Parasailor up, Thursday 25 April
Skippers Star Sight Blog, Night of Thursday-Friday 25/26 April
Another thing we do while ocean sailing is to take sights of Stars, Planets, Moon and Sun in order to determine our position, and in case the GPS goes wrong, or is taken out of service by malevolent forces.
The principles of Astro-Navigation are fairly straightforward- the heavenly objects are imagined to be mostly stable in relation to each other out there on the heavenly sphere, a kind of extension of the earth in the heavens.
If we imagine them in this way, then at any point of time, each heavenly object has a point on the earths surface where light coming from it would pass through on its way to the centre of the earth (if of course the earth were transparent!). If we can see the object from our own position, then it is above the horizon, and its altitude from our position can be observed, and, using a device for measuring angles from the horizon called a Sextant, we can measure its angle above the horizon. We can also measure its latitude, or Zenith, in the spherical hemisphere using the hand-held compass, up to 360 degrees (a complete circle), but this is really only to check that it is the star we think it is because the Zenith of each star and planet used for navigation purposes (the brightest and most stable in terms of light) for any date and time of day, and part of the globe, is given in the Almanacs, and a hand held compass will only give an approximation.
Given the star sight we can proceed to calculate our position by using the tables in a special almanac for each time (expressed in Universal time), date, hemisphere and our estimated position.
Would it were that easy in practice!
Several problems are faced when one tries to take a sight in the real galaxy.
First of all, the boat moves, sometimes violently! It is almost impossibly hard to stand up straight while using both hands to hold the precious sextant for long enough to take the sight of the celestial body and note the time accurately. Really hard to understand how these old salts found their way around at all, far less how they got home again! Of course the geezer who invented the device was desk-bound, and probably never took a sight from a boat.
Secondly, the idea of a sextant is that you look at the celestial object through a small eyeglass or telescope and move the image of the star down until it just rests on the horizon. Of course it is usually dark when this has to happen, and seeing the horizon at all is a challenge even for good eyes. It is at best a fuzzy line, usually obscured by mist, fog, or haze. Also, seeing a star (as opposed to the Moon or Sun).
It is recommended that sights of stars and planets are taken around half an hour either side of sunrise or sundown, since at those times the horizon is supposed to be clear. The Almanac allows you both to check when this is each day, and to discover in advance which of the stars and planets used for navigation will be visible at those times in any place on earth (assuming of course that the sky is cloudless!). However, it happens also to be very hard to see the star and planets at dawn and dusk! They are easiest to see in the middle of the night when it is dark. The sensible solution should be to use what is called a “bubble sextant”, which has a device like a masons level to indicate the horizon. However, such devices are used by those nasty airmen in flying machines, and not by sailors, who are only allowed to use “real” sextants requiring real horizons!
Then again, there is a list of errors to be factored into the calculations, such as “sextant errors”, reflecting the facts that they are not always made with 100% precision, or that they have been knocked around a bit on a ship, etc. There is also a correction for the height of the eye above the ocean, but how do you measure that when the boat is going up and down like a yo-yo? These are local errors, but there are more, attached to specific planets or objects, such as Venus, or the Moon, all of which have to be computed (with the right sign, for the right date and hour, etc) and factored into the calculation.
When you have finished it all, you arrive at a magic result that tells you how far, and in what direction, you are with respect to the estimated position you started with.
Of course all that was some time ago! So now you have to make corrections for your speed and direction of travel since he sight were actually taken. Maybe its time to start again, or take that noon sight of the Sun. At least you can see it on a clear day!
But seriously, it is one of the great pleasures of night sailing far from those over-lit urban conurbations that one can see, and reflect on, the stars and planets above. There are so many of them out there, visible to the naked eye. But where to start? Its all so massive! Before doing anything, though, just gaze. The more you gaze, the more stars there are up there, at least on a clear night. Take in the wonder of it all. While doing that, you will notice that some stars are brighter than others, and some make interesting shapes with other nearby stars, such as bears, horses, dogs, bows and arrows, necklaces, snakes, fish, etc etc. In ancient times both the Arabs and the Greeks named the stars and galaxies after such resemblances, and incorporated them into their mythical worlds of gods, goddesses, heroes and so on. For the Greeks in particular the mythical adventures and battles of their Gods and Godesses, their lovers, bulls, goats, horses like Pegasus, and dogs, and so on are reflected in the heavens above for all to see, free, for all time. What a great gift to humanity.
Thus stars like Aldabaran, one of the 57 navigation stars, was named by the Arabs. It is “the Eye of the Bull” in the galaxy Taurus, meaning the bull. The galaxy, however, was named after the flying bull that Jupiter transformed himself into in order to escape with Europa, the King of Crete’s daughter. Aldebaran is the brightest star in the galaxy.
One of my favourites, part,y because it was nearly always present on the way to Brazil as well as on the way back, is the constellation of Orion. It is easy to spot. Patrick Moore thinks it is the “most splendid Galaxy in the entire sky”, and I agree with him on this.
Three bright stars form the core of Orion’s belt. Another two bright stars lie some distance from the trio, and perpendicular to it- The upper one is Betelgause, and the lower one is Rigel, both good bright navigational stars. To the left of Rigel and below left from Orions belt we find Saiph, and between Saiph and the belt we have what looks like three small stars in line, well clustered together. In fact this is the Great Nebula or “Trapezium”. If the eye follows a line to the upper right from Saiph and through the lower of the three stars of Orions belt, one finds the star Bellatrix, also a bright navigational star. Last night (24/4/2019) I could see them all very clearly in mid-Atlantic at 2340 UT.
It is so much fun trying to find the patterns of the Galaxies in the stars, identifying them, and finding out how far away they are. More fun would be to write a book about this heavenly playground of the Gods, the story of the naming of the stars, and to be equipped to follow those nightly ancient battles and drama for all time. It can absorb hour and hours of pleasurable time on the part of sailors.
Even without all the fuss and bother of Astro navigation, we use the stars as guides to the direction to steer in at night – making life much easier than it is with a dark compass. This is an ancient skill, and nearly universal among seagoing peoples of the world.