A note from Frans Loots, Jeremy‘s team manager and friend. They have known each other since 1980 when Jeremy was a timid young first-year law student at the (then) University of Port Elizabeth and Frans was in his third year.
Frans is an incredibly experienced and talented sailor in his own right with numerous South and North Atlantic crossings to his name. One of his South Atlantic crossings in 1982 was singlehanded in a Petersen 33 using only sextant to navigate, and with no self-steering device. He was kind enough to give us all some background to Celestial Navigation (sometimes also referred to as sextant navigation).
We have been reading Jeremy’s updates and accounts of mastering the art of celestial navigation as he heads North up the Atlantic on his solo passage to France. Celestial navigation is easily learnt, but it takes time and practice to perfect. And that is what Jeremy is doing right now. Most of us have various devices with built-in GPS ‘s which give us our position at any given time and are accurate to within a meter or two. Not so for Jeremy. The use of a GPS or any device with a built-in GPS is strictly forbidden in the GGR race rules. Skippers in the race must navigate and determine their positions by using a sextant and paper charts.
The navigator’s daily routine would be to take a sun sight with the sextant between 9 and 10 am ship’s time. A chronometer is used to time the exact moment of the sextant reading of the angle of the sun above the horizon, down to the last second. Then it is down to the chart table. The sextant angle and recorded time are then entered onto a worksheet and the tables from various Sight Reduction Tables and Nautical Almanacs are used to derive a “Line of Position”. The position line is then drawn on the chart. But the position line itself will not tell you where you are. You are simply somewhere along that line. To get a “position fix” a second position line is required. For this, the navigator will wait until Local Noon, i.e. that moment when the sun is at its maximum height above the horizon wherever the yacht could be. To take a noon sight the navigator can predetermine from the Nautical Almanac the approximate time for the Noon Sight when the sun will be at its highest point. He then takes the sextant and follows the arc of the sun until the very moment it starts dipping down again. Getting this done accurately takes a bit of practice and skill.
This reading, again worked in with the sets of tables from the Almanac, will give you your Latitude and the Latitude will get plotted onto the chart. Your newly plotted Latitude will get drawn by a pencil line across the chart and it will cross the Position Line which you had drawn earlier that morning. So, if you have not moved anywhere, and remained stationary, there where your two lines cross, will be your position. But hopefully, you would have been sailing at a good rate of knots and have to transfer your morning position line according to your compass course and speed to your new estimated position. This is a “Transferred Position Line” and that will be drawn on the chart to give you a triangle within which your yacht will be. This will be your present position of “Fix”. The smaller the triangle, the more accurate your fix. Your 24-hour daily progress will be determined by your run from between consecutive Noon Fixes. For more accuracy, the entire process can be repeated in the mid-afternoon for another Position Line to confirm your earlier calculations. A skilled navigator will get his sextant out, pop up on deck, take a reading and be done with his plotting in less than 15 minutes. The most difficult part of celestial navigation is to get an accurate sextant sight of the sun while the boat is rolling under you and the horizon disappears behind waves and swell if there is a sea running. With time your skill improves and so does your accuracy. Keep in mind, no sun, no position fix.
Being accurate to within 14 nautical miles is pretty good for a navigator working from the low deck of a rolling sailing vessel. Even being accurate to within 30 nautical miles will do after a few days of no sun. Navigating by star sights follows a slightly different method, but it is also done by sextant and tables. Star sights do have the advantage that you can take sights of more than one star at a time and get a fix immediately, without waiting for a second position line. Jeremy has by now sailed approximately 3 000 nautical miles and is fast approaching the halfway mark of his voyage to France. In doing so he has completed his 2 000 mile qualifying sail and deserves an ice cold beer!
Disclaimer :If some expert navigator reads this, please don’t shoot me down, it’s been 25 years since I’ve navigated by sextant!Frans