Modern Australian

what happens when magnetic north and true north align?

  • Written by Paul Wilkes, Senior Research Geophysicist, CSIRO

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 1,000km away.

what happens when magnetic north and true north align? 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.

what happens when magnetic north and true north align? 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.

what happens when magnetic north and true north align? 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, Author provided (No reuse)

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.

Read more: New evidence for a human magnetic sense that lets your brain detect the Earth's magnetic field

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 minimise fuel use by taking the shortest route. All GPS navigation uses geographic location.

Read more: Five maps that will change how you see the world

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.

Authors: Paul Wilkes, Senior Research Geophysicist, CSIRO

Read more http://theconversation.com/explainer-what-happens-when-magnetic-north-and-true-north-align-123265

NEWS

what makes Jacinda Ardern an authentic leader

The qualities that have made Jacinda Ardern New Zealand’s most popular prime minister in a century were on display this week as she took an earthquake in her stride during...

The poorest Australians are twice as likely to die before age 75 as the richest, and the gap is widening

ShutterstockPeople living in socially disadvantaged areas and outside major cities are much more likely to die prematurely, our new research shows. The study, published in the journal Australian Population Studies...

The government says artists should be able to access JobKeeper payments. It's not that simple

Many workers in the film industry are excluded from JobKeeper.The Nightinggale/Transmission FilmsThis week, Australia’s finance minister Mathias Cormann told ABC radio he didn’t “accept [the] proposition” workers in the arts...

Why the coronavirus shouldn't stand in the way of the next wage increase

ShutterstockIn the early 1970s, when rising inflation and unemployment tore through the economy, someone coined the aphorism “one man’s wage increase is another man’s job” (unfortunately, most of the talk...

how media mythbusting can actually make false beliefs stronger

ShutterstockAs the COVID-19 pandemic has swept the world, politicians, medical experts and epidemiologists have taught us about flattening curves, contact tracing, R0 and growth factors. At the same time, we...

Why Trump's Make America Great Again hat makes a dangerous souvenir for foreign politicians

ShutterstockIt looked just like any posed political picture. The politician, in this case the National Party’s newly elected leader, Todd Muller, standing by a bookcase. So far so normal. It...

now he has an election to win and a country to save

AAP/EPA/Tracie van AukenAt age 77, in his twilight years, the third time was the charm for Joe Biden. He prevailed over a field of 24 Democrats from across the political...

If you took to growing veggies in the coronavirus pandemic, then keep it up when lockdown ends

Lynda Disher/ShutterstockThe COVID-19 pandemic produced a run on the things people need to produce their own food at home, including vegetable seedlings, seeds and chooks.This turn to self-provisioning was prompted...

public transport is key to avoid repeating old and unsustainable mistakes

ShuuterstockThe coronavirus pandemic has affected our cities in profound ways. People adapted by teleworking, shopping locally and making only necessary trips. One of the many challenges of recovery will...

P is for Pandemic: kids' books about coronavirus

NSW HealthWith remarkable speed, numerous children’s books have been published in response to the COVID-19 global health crisis, teaching children about coronavirus and encouraging them to protect themselves and others...

Australian economy must come 'out of ICU': Scott Morrison

Scott Morrison says it is vital to get the Australian economy “out of ICU” and “off the medication” of government support “before it becomes too accustomed to it”.In speech on...

Eden-Monaro byelection to be on July 4

Speaker Tony Smith has announced July 4 for the byelection in the Labor NSW seat of Eden-Monaro, which will be the first electoral test between Scott Morrison and Anthony Albanese.For...

Popular articles from Modern Australian

If you buy virtual currency, use a safe and secure exchangeUrban Development: Trends Shaping The Future of CitiesThree cities worth visiting in PolandUpgrade your career in beauty therapy with these short beauty courses6 Ways To Treat Obstructive Sleep Apnea (OSA)Top Tips for the Best Camping TripHealthy Cooking at Home - Tips & TricksMental Health and Covid-19: How Effective are Health- Supplements?Know the Best Times to Eat Protein BarsEnhancing Self Sense of HumorPlanning a wedding overseas? Keep your spending in check with these simple tipsNutri-Grain is launching 'Gold Honey Crunch', a limited-edition flavourTammy Hembrow Launches Mini Saski In Time For Mother's DayBad Cooking Habits You Must BreakTeaching Kids to Brush Their Teeth and Floss Correctly