Dr Ciarán Beggan is a geomagnetic specialist working at the British Geological Survey. Here, he explains how the UK's compasses will point 'true north' for the first time in around 360 years at some point over the next two weeks...
The angle a compass needle makes between True North and Magnetic North is called declination. As the magnetic field changes all the time, so does declination at any given location.
|Greenwich zero declination map|
For the past few hundred years in the UK, declination has been negative – meaning that all compass needles have pointed west of True North. The line of zero declination, called the agonic, is moving westward, presently at a rate of around 20 km per year. In 2017, it passed the easternmost side of the Britain when declination became positive (pointing east of True North). By September 2019, for the first time in about 360 years, the compass needle will point directly to True North at Greenwich, London before slowly turning eastwards (Figure 1). The last time this happened was during the reign of Charles II, around 1660.
By coincidence, it was Charles who established the Royal Observatory at Greenwich in 1675. Occasional measurements of the declination angle were made from 1680 onwards (Figure 2). In 1836, continuous measurements of the declination and inclination angles of the compass needle were started under the Astronomer Royal, George Airy. This continued until1926 when the electrification of the railways finally made the site too magnetically noisy to be scientifically useful and the observatory moved to Abinger.
In 1884, the great circle line between the North Pole and the Airy Transit Circle at the Observatory became the definition of ‘zero’ longitude by international agreement. This paved the way for a standard global reference system for maps and navigation to be established, creating the Greenwich Meridian and with it Greenwich Mean Time.
At some point in September, the agonic will meet zero longitude at Greenwich, thus marking the first time in since its creation that the geographic and geomagnetic coordinate systems have coincided at this location.
The agonic will continue to pass across the UK over the next 15 to 20 years - by 2040 all compasses will probably point eastwards of True North. It is, at present, impossible to predict how the magnetic field will change over decades to centuries, so the compass may well point east of True North for another 370 years in the UK.
Macmillan, S. and Taylor, T., A magnetic prediction comes true, Astronomy & Geophysics, 60, 2019, 1.16, doi:10.1093/astrogeo/atz041
Malin, S. C. R., & Bullard, E. (1981). The direction of the Earth's magnetic field at London, 1570-1975. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 299(1450), 357-423. doi: 10.1098/rsta.1981.0026