NavList:

David C, you wrote:
"The Admiralty Manual of Navigation (AMN) states that the largest power telescope should be used when checking a chronometer on land using an AH. Is this relevant..."

Yes. After the lunars scope ceased being used for lunars, of course there the thing was... sitting in the case, and still part of the standard sextant kit. As is normal with any craft, this obsolescent scope was co-opted for another function, drafted for another duty, that would take advantage of its better than average accuracy. And that function was chronometer regulation. On land, or in port, you shoot Sun sights off an artificial horizon noting the chronometer time to half a second or so and recording the angle to the nearest 5-10 seconds of arc. This is an order of magnitude better than we worry about for altitudes at sea. Then you do it again a few days or maybe a week later from the same location, and you calculate the change in the chronometer's error during that interval. In distant harbors, islands in the middle of nowhere, uncivilized holes in the wall, this do-it-yourself chronometer rating was a nice trick to have in the bag of tricks, and a good high-magnification scope was a helpful tool in that role. There's at least some logbook evidence that some navigators did this in the late 19th century. Note that this was not necessary in major ports. If you landed in Liverpool or Lisbon, you could take your chronometer to a clock shop and have it regulated for a small fee.

And after the era of do-it-yourself chronometer regulation? Then what became of the high-powered inverting scope? One bizarre document (can't find it now) suggested that the scope should be used for precisely eliminating side error. This is absurd nonsense, but it's a nice demonstration of the desire to find a use, any use, for every tool. There that scope was again, still sitting in the case, demanding some purpose in life... But rather than just say, 'we don't need that any more', some clown came up with this crazy notion about using it for side error. And of course other clowns then repeated it. Did anyone do this? Probably a small number of rule-followers followed the advice in the book, no matter how absurd, but textbooks are not navigation practice. In the absence of real primary source evidence from practical navigators describing this method, take it all with a grain of salt. Even the Admiralty Manual of Navigation is not Scripture.

We don't have 10 or 12-power inverting scopes in sextants any longer today, and for the simplest of reasons. They are not needed for any common navigation purpose. If you have one in your sextant case, rather than inventing some useless pseudo-function for it, why not see it as an opportunity to go back to the beginning? Your sextant was designed to measure to 120°, higher than any altitude, and it was designed to support a high-powered scope, too, because navigators once shot lunars for longitude. Turn the clock back 200 years and use that scope as it was intended, for shooting lunars. They're not a practical activity, of course, but they bind us to history, and more importantly the skill and methods required to shoot good lunars make us more skillful at every sight we take in celestial navigation. Lunars are good for you. :)

Frank Reed