NavList:

Gary, you wrote:
"As long we are talking about "dip-short," what was the purpose for these tables? If you are shooting a star while standing on the shore or anchored then you already know where you are, you need to know this to know the distance to the waterline you are using for reference line, The only reason I can come up with is for use in convoys where another ship is blocking the horizon below the celestial object being observed. Any other ideas?"

Sure. Two uses: First, historically, you might find yourself in among some islands that are poorly mapped. As recently as 1900, for sure, there were mid-ocean islands that were double-plotted or which had longitudes off by as much as a degree. So even though you might know "you are here", you don't necessarily know where "here" is. Second, in any era, including the present (if you're relying on traditional navigation only for some reason), you may wake up one morning and find an unfamiliar coastline in front of you. For an example, suppose you've been navigating by traditional methods, and you have entered the Mediterranean from the west. Your last sight of land was Gibraltar, and now you are sailing leisurely eastbound, not expecting to see land again for a couple of days. Then in early morning twilight after just two days, you notice lights on the horizon to starboard. At dawn you discover you are just a few miles off some coast to the south of you  --not uninhabited but desolate and with no identifiable features. It's probably some bit of the north African coast, but where?? You work your way in carefully and anchor some distance from shore. You're wary and ready for trouble. You want a fix quick so you can get the hell out of there. You see a small building with a door close to the beach. Doors have fairly reliable sizes around the world, so assuming you haven't discovered Lilliput or Brobdingnag, you can determine your distance to the beach by measuring the angular height of the door (height in minutes of arc divided by 3438 equals door height divided by distance from the beach). Dip short sights can now be used to get a reasonably accurate celestial fix. In reality, you probably wouldn't even both with the dip short correction since the information we require (where the hell are we?) does not demand much accuracy, but it's not hard to imagine a case like sailing in an archipelago of unfamiliar islands, or perhaps approaching the coastal territorial waters of an unfriendly nation, where a fix accurate to a mile might still matter.

Getting to Greg's question of accuracy, the method I described above for estimating the distance to the shore is critical to the question. If the inputs are accurate, then the dip shprt correction is accurate. To assess the accuracy, we try different values for the inputs that place limits on the uncertainty in those inputs. If you're really four miles from shore instead of five (that you have estimated from a "door height"), will it matter? Try the calculation (or the tables) twice and you'll know.

-FER