ANYONE who has ever stood on the Prime Meridian line at Greenwich Observatory in London holding a compass will know that True North and Magnetic North do not align.

Earth’s wandering magnetism means that the magnetic North Pole is constantly shifting, sending metal compass needles darting away from the geographical North Pole – a problem that requires hikers to make continual adjustments when navigating by map. But next month, for the first time in 360 years, True North and Magnetic North will line up.

It means that for traditionalist explorers in the capital and its surroundings, no alterations to compass readings from True North will be required.
The occurrence is so rare that it has not happened since before Greenwich Observatory was built in 1676.

800px MuseeMarine compas p1000468In fact, when the Prime Meridian of zero degrees longitude was established 1884 – marking an imaginary line between the North and South poles – magnetic north was nearly 25 degrees further west, one of the biggest discrepancies in hundreds of years. The line pointing directly to the magnetic North Pole is known as the agonic and the angle between True North and Magnetic North is called the declination. As the magnetic field changes all the time, so does declination at any given location.

Dr Ciaran Beggan, a geomagnetism scientist with the British Geological Survey, said: “At some point in September, the agonic will meet zero longitude at Greenwich. This marks the first time since the observatory’s creation that the geographic and geomagnetic coordinate systems have coincided at this location. “In and around London, it means that people navigating using compasses do not have to make a correction for the offset away from True North – for a few years at least. The agonic will continue to pass across the UK over the next 15 to 20 years.”
She added: “Compasses and GPS will work as usual; there’s no need for anyone to worry about any disturbance to daily life.”

The Prime Meridian at Greenwich was set in 1884 using the large Transit Circle telescope built by Sir George Biddell Airy, the seventh Astronomer Royal. The telescope tracked the movement of “clock stars” – circumpolar stars that never rise or set.

Because these stars are always present in the sky and transit the meridian twice each day, their appearance in the telescope cross hairs can be used to set time and longitude.

It created the definition of zero longitude which paved the way for a global reference system for maps and established Greenwich Mean Time.

When GPS was switched on in 1984, experts realised that the real meridian line is actually 334ft to the east of the metal marker laid into the ground at Greenwich because 19th-century astronomers had failed to factor in the impact of gravity.