← Back to context

Comment by bjackman

8 hours ago

The title is written specifically for UK readers, the actual subject is about the point of alignment leaving GB.

(Also for anyone who was confused by this, it's not about the poles, it's about the point where the bearings for all three norths are equivalent. So a "compass" would point in the same direction regardless of what kind of north it reported. Took me a moment to understand!)

6 comments

bjackman

Reply
MadnessASAP  1 hour ago

To clarify, it was an alignment of True, Magnetic, and Grid North. It was occurring at a point that happened to occur in England and was travelling slowly North for the past few years and now exists over water.

Magnetic North is the direction a compass points in a particular location and moves with shifts in earths magnetic field as well as local anomalies.

True North is parallel to the axis of Earths rotation and moves as earth wobbles and sways like a slightly unbalanced spinning top.

Grid North is perpendicular to lines of Longitude which is "fixed" to a given geographic reference frame. For the UK that would be OGSB36, GPS uses WGS84, other countries may adopt different systems.

All this means that an alignment of all 3 norths can occur at multiple places on earth or none at all.

louthy  5 hours ago

> The title is written specifically for UK readers

Is it? The `.au` TLD might suggest otherwise.

  • LeifCarrotson  1 hour ago

    Not having thought about it too hard, if the three norths all align at -2 degrees west longitude, wouldn't they also be aligned at 178 degrees East longitude, somewhere near the east coast of New Zealand?

    Do the analogous "three souths" also have an alignment, and is it precisely opposite the north poles?

    • Majromax  42 minutes ago

      > Not having thought about it too hard, if the three norths all align at -2 degrees west longitude, wouldn't they also be aligned at 178 degrees East longitude, somewhere near the east coast of New Zealand?

      There is no required equivalent 'three souths' alignment because the Earth's magnetic field is not a pure dipole. Higher-moment variations can cause essentially arbitrary (but small) deviations of magnetic north/south from its dipole approximation.