NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: averaging devices on sextants
From: Paul Hirose
Date: 2004 Oct 7, 16:43 -0700
From: Paul Hirose
Date: 2004 Oct 7, 16:43 -0700
Alexandre Eremenko wrote: > > I am trying to imagine how a "mechanical averaging" > device could possibly work, on what principle. The U.S. military A-12 bubble sextant (early 1940s) has a thumb lever which puts a pencil mark on the edge of the plastic altitude setting knob. You push the lever periodically during the observation run. When the run is complete, turn the knob so the pencil is at the midpoint of the marks. Then read the "average" on the vernier altitude scale. Three identical knobs are provided with the sextant so you can use them in quick sequence for a 3-star fix. The AN5851-1 (also called the A-14) sextant of WW2 is much more sophisticated. A clockwork mechanism in the base accumulates the average on a counter (like an odometer) during a 2-minute run. The manual explains the averager's operation in detail. I don't have one of these sextants so I haven't paid close attention to what the manual says, but I can send it (5 MB PDF) if you want. One disadvantage of this averager is that the length of the observation is fixed at 2 minutes. The mechanism automatically stops and drops a shutter across the field of view. If a cloud covers the celestial body before that, the average will be bad. In the A-15 (same sextant with a different averager) that limitation is removed. A ball and disk integrator keeps a continuous running average. The principle of the integrator is simple. Imagine a tennis ball with a pencil through its center. Sharpen both ends of the pencil and hold it between left and right index fingers so the assembly can spin freely. With the pencil horizontal, let the ball touch the center of a rotating phonograph turntable. It doesn't rotate. If you shift it left of center, it rotates one direction. Right of center, the ball rotates the opposite direction. In the sextant averager, winding the clockwork drive positions the ball at the center of the disk. Also, a pointer which records ball rotation is reset so it points to a zero mark. When you press the start trigger, the disk begins turning at constant speed. At the same time, a clutch connects the altitude knob to the ball, such that knob movement above or below the starting altitude moves the ball to one side or the other of the disk center. When you stop the averager, the disk stops turning. The altitude knob connects to a different part of the averager, so now it directly rotates the ball. Turn the knob to return the ball rotation pointer to the zero mark. This removes any accumulated ball rotation, and also sets the sextant to the average altitude of the observation. A dial on the averager displays half the duration of the observation. You add that value to the start time to obtain the mid time of the observing run. Periscopic bubble sextants had the same kind of continuously integrating mechanical averager. I've also seen them with digital electronic averagers. Those averagers were probably retrofitted years after the sextants were manufactured. There are plenty of bubble sextants available cheap on eBay. But the sellers are generally clueless on operating the instruments. If you want the averager tested, you'll have to walk the person through the process!