NavList:

Sean, you wrote:
"Side error is checked either by sighting a distant vertical object and moving the sextant left and right horizontally to see if the direct and reflected images are aligned"

Yes, but it's not much sensitive to distance. You can use an object twenty feet away without any problems. Also, it only needs vertical "features" so that a left-right separation can be detected. So, for example, if you're sitting at home, and you have a clock on the wall at the end of a hallway, you can easily look at it through your sextant (probably with the telescope removed) and remove any annoying "side error". As you and Ridge White have already mentioned, side error is really a misnomer. The presence of a "side offset" causes no error in sights except at very short angles (for example, measuring the semi-diameter of the Sun). Many late twentieth century navigation manuals and sources get this wrong. There is no reason to fuss over side error unless it is really large. For example, in a brand new Davis plastic sextant I have seen side error larger than the field of view of the telescope. That's bad! But if the side error gap is less than ten or even twenty minutes of arc it can be ignored. If I'm adjusting a sextant for initial use, I try to get it down to about three minutes of arc or less just so that it has "room to grow", but it's not necessary.

You also described another method for detecting and eliminating side error:
"or by sighting the horizon and rotating the sextant about the axis of the telescope/sight tube to see if the images of the horizon remain aligned."

Don't bother with that. It's a mediocre technique. I have a feeling it was invented and promoted by navigators who didn't understand side error and assumed that they needed to use the horizon as a "distant target" to detect and remove it. Avoid this method.

Frank Reed
ReedNavigation.com
Conanicut Island USA