Correcting the dead reckoning position we use the sextant. This instrument is nothing more than an angle measuring device. If we would be on a certain distance from a church we would be able to measure the angle between the doorstep and the top of the church. With knowledge of the height of the church we can, with some goniometric magic, determine the distance to this church.

If measuring a larger angle on the next measure, we are closer, in case of a smaller angle, further away. In the middle of the ocean churches aren’t that widespread, so we are measuring the angle between the horizon and a celestial body; sun, moon, star or planet. Using the nautical almanac and some calculations we can decide how high we should have measured this celestial body form the dead reckoning position. The difference in angle determines the distance further or closer to the dead reckoning, seen from the position of the celestial body. 

Since people miles away from me are measuring the same angle, in the same direction (these celestial bodies are far away), I can draw a line on the chart tracking all these pitches. If I succeed in another measure, that results in another line, that leads to a crossroad  which will be our new starting point for correcting the dead reckoning…until we reach the destination…or somewhere else. In case of the latter, you might have expected this to be a full Astro Navigation course, although it is only meant to be a short explanation of the principle. A bit of ‘abacadabra’ it may remain…

Richard Slootweg
Captain Clipper Stad Amsterdam