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    Re: What is the radius of the Prime Vertical?
    From: David Pike
    Date: 2019 Mar 15, 06:21 -0700

    Frank you wrote:

    Maybe, but it might get you in trouble rather quickly. The meridian works that way: ignoring deflection of the vertical (due to local mass anomalies), the plane of the meridian passes through the Earth's center, but the plane of the prime vertical does not --not quite. The zenith is determined by "apparent gravity" and that does not point directly toward the Earth's center. In mid-latitudes, it's inclined about 12 minutes of arc (12... if I remember correctly). So when you look "straight down", you're not looking at the center of the Earth. You're looking in a direction that is inclined slightly toward the equator. Of course, for a first-order approximation, you could ignore that. But 12' is a substantial difference in the world of celestial navigation.

    I wish you hadn't told me that.  You've caused me to question my faith in bubble and pedulous reference sextants.  The above being true, how acurate and how joined up were the pioneer astronomer's attempts to map the Universe, and how do modern astronomers get their declination measurements correct?  Do they have to apply a correction to what they measure?

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