A number of our early compasses have a small arrow or some other mark indicating True North at the time of their making – for example, in 1790, declination – the angle between True and Magnetic North – was 23.5 degrees West.
The Royal Observatory has announced that, at some point over the next two weeks, compasses at Greenwich will point true north for the first time in about 360 years. And for some parts of the UK, this may not happen for another 20 years. Either way, it is a once-in-a-lifetime event.
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. Over 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 at a rate of around 12 miles (20km) a year, experts say. By next month, the compass needle will point directly to true north at Greenwich in London, before slowly turning eastwards.
The Royal Observatory Greenwich was established in 1676 and, from 1839, hosted the specialised magnetic observatory that made continuous measurements from 1840 onwards. In 1926, the instruments were moved to Abinger in Surrey, as electrified railway lines had made it impossible to measure the magnetic field.
Dr Ciaran Beggan, a geomagnetism scientist at the British Geological Survey’s Lyell Centre in Edinburgh, 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.
“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 360 years in the UK.”
However, the experts have stressed that zero declination will have no impact on daily life. Beggan added: “Compasses and GPS will work as usual – there’s no need for anyone to worry about any disturbance to daily life.”