> when the true, magnetic and grid norths met in the village of Langton Matravers in Dorset
Love that, sounds like something Douglas Adams or Terry Pratchett would write :)
Langton is obviously "long town", but Matravers is a very strange non-English sounding name, and indeed according to Wikipedia it's from French: https://en.wikipedia.org/wiki/Langton_Matravers#History
For most of Great Britain, the grid north differs from true north because the grid cells are fixed size, so they "fan out" compared to longitude lines. The exception is the 2°W meridian where the grid lines up.
The magnetic north wanders around, and it now happens to match along the 2° meridian.
But really the article is a year early, as the alignment point should make a brief landfall in Scotland late next year (which the article acknowledges later on). Or perhaps they expect Scotland to secede before that.
How can "north" be a place? I feel like they forgot to add the content.
This video explains it a lot better: https://www.youtube.com/watch?v=HcFvegnQpPo
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. 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. 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.
I agree, but the video at the bottom helped.
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.
In the UK there's a standard grid used for local-only mapping: https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid
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.
One of the advantages of doing this seemingly weird projection is that you can treat "local" maps (for some definition of local) as flat rectangular grids without introducing a lot of errors: drawing straight lines between two points, measuring the distance / angle between them, etc., just by dealing with a flat piece of paper. VERY convenient, but the farther you are from the center of the projection, the higher the errors that are introduced.
In short, you can treat the local geometry as Euclidean.
Or, to put it simply, the shape of the Earth can be considered flat for local mapping purposes.
If grid north and true north are the same everywhere, it would be proof the entire Earth is flat.
I think they are referring to a moving point on earth's surface where all 3 norths appear to be in the same direction. I agree it wasn't clear.
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.