The Great Conga in the Sky

It has taken me a fortnight to get to grips with sun run sun, Moon and planet sights.  I still have around 800 miles of the trip left to fathom star sights.  I’ve been shying away from this somewhat as the pro-forma given in Mr Cunliffe’s book is basically a page containing seven columns of numbers and it looks like a lot of heavy calculations.

  My Favourite Book gives a list of 57 Stars, the brightest ones, thus they are the most useful to the navigator.  I learn that there are more we could use, but 57 seems like plenty to me.  I had been spoiled in the early part of the trip with a huge full Moon lighting up the horizon for hours.  The horizon is an essential part of sextant operation, through a clever series of sights and mirrors we can look at both a celestial body in the sky and the horizon in the same viewfinder, no horizon – no sights.  On these bright nights I took sights on everything and anything that I recognised, all of them featured on the list of “The 57 Stars”, thus I had assumed that I would be able to deduce my position from any or all of them at any time I could see them.  

  Lesson number one, I need the horizon.  Lesson number two, taking sights on any random stars because you know their names, is not how this works!  It probably is possible,  I think the maths based book I had brought with me has all the right ingredients, but the language of cos, tan, sin, P, Z, X, and other Greek, was not one that I had ever learned or understood.  Throw in the added complication that we are not dealing with ordinary, regular straight edged triangles, no, our triangles have curved edges and this is all too much for my little brain.

  I left the stars alone for a couple of weeks and focused my efforts on the Sun, Moon and planets.  Eventually I return to my best friends Mr Rodgers and Mr Cunliffe, they make a fix from the stars sound so simple and efficient, but the process looks so lengthy.  We must begin somewhere…

  Aries, The First Point in Aries, to be precise, is where we start.  Already this couldn’t be straightforward, The First Point in Aries is just a point in the sky (more accurately a line of longitude on the celestial sphere).  

  Aries…. Longitude…. Celestial sphere…. Confused already?  Yes, me too.  It gets easier when you realise that it is all just painted on a dish and most importantly that dish is spinning around us.  On that dish there are paintings, a sheep, a cow, a giant and his dog, then a unicorn, a pair of twins, a serpent, a lion and more, all locked in an eternal conga across the heavens.

  As the dish spins around us we see the sheep pop its head up over the eastern horizon, over the course of the night the sheep will rise up, pass above our head and then descend in the western sky, the last thing we see is its tail dipping below the western horizon.  The cow follows the sheep, the giant is chasing the cow and the giant’s dog faithfully runs after the giant…. All night…. Every night…. Since long before we were around to observe them.

  Back to the Point of Aries.  The point is this is our sheep.  The sheep passes exactly overhead at Greenwich in London at known (but slightly different) time each day.  The Favourite Book gives us “Sheep o’clock” for every day of the year, so that part is straightforward.  Provided I remember to read the “Sheep” column in the book of numbers, and not the “Sun” or “Moon” or one of the other numerous options given there.

  Now, if I were at Greenwich, at Sheep o’clock, I would go to my new found friend The Stars Epoch book, find the page for my latitude, find the section for Sheep o’clock and the book will give me a list of seven stars.  If I use these seven stars to take my sights I can compare my sights directly to the numbers given in the Stars Epoch and from that I can work out where I am.  Once the correct numbers are there, the mathematical process is along the same lines as for Sun, Moon and Planets.  Seven stars and seven sights looks and sounds like a long and literally numerous task.

  The next consideration is that I am not standing at Greenwich at Sheep o’clock, I am quite a long way west of Greenwich (two time zones different), so the sheep will pass over Greenwich two hours before it passes over me.  After all, they are just pictures painted on a dish, luckily the dish spins at a nice constant speed and the pictures are always the same distance apart.  If I’m standing in Greenwich looking at a sheep over my head, someone in Estonia would be looking directly up at, say, the unicorn.  

