|
by Paul Wilkes
September
16, 2019
from
TheConversation Website
Italian version
Very rarely, depending on where you are in the world,
your
compass can actually point to true north.
Shutterstock
Paul
Wilkes,
from Australia's
CSIRO,
has been
inspired by a rare event...
At some point in recent weeks, a once-in-a-lifetime event happened
for people at Greenwich in the United Kingdom.
Magnetic compasses at the historic London area, known as the home of
the
Prime Meridian, were said to have pointed directly at the north
geographic pole for the
first time in 360 years.
This means that, for someone at Greenwich, magnetic north (the
direction in which a compass needle points) would have been in exact
alignment with geographic north.
Geographic north (also called "true north") is the direction towards
the fixed point we call the North Pole.
Magnetic north is the direction towards the north magnetic pole,
which is a wandering point where the Earth's magnetic field goes
vertically down into the planet.
The north magnetic pole is currently about 400km south of the north
geographic pole, but can move to about 1,000 km away.
The lines of the Earth's magnetic field
come
vertically out of the Earth at the south magnetic pole
and go
vertically down into the Earth
at the
north magnetic pole.
Nasky/Shutterstock
How do the
norths align?
Magnetic north and geographic north align when the so-called "angle
of declination", the difference between the two norths at a
particular location, is 0°.
Declination is the angle in the horizontal plane
between magnetic
north and geographic north.
It changes with time and geographic
location.
The declination angle varies between -90° and +90°.
Author provided
On a map of the Earth, lines along which there is zero declination
are called
agonic lines. Agonic lines follow variable paths
depending on time variation in the Earth's magnetic field.
Currently, zero declination is occurring in some parts of Western
Australia, and will likely move westward in coming years.
Locations on this 2019 map
with a green contour line have zero declination.
Lines along which declination is zero
are called agonic lines.
Author provided
That said, it's hard to predict exactly when an area will have zero
declination.
This is because the rate
of change is slow and current models of the Earth's magnetic field
only cover a few years, and are updated at roughly five-year
intervals.
At some locations, alignment between magnetic north and geographic
north is very unlikely at any time, based on predictions.
The
ever-changing magnetic poles
Most compasses point towards Earth's north magnetic pole, which is
usually in a different place to the north geographic pole.
The
location of the magnetic poles is constantly changing.
Earth's magnetic poles exist because of its magnetic field, which is
produced by electric currents in the liquid part of its core. This
magnetic field is defined by intensity and two angles, inclination
and declination.
The relationship between geographic location and declination is
something people using magnetic compasses have to consider.
Declination is the reason
a compass reading for north in one location is different to a
reading for north in another, especially if there is considerable
distance between both locations.
Bush walkers have to be mindful of declination. In Perth,
declination is currently close to 0° but in eastern Australia it can
be up to 12°.
This difference can be
significant.
If a bush walker following a magnetic compass
disregards the local value of declination, they may walk in the
wrong direction.
The polarity of Earth's
magnetic poles has also changed over time
and has undergone
pole reversals.
This was significant as
we learnt more about plate tectonics in the 1960s, because it linked
the idea of seafloor spreading from mid-ocean ridges to magnetic
pole reversals.
Geographic north
Geographic north, perhaps the more straightforward of the two, is
the direction that points straight at the North Pole from any
location on Earth.
When flying an aircraft from A to B, we use directions based on
geographic north. This is because we have accurate geographic
locations for places and need to follow precise routes between them,
usually trying to minimize fuel use by taking the shortest route.
All
GPS navigation uses
geographic location.
Geographic coordinates,
latitude and
longitude, are defined relative
to Earth's spheroidal shape.
The geographic poles
are at latitudes of 90°N (North Pole) and 90°S (South Pole),
whereas the Equator is at 0°.
An
alignment at Greenwich
For hundreds of years, declination at
Greenwich was negative,
meaning compass needles were pointing west of true north.
At the time of writing this article I used an online calculator to
discover that, at the Greenwich Observatory, the Earth's magnetic
field currently has a declination just above zero, about +0.011°.
The average rate of change in the area is about 0.19° per year,
which at Greenwich's latitude represents about 20km per year. This
means next year, locations about 20km west of Greenwich will have
zero declination.
It's impossible to say how long compasses at Greenwich will now
point east of true north.
Regardless, an alignment after 360 years at the home of the Prime
Meridian is undoubtedly a once-in-a-lifetime occurrence.
| 
