Tuesday, March 20, 2018

Medieval Navigation Part II, Steering by the Stars

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In our last post, we were discussing the early ways people navigated.  We concentrated on how these sailors kept to the coast line concentrating on geographical objects such as mountains.  When the sailors ventured outwards away from shore, they needed tools to find their way to their destination and then back home.
Before we get into celestial navigation, aka astronavigation (a means of navigation using the sun, moon and stars as reference points), we first must understand the earth itself. We live on a sphere, but we observe the earth as flat. As a result of our interpretation of what we see and what is reality, we have broken down the earth into two ways to move around:  Latitude and Longitude. (We will discuss this more next time).
Latitude:  The invisible lines on a globe that circumnavigate the earth.  As a quartermaster, I always remembered Lat=Fat.  So, imagine the earth having a waistline, and its pants size is latitude.  Looking at the latitude, you can determine how far north or south you are.
Longitude:  The lines created by man that run the lengthways of the planet from north to south.  This will determine how far east or west you are.
Being on the sea, with these two points you can determine where you are and hopefully the location of where you want to be.
Sailing by the sun is basic, if you know the sun rises in the east, and sets in the west.  However, during the day you also need to find your location. Many tools were designed to do this function.
The astrolabe is one of the first going back to ancient Greek times, but perfected by Arab navigators.  It is a form on an inclinator, that gives the angle of a known celestial object such as the sun or a star.  It can be flat, or a circular device.
One of the first tools to measure an angle to a star is the Kamal.  This is simply a board (the width is vital), with a string in the center or bottom. The navigator would hold the string in his mouth, and hold the board at arm’s length.  Looking at an object, if it was just sitting on top of the board (remember the width is vital) meant he was on the right direction toward it.  If it was lower or higher, his vessel’s heading would be adjusted. The north star (Polaris) was used often since in the Mediterranean (closer to the equator), it is near the horizon than northern countries.
Another primitive navigational devise the Norse gnomon.  This was a board with a nail driven into it, resembling a sundial.  Small marking formed a half circle around it.  Letting the sun cast a shadow on the board, and comparing it to the markers on the semi-circle you could determine the altitude of the sun. 
In order to do this, you need to know when the sun is highest in the sky.  This is fine during a sunny day, but is hard when it is cloudy.  The Vikings used a sun stone, made from the crystal Iceland Spar located in their country.  It is transparent, and when looked through allowed you to visibly see the sun, even when cloudy.
In the last post, you remember us discussing the cross staff or Jacob’s staff, this too was used in celestial navigation similar to those I just mentioned.
There is one major problem with many of the navigational aids I mentioned above. They show how far north or south you are, this allows you to travel accurately in an east or west direction. This is the reason why when the Vikings traveled directly into the open ocean, they discovered Iceland and Greenland; because they are directly west of their home countries.
One of the earlier but more advanced form of navigation was the Lunar Distance.  This was to calculate the angle between the moon and another celestial body. This was very popular form of navigation, until an accurate time piece was created, this will be discussed below.
So humans now figured out a better way to navigate.
Astronomy has been with the human race for almost its entirety.  Sometimes, it was mystical, other times based on science.  But the location of the stars and sun, would be the basis of travel around the world. An example is in the bible, the Wise Men are guided by a star to Bethlehem. 
For the most part of navigation, the North Star (Polaris) was a constant for it was located over the North Pole.  All other stars appear to rotate around the North Star. So, if you could locate this star, you would know where North Was. Note: The earth has a wobble in it, and slightly changes over the years.  In ancient Greek times, the North Star was not located over the North Pole. In fact, around 1,000 B.C. the constellation Draco would have been closer to the North Pole than Polaris.
Staring at the North Star was good for when you were in the Northern Hemisphere, but soon expansion, exploration and trade reached beyond the sight of Polaris.
Some stars are located over a specific part of the earth.  If you were at the North Pole, and looked directly overhead, there would be the North Star (or pretty close). So almost every star has a location on earth it is associated to.  However, as I started earlier they appear to move in a circular motion around the sky and their position over a specific place on earth changes. So, to navigate by the stars, you need to know the time a star is located over a specific point on earth.  It does get more complicated than this, you need to factor in curvature of earth etc, but for this discussion we will keep it simple.

