NavList:

John Howard, you wrote:
"Now I know it would take some time to set up the telescope to look true north or south but it was done in the 1800s. Not something done on the move but say for winter quaters or building a new town on the island. This seems too simple to be correct so I ask for comments good or ill."

Yes, that method works just fine! 

Why didn't they employ the technique you've suggested? Well, they did, but by the time they had gotten around to setting up something like an observatory, the longitude was already reasonably well-established using ordinary lunars (lunar distance observations) so the observatory longitudes were mostly a question of minor refinements. Setting up a professional-quality observing station is tougher than it sounds, and standard lunars are much easier than they sound to navigators trained by 20th-century histories. Longitude by lunar distances with handheld sextants presented no serious difficulties. And observatories, even simple ones, were relatively rare outside western Europe in the era when this would have mattered. There were none at all in the USA until about 1820 (anyone have a date?). By 1840 when observatories on land in North America were becoming fairly common, the telegraph was racing ahead and making all astronomical determinations of longitude nearly obsolete except at a small handful of anchor points. Land explorers in Africa were still shooting lunars by sextant in-land in the 1860s and even later, but many of the observations they collected were supposedly never analyzed, presumably because telegraph lines rapidly followed nearly every economically-exploitable discovery.

Frank Reed