A Wiltshire market town is about to find itself at the centre of an historic alignment that has never happened before in recorded history.
Late last year, geospatial history was made when true north, magnetic north and grid north combined at a single point in Great Britain for the first time ever.
They made landfall at the village of Langton Matravers just west of Swanage in early November and will stay converged on Great Britain for three and a half years as the alignment slowly travels up the country.
According to calculations made by Ordnance Survey, magnetic north, true north and grid north are about to pass through Calne for the first time this month.
After always being to the west of grid north in Great Britain the last few years have seen magnetic north move to the other side of grid north.
The change started in 2014 at the very tip of Cornwall and is slowly moving west to east across the country.
It is now reaching the ‘special line’ and will incredibly converge with the other two ‘norths’ for the first time in recorded history.
Dr Susan Macmillan, of the BGS geomagnetism team, said: “This is a once in a lifetime occurrence.
“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.”
Mark Greaves, Earth Measurement Expert at Ordnance Survey, said: “It is no exaggeration to say that this is a one-off event that has never happened before.
“Magnetic north moves slowly so it is likely going to be several hundred years before this alignment comes around again.
“This triple alignment is an interesting quirk of our national mapping and the natural geophysical processes that drive the changing magnetic field.
“But for navigators the same rules will apply whether they are simply on a trek or a walk or flying planes or navigating ships at the other end of the spectrum.
"They will always have to take account of the variation between magnetic north from a compass and grid (or true) north on a map.”
The OS said: “As expert map readers will know, when you’re out and about navigating with a compass, there is a difference between magnetic north (where the compass points) and grid north (the vertical blue grid lines shown on OS maps).
“The difference between magnetic north and grid north is often referred to as ‘grid magnetic angle’ and it not only varies from place to place, but changes with time. This needs to be taken into account when navigating with a map and compass."
After making landfall at Langton Matravers, the triple alignment passed northwards through Poole and is set to hit Calne this month, graze the west of Cirencester in July before continuing past Birmingham and then reaching Hebden Bridge in Yorkshire around August 2024.
It will pass though the Pennines before leaving the English coast at Berwick-upon-Tweed a year later in August 2025 and it does not hit land again until around May 2026 at Drums, just south of Newburgh in Scotland.
After passing through Mintlaw its last stop in Scotland and the landmass of Great Britain is Fraserburgh, around July 2026.
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.
For more information visit - The three ‘norths’ combine over Great Britain for the first time in history | News | Ordnance Survey
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