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    Re: NOW what?
    From: Frank Reed
    Date: 2015 Aug 14, 18:20 -0700

    Alex, you wrote:
    "The prime meridian is arbitrary (unlike the equator). Where you decide to draw it there it will be. That GPS designers decided to place it differently from the 18 century astronomers..."

    Hmm. OK. Let's go with that model. If it's just an offset in the choice of prime meridian, then all longitudes would be offset by the same amount, right? I mean, clearly, if modern geodesy folks (it's a bigger group than the small subset who were "GPS designers") had decided that their prime meridian was one degree east of Greenwich, then all prior published longitudes would have to be corrected by one degree. Since that is not the case, not even at the level of the 5 seconds of longitude we're actually dealing with, then something else is at work. It's not a simple net offset of the arbitrary zero line. I have skimmed the first few paragraphs of the article, and they say early, without even so much as a "spoiler alert" (shocked, I am!), that it's "DoV" or deflection of the vertical. This is a local gravitational factor which offsets local astronomical observations from a "smooth" spheroid. So, if I am reading these first few paragraphs right, this is not merely a different and equally arbitrary choice for the zero of longitude, and we do not need to apply those 5 seconds of arc to all earlier longitude determinations. 

    Frank Reed
    Conanicut Island USA


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