An example of a nautical almanac page.
Soon nautical almanacs were created showing the location of stars at specific times and dates. The most famous was The American Practical Navigator, originally written by Nathaniel Bowditch. Using the precursor, The New Practical Navigator by John Hamilton Moore of the Royal Navy. Bowditch expanded on his works and recomputed and expanded the tables within, also correcting many errors that were in the almanac.  The new book not only included the tables used for celestial navigation, but also how to use the information and basic piloting of a vessel.


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The above named navigational aids (kamal, Jacobstaff) were the precursor to the sextant (created around 1757). The difference being is the later has mirrors to help you find the angle from the horizon to your celestial object.   However, I want to point out one trivial point which is missed in many movies or TV shows that include the sextant.  You will see the navigator use them during the middle of the night.
At first this might seem to make sense; the stars are brighter at night.  Remember, when I said the sextant measured the angle between the object and horizon, the horizon disappears after dark.  The sextant, and other variables, were used twice a day, right after sunset and before dawn.  This way the navigator had a visible horizon to measure the angle.
The stars and the sun’s location would be written down and the time they would be located there.  But, to navigate with this information, you needed an accurate time piece.
Clocks during the time were mechanical, but also were subject to error due to violent movement, humidity etc.  At sea, these factors could dramatically change the time of a clock.  And time, as I stated is important in determining the location of a star.
At many trials and errors, the chronometer was invented.  This was a sealed clock (which limited humidly and temperature variations to occur), it also had gimbles which allowed it to maintain a level position (this allowed limited movement to its internal workings) when the vessel moved side to side on the waves. Having a standard time to wind the chronometer, meant it had a constant and even mechanical movement.  There were many trial and errors inventing this clock, but John Harrison is considered the first to invent the truly first working and accurate model.
Now, the chronometer did loose time, but it lost it at a precise constant amount. In one day, it might lose ¼ of a second, but as I said it was consistent. So, if I knew the chronometer was correct at noon when I left port, 4 days later it would be 1 second off the correct time. The quartermaster, or someone similar, would keep a record of when it was exact time, and how many days since that period, and its difference from the correct time.
When you use a device such as a sextant, it gives you one line (referred to as a Line of Position).  This imaginary line goes around the earth and your location can be anywhere one that line. With this one line, if you knew your past position, and speed you could do simple navigation (see the dead reasoning section in previous post).  So, to get a position with the stars, you would need to get the position of two stars, this gives you two lines, and where they intersected, would be your position.  Using three stars is considered an accurate position.
The chronometer was such a huge invention, that during the War of Austrian Succession, the device was not allowed on British ships in fear that if a ship lost a battle, the chronometer would fall into enemy hands.
The chronometer eventually did spread to other countries. With this new invention, the entire world was now accessible for navigation by sea. The compass and the chronometer are two of the major inventions in terms of navigation. Britain, Spain, Portugal, and sailors form the Netherlands soon moved across the world using these tools. It was with navigation that they expanded trade and built empires.
So, you are a captain, or navigator of a ship.  How do you keep all this information?  This is our topic in our next post.

Next Time:  The Logs of a Captain.
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W.A. Rusho is an author, professional wrestler and historian.  You can reach him by his website, or via email.



  1. This is an amazing post, William, that you clearly has researched to precision. I had no idea that the earth has a wobble in it and that is changes the position of stars over time. Very fascinating!

  2. This is the kind of technical information that boggles my mind, William! How did they keep it all straight? I'd be Gilligan and lost at sea for a long, long time!

  3. William - you write with such depth and take your subjects very seriously. A lot goes over my head as ckearly my brain is wired differently. Thank goodness for people like you.