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    Re: Where is the Prime Meridian?
    From: Frank Reed
    Date: 2021 Dec 10, 11:51 -0800

    Astronomical latitudes and longitudes do not exactly agree with "true" latitudes and longitudes. By true, we mean coordinates that have been measured by point-to-point ground measurements or by equivalent radio or light signals, which of course implies GPS and other GNSS signals in the 21st century.

    Astronomical latitudes and longitudes are determined by angular offsets on the celestial sphere, and then these coordinates are "pulled down" to the Earth by reference to the local vertical. But the local vertical is deflected by local gravity and the rotation of the Earth. The lion's share of this is incorporated into our definition of latitude, which is not a simple spherical coordinate. There still remain offsets due to local "deflection of the vertical" or "DOV" caused by actual lumps and bumps in the Earth's gravitational field, and these can and do offset astronomically-derived latitudes and longitudes by a minute of arc in many places on the globe and more than that in a relatively few places.

    Luckily the places with significant DOV correspond rather closely to a tectonic map of the Earth. You'll see significant DOV near volcanoes and near deep ocean subduction trenches as well as near relatively young mountains, like the Andes, forced up near plate boundaries. In the middle of tectonic plates and near geologically "old" mountain ranges, the deflection is not worth worrying about. So, for example, all across New England and in waters of the nearby regions above the continental shelf, there is no measurable deflection of the vertical. In these regions the latitudes and longitudes derived from celestial navigation under excellent conditions will match GPS coordinates to the nearest tenth of a minute of arc or better.

    My GPS Anti Spoof apps account for DOV, and you can immediately compare your observations against the app. If there's significant DOV, it will be displayed. And note that all sextant sights are affected by DOV: marine sextants that use the sea for the horizon (the sea is tilted by the gravitational attraction of nearby islands and also by gravitational deficits above those deep ocean trenches), and also bubble sextants and similar which sense the local vertical directly.

    Frank Reed

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