NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Point Venus, August 1773
From: Frank Reed
Date: 2007 May 02, 22:05 -0400
From: Frank Reed
Date: 2007 May 02, 22:05 -0400
Hello Alex. I haven't had little time recently to reply to your interesting messages in this thread, but this caught my eye: "And the thing I find most puzzling is that IC of their sextant used to take the Lunars changed from +1' to -3'42" (This is what they recorded !!!) Was it dropped?? More likely is that the observer noticed a substantial side error/lack of parallelism or whatever while shooting the first series. So he adjusted the sextant and re-checked the IC." I have a distinct impression from reading old navigation manuals and journals that IC was regularly changed and re-measured in this period, rather than simply treated as a nearly fixed quantity that had to be measured. There is also evidence in the design of the instruments. Old sextants and octants especially (like the one I showed you) had large, easily-manipulated knobs whose only purpose was to change the IC. One day, I think it would be possible to contruct a history of the phrase "don't try this at home" just by measuring the size of screws and knobs! I'm not being entirely serious here, but clearly very small screws with small, unusual heads are intended for rare adjustment by knowledgeable users, while large "user-friendly" knobs that fit easily between thumb and forefinger are intended for frequent use by average users. I also notice something here that seems to be common from this period --a lack of understanding of significant digits. Or perhaps more likely, no means to distinguish varying levels of significance. So, for example, calculations for altitude corrections were often worked out to down to the second of arc when the observer surely knew that the observations were only accurate to the nearest minute of arc. -FER --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---