  We know for certain that in a particular position, at a precise time the angles between the seven stars and us the observer, would match those angles given in the book.  Essentially from our estimated position we assume we are at a nice round number latitude and longitude that is nice and close to our estimated position.  We work out whether it is sheep, cow, giant, dog, or other o’clock with us and refer to the Stars Epoch book, to find the seven stars to use.  I frequently end up repeating this process, mostly through my own mathematical ineptitude, but practice makes perfect, so they say.  Next step is to go and find those seven stars, then wave the sextant at them, write the numbers down, do the maths and hope for the best.

  Under the brightness of the moonlight, with the horizon visible this is feasible for many hours during the night.  But the full moon soon wanes and the nights get darker and darker as the weeks pass by.  The challenge now is that there are only two very brief times of day where both the stars and the horizon are visible together, the twilight at dawn and at dusk.  Now we need to know whether it will sheep, cow, giant, dog or something else o’clock during the short twilight spells.

  I quickly learn lesson number three, prior preparation is essential before attempting star sights.  Then I come up against lesson number four when I discover that taking star sights at twilight is a constant race against the rising sun or the growing darkness each day.  There is no pause button, the dish keeps on turning.

  I find it easier to take the morning sights.  Once the pre planning is done below deck, I get my head outside into the night while it is still nice a dark and all the stars are out.  There is time to sit on deck, stare at the pictures on the dish that is the sky and find the seven stars listed in the book, for my present time – Lion o’clock.  As the dawn starts to break and the horizon appears it doesn’t take long to get the stars lined up in the sextant, down to the horizon and the numbers recorded quickly.  

  At dusk twilight, things are a little trickier.  I have been through the pre-planning, found which seven stars I should use for my local hour angle (which today is Fish o’clock).  Next  I need to shoot them before I lose the horizon to darkness, the race is on.  

  To make matters more complicated I only know and recognise two of the seven stars and I have no other stars to use as reference, they are not out yet.  The seven listed stars are bright and so should appear first, but of course more than seven appear at the same time.  To solve this the Stars Epoch does give a bearing to each of the seven stars and a very good idea of roughly what angle above the horizon it should be.  So now armed with hand bearing compass, sextant and Star Pocket I search for my stars.

  Vega is the first to show herself in the north west followed by Deneb in Cygnus.  Now for the others I don’t recognise, I manage to find Altair in Alquila and Fomalhaut (not sure which constellation). The weather is now working with the nightfall to thwart my plans and the stars in need in the east are lost in clouds.

  It transpires that the daunting page of several columns of numbers is actually not as daunting as it first appeared.  Having followed the practical instructions of Mr Rodgers this far, I go to the pro-forma given by Mr Cunliffe, and copy his method with my numbers, I begin to see the simplicity of this final part.  We have been through all the complicated bits to reach this point, it is all downhill from here.  With numbers transferred to their corresponding columns, index error, height of eye and corrections all applied we are left with that magic number “Ho” for each star.  The mantra Ho Mo To returns again.  

  The numbers lead a dance and again this mathematical pattern is trying to appear in the periphery of my mind.  I can’t see it and I doubt I ever will, it is far too complex for my little mind.  But I can see when my numbers look right and when they look wrong, dare I say it is starting to make sense.   

  The final step is to draw the results on a Plotting Sheet and unlike Sun navigation which needs around four hours to produce three lines on a sheet, star plotting is a dream.  In one hour the sights are taken, calculated and there on the Plotting Sheet you have a position, with up to seven lines!  Our position is there, staring me in the face, that small triangle where most of the lines cross.  This whole process is super satisfying, I am totally confident that fix I proudly mark on the chart is where we are to within the thickness of a pencil line.

  Occasionally on the Plotting Sheet I have a line which looks a long way off, I accept this human error. The stars all look pretty similar and I am battling the spinning of the dish, clouds and my sextant, with the useless telescope eyepiece removed and having to work the whole thing backwards.  I am not surprised that I lose a star every now and then.  Of course by now it is too dark or too light to go and take another sight, the dish has moved on, so have we, I can only accept the error for what it is and move on too.

Leave a Reply

Your email address will not be published. Required fields are marked *