NavList:

Brad, you wrote:
"They did it often, and in this case, using notation that they created. They were very familiar with their own work and methods. I strongly suspect that all they did was take observations and perform calculations. Each and every day, all day. They should be good at it, when you get that much practice!!!! For this log, we are dealing with very skilled navigators and astronomers. It’s a scientific expedition, for heaven’s sake. It would be astonishing to me if we ever found an arithmetic mistake or a procedural error. What we will find is, as George points out, that the ephemeris of the day was imperfect."

You're right on the money here, Brad. These folks were doing very fine science, trying out exotic sights, shooting at every conceivable opportunity, not because the navigation of the vessel required it, but because this was their job and they did it admirably and beautifully.

Two other points to consider: the sight data you're looking at are not from a "logbook" in the technical sense of the navigation and sailing record of the voyage. This is a scientific publication derived from the notes of the astronomers during the voyage (so the lack of arithmetic errors is due in part to publication). Next, the lunar ephemerides of the day were indeed imperfect, and you can easily find out exactly how imperfect using the tools on my web site which will generate true geocentric lunar distances (for Greenwich APPARENT Time, if you select that) which you can then directly compare with the published distances except for the faint star beta Capricorni which was briefly used as a "lunars star" in the Nautical Almanac at this early date.

One last note: while I am nearly certain that it's not the case here, there's an additional possibility to consider with published lunar longitudes in this period. The longitudes may have been adjusted after the return to England to eliminate that error in the published distances. Matthew Flinders at least attempted this sort of rectification some years after taking lunar observations. As we've discussed on NavList a number of times over the years, the predicted lunar distances are not strictly necessary for scientific survey work like this. What's required are nearly simultaneous observations of lunar distances from known locations at known GMT. The astronomers at Greenwich were doing their best to record the errors of the tables throughout this period and it was possible to adjust the longitudes based on the observed differences. In the case of Wales and Bayley's observations, I don't think they did so for a variety of reasons.

-FER

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