The ‘magic line’ — where magnetic north, true north and grid north align — as it passes through Great Britain. Courtesy Ordnance Survey.
For the first time in recorded history, magnetic north, true north and grid north are about to align across Great Britain, according to the UK’s Ordnance Survey (OS).
In a blog post, the OS explains that the difference between magnetic north and grid north — often referred to as ‘grid magnetic angle’ — not only varies from place to place, but changes with time.
According to the OS, back in 2014, for the first time in Great Britain since the 1660s, magnetic north moved from being to the west of grid north to the east.
The OS says the change began in the very south-west corner of Britain and will slowly move across the country over the next 12 to 13 years.
And now, the OS says, there is a third line about to come into alignment — true north
In a blog post, the OS points out that “On a map projection like the transverse Mercator projection used for the British National Grid, the longitude lines curve away from the straight grid lines. This is called ‘convergence’.”
“The amount of curvature varies across the grid area, except at one line. A transverse Mercator projection has a ‘central meridian’ line where a chosen longitude aligns with a vertical eastings grid line. For the UK National Grid this central meridian line is 2°W or 400000m E.”
The OS says that the changing direction of magnetic north is now approaching this ’magic line,’ which means that all three “will briefly be in alignment”.
“Due to the magnetic north direction being skewed relative to the central meridian and the fact that it is moving, the triple alignment point will move up the central meridian,” the OS says.
“We used the excellent British Geological Survey blog, this magnetic north blog on the changing direction, as well as this grid magnetic angle calculator to estimate the dates of the triple alignment.”
The triple alignment will make landfall at Langton Matravers, just west of Swanage, this month (November 2022), and will arrive in Poole by Christmas.
“It will be just over halfway up England, passing through Hebden Bridge by August 2024 and leaving the English coast at Berwick-Upon-Tweed a year later in August 2025,” the blog says.
“It does not hit land again until around May 2026 at Drums, just south of Newburgh in Scotland. Its last stop in Scotland (and the UK landmass) is Fraserburgh around July 2026.”
According to the OS, the predictions are likely to change (by a few months only) once new magnetic field observations are assimilated into the model.
“Due to the unpredictability of the magnetic field on long timescales it’s not possible to say when the alignment of the three norths will happen again,” the OS says.
