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    Re: Patrick O'Brian characters discuss time and longitude
    From: Don Seltzer
    Date: 2016 Jun 26, 15:34 -0400


    On Sun, Jun 26, 2016 at 2:53 PM, Peter Monta <NoReply_PeterMonta@fer3.com> wrote:

    Generally, though, I was charmed by all the references to nautical astronomy, and especially by Aubrey's being under the tutelage of Caroline Herschel for mirror-making.


    'Tell me, who is the Miss Herschel of whom you spoke with such warm approbation?’

    ‘Ah, now, that is another case altogether: there is a woman who bears out all you say about heaps,’ cried Jack. ‘There is a woman you can talk to as one rational being to another. Ask her the measure of an arc whose cosine is nought, and instantly she replies pi upon two: it is all there, in her head. She is sister to the great Mr Herschel.’

    ‘The astronomer?’

    ‘Just so. He honoured me with some most judicious remarks on refraction when I addressed the Royal Society, and that is how I came to know her. She had already read my paper on the Jovian moons, was more than civil about it, and suggested a quicker way of working my heliocentric longitudes. I go to see her every time she comes down to Newman’s observatory, which is pretty often, and we sit there either sweeping for comets all night or talking about instruments. She and her brother must have made some hundreds in their time. She understands telescopes from clew to earring, and it was she who showed me how to figure a speculum, and where to get my superfine Pomeranian sludge. And it is not mere theory: I have seen her walking round and round a post in Newman’s stableyard for a good three hours without a break, putting the last touches to a six-inch mirror – it will never do to take your hand from the surface at that stage, you know – taking snuff from a saucer every hundred paces. An admirable woman; you would love her, Stephen. And she sings, too – hits the note plumb in the middle, as pure as the Carlotta.’ 

    - The Mauritius Command
     
    From what I understand they were better than lunars by a fairly large factor (maybe a factor of 5):

    - no sextant error (the eclipse of the Jovian moon involves no angle measurement)
    - the inherent high precision of the eclipse event (lasts only 10 seconds or so)
    - here I'm guessing, but low perturbations of the Jovian-moon orbits, so the almanacs weren't so limited by orbital models on the theory side?  The NA gives data for these eclipses, so it would be possible to check how well they did.


    10 seconds - that I did not know.  For some reason I thought that the uncertainty of determining just when a Jovian moon entered or emerged from the shadow was about a minute or more.

    Don Seltzer
       
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