# From sailing on a rhumb to flying on a geodesic

## That’s Maths: Pedro Nunes, a Portuguese cosmographer and one of the greatest mathematicians of his time, was first to develop a mathematical basis for navigation

If you fly 14,500km due westward from New York you will come to Beijing. The two cities are on the 40th parallel of latitude. However, by flying a great circle route over the Arctic, you can reach Beijing in 11,000km, saving 3,500km and much time and aviation fuel. The route following the parallel of latitude maintains a compass bearing of 270 degrees relative to true North, crossing every meridian at a right angle. In contrast, the great circle route provides the shortest distance between the two cities, but the compass direction varies along this route.

A route that crosses every meridian at the same angle is called a loxodrome, a word that means “running obliquely”. For short voyages at low or middle latitudes, a loxodromic course is close to the minimum distance route, as it deviates only slightly from a great circle. However, at high latitudes, loxodromes are not suitable for navigation: they ultimately spiral repeatedly around the poles.

Pedro Nunes

Loxodromes were first discussed by Pedro Nunes in 1537 in his Treatise in Defence of the Marine Chart. Nunes, a Portuguese cosmographer and one of the greatest mathematicians of his time, was the first person to develop a mathematical basis for navigation. This was a subject of vital importance in Portugal during the 16th century: sea trade was the main source of Portuguese wealth. By means of such mathematical advances, new worlds were discovered and vast riches were found. As Nunes wrote, "The Portuguese dared to take on the great sea and ocean. They discovered new islands, new lands, new seas, new peoples; and what is more, new heavens and new stars."