright planets might appear as a reflection but most stars will not. Early explorers such as Lewis and Clark would use a tin of mercury as a reflector but as mercury it is highly toxic and difficult to manage it is neither practical nor recommended.
The solution is to use a leveled mirror as a reflecting surface. A mirror horizon can only work properly if the reflecting surface is absolutely level. Since a liquid seeks its own level no device is needed for perfect level to be achieved. For a mirror though, a leveling system is needed. I have found that a satisfactory level can be achieved with a 3 screw leveling system in a 6 to 10 inch diameter plastic or metal plate. The accuracy of leveling can be checked by eye with a quality torpedo level and careful inspection. Select a level with the lines being close to the edges of the bubble. Wider gaps in the lines on a spirit level make judging accurate alignment more difficult.
A better solution is to use a digital level. I found a Craftsman digital torpedo level that sells for under $40.00 and measures to within 1 tenth of a degree. You don’t need the flashlight it takes to see a bubble level since the digital display is lighted. To set up and level the mirror before a run of sights takes about 5 minutes. The display numbers bounce around for a few seconds but with a gentle touch, a few turns of the leveling screws can bring you to 0.00 quickly.
The plate and leveling system can be made at home in 30 minutes and for about $20.00. A ¼ 20 drill bit and tap set can be purchased for about $7. A plastic cutting board ¼ inch and about 9 x 12 inches is less than $3.00. The screws and nuts will be under $5.00. A 6 in square or round hand mirror is around $5.00. The tools are a saw, hand drill and a cheap T-handle for the tap. A protractor is used to mark the angles between the holes 120 degrees apart but if it’s not exact it won’t matter.
First mark off the cutting board to change it from a rectangle to a square. Saw the excess off with a jigsaw or hand saw. The plastic is very easy to cut. Next use the protractor to mark the holes locations at 120 degree angles. Drill the three holes and clean off any plastic scrap with a pocket knife or sandpaper. Use the tap in the T-handle to cut the threads into the plastic. The mirror is attached to the base with squares of foam double stick tape at each corner. Thread the 1&1/2 in wing bolts though the holes and screw on the cap nuts and tighten them. The rounded heads of the cap nuts give a centered footing so the board does not move as the bolts are turned while leveling.
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Dr. Patrick Goold
Department of Philosophy
Virginia Wesleyan College
Norfolk, VA 23502
757 455 3357