Line of No Variation

An imaginary line can be drawn be drawn through these places which today is called the agonic line.

In the eighteenth century, the agonic line was known as the 'line of no variation.'

As one moves west of the agonic line, easterly variation will increase. This is because the compass will increasingly point to the east of true north. When east of the agonic line, westerly variation will increase.

Over time the agonic line has had to be re-drawn as magnetic variation has changed, due to fluid motion in the molten metallic region of the outer core of the earth that lies from 2800 to 5000 kilometres below the planet's surface. The complexity of fluid motion has also meant that magnetic declination has changed in unpredictable ways.

The lack of magnetic variation in certain places was first noted in the late fifteenth century during the course of voyages to America. Colombus, for example, in his account of his third voyage of 1498, observed that 100 leagues west of the Azores ''the needle of the compass, which hitherto had turned toward the northeast, turned a full quarter of the wind to the northwest' (Columbus, cited Benjamin, History of Electricity, p. 199).

This led to speculation that there existed a 'true meridian' that could be used to determine longitude accurately.

By the end of the fifteenth century, however, it had become apparent that there was no readily discernible patten to variation. Mariners needed to make daily celestial observations to determine the variation of the compass at the point they were located and correct their course accordingly. By the mid-sixteenth century it was also clear that the amount of variation recorded in any one place was subject to change over time.

Even so, scientific interest in magnetic declination led to various hypotheses suggesting the phenomena might provide a means of determining longitude at sea.

By the turn of the eighteenth century the most influential hypothesis in English scientific circles was that propounded by the astronomer Edmond Halley. Halley suggested that the earth possessed four magnetic poles, two in the outer shell of the earth, and two in the core of the earth that revolved every 700 years.

In 1698-1700, Halley undertook two voyages sponsored by the Royal Society, during which he made around 150 measurements of magnetic variation at various points in the north and south Atlantic. He then employed this data to produce a chart, published in 1701, showing how the earth's four poles influenced magnetic variation in the Atlantic. Halley took care however to stress that the chart would require updating because the influence of the poles would change over time, altering the degree to which compasses were affected by magnetic variation.

Combining his Atlantic data with information drawn from journals of voyages to the Indian Ocean, Halley then produced a sea chart of the whole world the following year. This chart, published in 1702, was republished in various editions and revisions through the course of the eighteenth century. It was widely used by European voyagers including Cook.

Halley plotted a line of no variation running southeast from the coast of Carolina that passed westwards of the Cape Verde islands and east of St Helena.

By the time of Cook's first voyage, this line of no variation was perceived to have shifted further westward.