It turns out that during the day a sailor has only one real chance at confirming the ship’s latitude, this happens at noon each day. When the Sun is at her highest point in the sky we can take our sextant, measure the angle between the Sun and the horizon and after applying some maths we can calculate our latitude.
It also turns out that noon is not at noon, so already the whole affair becomes a little more complicated. This is where my new favourite book to be, The Admiralty Almanac, shows me exactly why it carries the hefty price tag, all the answers are in this book, I’ve just got to decode it. Mr Rodgers and Mr Cunliffe provide sufficient guidance and I’m away. Even standing still the time of noon varies by up to 16 minutes away from clock noon over the course of a year, because of something called the Equation of Time.
The beauty of the navigation side to this is that you don’t need to know why this is, one simply needs to know where to look in the new Favourite Book. Today noon is actually at 1144 if you’re standing on the Greenwich Meridian, which of course we are not. . So… I know the Sun will pass over Greenwich in London at 1144, from my dead reckoning position I have an idea of how far west we are, so the Sun will pass over my head some time after 1144. I can now refer to the favourite book and estimate what time the Sun should reach her highest as she passes over my head (or my meridian on the globe if you prefer).
Then it is off out on deck in the heat of the midday sun, armed with sextant, watch, notebook and pencil. First thing, check and write down that sextant error bit, once that is done it is time to start watching the world spin round using a small telescope and a couple of mirrors. I am on deck 20 minutes before I expect my local noon to happen (since mine is only approximate based on my estimated position), and so I measure and watch as the sun stops rising in the sky, now she seems to steady on the horizon for a few minutes before she begins her slow descent toward the western horizon. And that is it, it is all over, we have our time and measurement taken with the sextant written down and it is time to get back into the shade.
Now I am getting more acquainted with my new Favourite Book, it will help me do a handful of calculations to (hopefully) ascertain our latitude at noon. Two important things happen here. First off the world tilts on its north south axis, hence we get longer days and the sun higher in the sky during the summer and now, in the winter our days are short and the sun is low in the sky. In the celestial navigation world this is called declination. And that, is more than we need to know about it, we’ve just got to ask the Favourite Book for the answer. We do however have to ask it the right questions, mostly “what time is it at Greenwich” and “what time is it with me here and now”, the book reveals differences between the two that we must take into account to find our precise declination. Then a very quick sum and we have our latitude.
At this point I don’t think it is cheating to now check our noon position according to the GPS to see if I am close or not. In a beginners kind of way I probably am close, on the scale of the ocean chart it looks close, but for the first three days my noon sights were not accurate enough.
I continued this process like a devoted student, on day three I realise the error I have is a consistent one, so this time I take the noon latitude from the GPS and do the maths backwards to work out my errors. Operator error and operator understanding are definitely the cause, looking at the sextant instructions on or before day one would have been an idea too. So with errors found and corrected and corrections applied to the previous sights too, my latitudes are now acceptably close to our GPS track and I am starting feel that the task I have set myself might still be achievable..