For the first time in several centuries, compasses in Greenwich will point at true north

World Magnetic Declination
Map of magnetic declination in 2015(Credits - Wikimedia Commons)

Compasses in Greenwich will be pointing towards the true north for the first time. This coincidence of magnetism and time has not occurred for almost 360 years. It is set to occur in the coming fortnight and it is a reminder of the fact that magnetic north pole of Earth is constantly wandering, unlike Earth’s geographic north pole. 
The angular difference between the geographic and magnetic meridian at any place is known as magnetic declination. Although this difference does not affect the normal citizens in a significant way, this disparity can last for a long time. For almost a hundred years in the United Kingdom, the compass needles have been pointing to west of actual north since the magnetic declination has been negative. 
This is not permanent. The agonic, an invisible line which joins the north and south magnetic poles of Earth which also represents the zero declination has been shifting in the western direction at a rate of nearly 20 kilometres or 12 miles every year
If this rate continues, it will be passing through Greenwich in this month which is the site of the Greenwich Royal Observatory thus creating a historic occurrence. Ciaran Beggan, geomagnetism researcher at British Geological Survey said that in the month of September, the agonic will meet zero longitudes at Greenwich. Since the creation of the Observatory, this is the first time that geomagnetic and geographic coordinate systems will be coinciding at this place. 

The Royal Observatory was founded at the decree of King Charles II in 1675. It is a coincidence that compasses in Greenwich due to zero declination also pointed towards true north. From that time, the agonic has been shifting as Earth’s magnetic north pole has been varying in its position due to changes in the molten outer core. 
Beggan said that the agonic will continue to pass across the United Kingdom for the next 15-20 years. So this phenomenon is expected to continue beyond circumstantial synchronicity of this September. By 2040, compasses will most probably point east of true north. Beyond this scientists cannot provide any prediction as it is difficult to estimate the future magnetic movements. Beggan said that currently, it is not possible to predict how the magnetic field will change in the time span of several decades. For 360 more years in the United Kingdom, the compass may point towards the east of the true north direction. 


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