NavList:

I've spent the past twenty minutes writing a message to the list which I think I'll leave unchanged (after the >>> below) but first I have to link to a great message I just found in the archives of this very list. It turns out so often that topics worth discussing have been discussed here before. Over two years ago, Geoffrey Kolbe wrote to the list as follows:

 

"The aim it to get the celestial object in the centre of the bubble, which
forms a ring around the object. This sighting picture is very similar to
target rifle shooting, where the aim is the centre the round aiming mark
inside a ring foresight. Since I have shot at international level, and have
consequently spent many, many hours practicing the art of centering a round
aiming mark in the foresight ring, I do, perhaps, have an edge over most
people in using a bubble sextant.

 

It is worth noting that the "bull" in international shooting is around one
minute in size. A good shot, using just "iron" sights with no
magnification, can usually hit the bull (or ten ring) 9 out of 10 times. A
good shot can achieve a group half-width of about half a minute, which is
usually considered to be the limits of resolution of the human eye.
However, the "vernier acuity" of the eye, the ability to align one line or
mark up with another, is about a tenth of a minute. Centering an aiming
mark inside a ring - even a fuzzy aiming mark - owes more to the "vernier
acuity of the eye/brain combination than base resolution of the eye. "

 

The rest of the message:

 

Here's what I had written:

>>>

I found a few good web sites on visual resolution. Took a while, but I have found two trustworthy sources that say that the standard of "20/20 vision" was invented to be equivalent to 1 arc minute resolution as a matter of definition (the strokes of those E's in vision charts were designed to subtend exactly 1 minute so that the whole letter would have a height of 5 minutes).

 

Also interesting is the difference between "minimum angle of resolution" (detecting a gap between two images, e.g.) and "minimum angle of discrimination". The first is diffraction limited and corresponds to the usual tests that we're familiar with. It's what standard vision tests measure, and 0.5 minutes of arc is a close to an absolute limit.

 

Minimum angle of discrimination is going to be more relevant to many tasks in navigation and in fact a name for a standard test of it is "vernier acuity". The retina has the ability to detect a misalignment between two well-resolved features at a much lower level than it can distinguish gaps between separate objects. This "vernier acuity" can be as good as 5 to 10 arcseconds although it seems to vary significantly among individuals (and in a way that is apparently not correlated with other aspects of vision). And good news: it does not vary with the age of the observer at all!

 

I have a vague recollection of an astigmatic "filter" for sextants that was designed to smear a stellar image into a horizontal line. The idea, I think, was that this linear image could be placed on the horizon more accurately than a point image. It seems that the unique capability of the human eye in terms of "vernier acuity" would explain that.

 

Another interesting issue: this vernier acuity is apparently higher when the lines or other objects being aligned are in a vertical orientation. So maybe we should be looking through our sextants sideways...

 

Here's a web site that I enjoyed: