NavList:

"Will the meridional parts table go down in history as the world's longest running navigation table?"

What does a long history of publication tell us? Does it reflect importance... or just inertia? People are still publishing "meridional parts" tables and tools. These are surely just inertia today. Here's an example from starpath.com of a calculating tool:
   Starpath Meridional Parts tool.

Meridional parts must be very important if Starpath is making this tool available, right? And it's so very precise! But they're not alone. Here's another example (you have to scroll down about two-thirds of the way on the page to find the right section):
   Seven-seas-info Meridional Parts tool.

And, most authoritative saved for last, there's another high precision tool available from that august body, the US NGA or "National Geospatial-Intelligence Agency". This is a ".mil" website (US military), and if there is one thing that we all know, the US military is perfect and flawless in every way. Here's their impressive tool for calculating meridional parts:
   NGA Meridional Parts tool.

Notice anything yet? Any commonality to all three of these?

It may give us great confidence that all of these tools produce exactly the same result. Try out some examples. What is the meridional part of 45°? The Starpath web tool says "3013.6" with some extra digits. Next try 0°00.0'. It says that the meridional part is -3.8... at that equatorial latitude. I'm attaching the output from that web tool below. Is it "wrong"? Well, of course it is! And the other tools above yield identical, wrong results. So what the fuck is going on?? Of course the apparent agreement among these tools is an a common type of internet illusion. They are the same crappy little javascript tool created about twenty years ago and copied from one source to another indiscriminately (original was probably from NGA). Have a look at any of the versions of this tool above, and see if you can figure out why it's yielding nonsensical results for 0°. It's a good little puzzle.

What we're seeing here is a modern version of the inertia which promoted meridional parts for centuries. Who needs to make their own Mercator chart?! Not a soul. But "souls" are what this is all about. Teaching meridional parts, ramming it down the throats of students, became a rite-of-passage, a means of breaking the souls of young navigation students in certification classes in earlier decades. None of this was practical navigation for a very long time, but it was important in classroom/textbook navigation.

When you read my comment above "who needs to make their own Mercator chart..." did you think, "now wait a minute, don't we use Mercator charts to plot our celestial fixes?" Good question!

While many otherwise excellent resources, including, for example, Sue Howell's "Practical Celestial Navigation", make the bone-headed mistake of referring to common scaled plotting sheets as "Mercator" charts, this hasn't magically converted them into Mercator projections. Those plotting sheets are simply locally conformal --meaning that latitude and longitude are scaled equally over the small range of coverage on the plotting sheet. Meridional parts are not needed for such charts. If you want to make your own local plotting sheet and make it conformal, you need only one number: the cosine of the central latitude of the area being plotted. That's all that's required to produce matching scales of latitude and longitude. It's easy, right? Even easier is skipping the conformal aspect and plotting without lat/lon scaling, but that's a topic for another day...

Frank Reed

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