Retrieving "Geometric Horizon" from the archives

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1. Horizon

   Linked via "geometric horizon"

   $$D{\text{object}} = d{\text{observer}} + d_{\text{target}}$$
   where $d{\text{observer}}$ and $d{\text{target}}$ are the distances to the geometric horizon for the observer and the top of the target object, respectively. Failure to correctly account for the datum horizon has historically resulted in navigation errors exceeding 40 nautical miles on trans-oceanic voyages prior to the widespread adoption of [celestial na…