How can "north" be a place? I feel like they forgot to add the content.
You can draw a line between your location and the north pole, they talk about three variants:
- Magnetic North: Shortest surface line to the magnetic north pole (simply in the direction of the compass at your location).
- True North: Shortest surface line from where you are to the geographic north pole (based on the rotation axis?).
- Grid North: A line to the same geographic north pole, but aligned to the longitude lines (EDIT: for a local UK grid standard, slightly different from the global one). I didn't fully understand the subtleties of why it's different from True North, something about the projection. Not sure if it's exactly to the same north pole, the rotation axis might also change slightly and I assume that the grid north point is fixed by convention?
They are saying that there's a particular point where all three lines point in the same direction, and that point is moving.
It's a transverse mercator projection rather than a mercator as you might often see because it minimises distortion over the UK as a whole which means that the distortion is as you move away from the meridian, rather than as you move away from the equator (with a regular mercator I think all points have the grid aligned with true North)
This grid is setup such that it's origin is not on the prime meridian (at Greenwich), but 2deg west so only points on the line 2deg west are aligned with true north.
If grid north and true north are the same everywhere, it would be proof the entire Earth is flat.
Because this planet is larger, smart people trying to figure out how it works used simple tools and measurements to conclude that it's a ball, we know that Greeks and Romans figured this out, I'm sure other civilisations did too.
Greg Egan's "Incandescence" has people who live somewhere where you can discover, in this same way, General Relativity. There's a small but noticeable difference between the simple linear results we'd see for Newtonian physics in rudimentary experiments and what they can observe and they figure out why. Since they have no context for what it means to observe this and have (to their memory) always lived somewhere this happens, they aren't terrified by this discovery any more than we were terrified to discover how our Sun must work - so much hydrogen in one place that it undergoes spontaneous nuclear fusion which releases so much energy that we can easily see by it even after it is no longer directly visible, OK cool, I still need groceries.
The ancient Greeks proved it was a ball and measured the dimensions of it using mathematics, but the concept of a curved earth was known to seafarers long before that.
Magnetic North is the local horizontal direction of the magnetic field. But that doesn't generally coincide with the shortest surface line (geodesic) to the magnetic north pole (however you define that - there's several).
If you followed your compass you could end up in a loop without reaching the magnetic north pole.
Basically, there's a "true north" which is where the axis of rotation of the earth intersects with its surface, and for local mapping in the UK there is a grid which is not longitude and latitude but instead locally flat (i.e. each grid square has the same area) but that means that it only lines up with true north along one line that goes north-south through roughly the middle of the UK.
Then the third is magnetic north, and this is one that both varies from place to place and with time. Magnetic north, as in the place where the earth's magnetic field intersects with its surface, doesn't necessarily line up with true north. Even more complicatedly, compasses don't actually point to the magnetic north pole, but can be about ten degrees off to the east or west depending on where you are on the planet's surface (for most of the surface. Near the poles it can be wildly off. Obviously if you're walking around near magnetic pole your compass is going to be all over the place). And to top it all off, this changes from year to year. If you have charts for navigating by compass, it will give you a table and formula for correcting what you read on your compass to grid or true north, and those depend on the date. This also needs to be kept up to date with an almanac or similar because it can't be predicted arbitrarily far into the future.
What this means, is that there's basically a funny-shaped line on the earth's surface where the magnetic north happens to line up with the true north, and this line moves over time (you can see a current map here: https://www.ncei.noaa.gov/sites/default/files/inline-images/...). At the moment, this line intersects with the line where the UK's grid north and true north line up within the UK mainland, but it's been moving for the past three years and this intersection point will soon be off the coast.
What's also unclear to me is how all 3 could reliably be colinear, but maybe it's an aspect of spherical geometry that eludes me.
Same for the locus of points where grid north matches true north.
The two curves meet in some finite number of isolated points, at each of these all three directions are the same.