A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Patrick Goold
Date: 2011 Sep 29, 08:59 -0400
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