Between fixes a navigator keeps a dead reckoning (DR) position: the last fix, moved along the course steered by the distance run. Distance is speed × time: 6 knots for 3 hours is 18 miles. Every sight in this course starts from a DR, so it is worth seeing how it is made.

The compass points to magnetic north, which is not true north. The difference, the variation, is printed on the chart: with 14° W variation, a magnetic course of 065° is 051° true. The chart and the plotting sheet are drawn to true north, so correct the course before plotting it ("variation west, magnetic best": with west variation the magnetic number is the larger, so take it off to get true).

Course steered (magnetic): 065° M
Variation 14° W, so true course: 051° T
Distance run = speed × time: 6 kn × 3 h = 18 nm
Change of latitude: 11.3′ N (= 11.3 nm)
Change of longitude: 18.6′ E (fewer nm than that)

Measure distance on the latitude scale (the sides of the sheet), where 1′ is 1 mile. The dashed line shows where you would plot if you forgot the variation: 14° off over 18 miles puts you 4.4 miles wrong.

Course steered: 065° MSpeed: 6 knotsTime: 3 h

A DR is only as good as the course and speed: current, leeway and a wandering helmsman all add up, which is why a navigator wants a fix from the sky. The rest of the course is about getting one.

Dead reckoning: where you think you are