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Over the next two weeks, the Greenwich compass will show true north for the first time in about 360 years.
And for some parts of the UK, this might not happen before 20 years. Whatever it is, it is a unique event in life.
The angle that makes the needle of a compass between True North and Magnetic North is called declination. As the magnetic field changes all the time, so does the declination at a given location.
In the United Kingdom, over the last hundred years, the declination has been negative, which means that all compass hands have been directed to the true north west.
The zero declination line, called agonic, moves westward at a speed of about 20 km a year, according to experts.
Next month, the compass needle will point to the true north of Greenwich in London, before slowly turning to the east.
The Royal Observatory of Greenwich was established in 1676 and, from 1839, hosted the specialized magnetic observatory which carried out continuous measurements from 1840. In 1926, the instruments were transferred to Abinger, Surrey because the electrified railway lines had made it impossible to measure the magnetic field.
Dr. Ciaran Beggan, a geomagnetism scientist at the Lyell Center of the British Geological Survey in Edinburgh, said: "At some point in September, the agone will reach zero longitude in Greenwich. This is the first time since the creation of the observatory that geographic and geomagnetic coordinate systems coincide at this location.
"The agony will continue to cross the United Kingdom over the next 15 to 20 years. From here until 2040, all compasses will probably be directed to the true north east.
"It is currently impossible to predict how the magnetic field will change over the decades, or even centuries, so that the compass can very well point to the geographical north east for another 360 years in the UK. "
However, experts pointed out that zero declination would have no impact on everyday life.
Beggan added: "Compass and GPS will work normally – no one has to worry about the disturbances of everyday life."
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