Navigating with Sun and Moon

using a sextant to take a sunsight

using a sextant to take a sunsight

After having learned the theory of fixing my position using celestial objects during my Yachtmaster Ocean theory course, I was keen to try it for real during a trip from Spain to Morocco a few days later. The first two days there was no horizon, no sun, nowhere you could  safely jam yourself in to and no chance of my even thinking about doing calculations of any type. But on the third day the storm abated and the sun came out.

I was surprised at how well the sextant allowed me to measure the angle between the sun and the horizon despite the heavy swell. Whoever had this idea was a genius. Unfortunately I am not so clever and must have written down the measured angles false as I could not arrive at a position on this day.

plot of morning and afternoon sights as well as merdian pass

navigation plots

The next day (7. December) I took a morning sight, then a meridian pass and finally an afternoon sight. After doing all the calculations, I was amazed to find that I was only 3 nautical miles off our actual position per GPS. I was stunned to discover that I could actually derive my position using just the sun, a sextant and a set of tables.

The next day (8. December) I took both a sun sight and moon sight in the morning and then an afternoon sight. Using these three I was once again able to calculate our position to within three nautical miles. Yesterday had not been a fluke. I was a little shocked. The map shows the route my GPS plotted and the position of the two fixes. Either would have been accurate enough to have found our destination.

positions plotted per sun and moon

positions plotted per sun and moon

Once you have taken the sights, there is then a lot of calculating to be to done before you have the values you need to plot your position. I have created Google Sheets to make this work a little easier and reduce the risk of mistakes. These worked well but the experience has shown where they need to be improved.



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