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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.
Sextant. Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License
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. Like what you are reading? Then support your blogger by selecting one of the wonderful advertisements on their website. W.A. Rusho is an author, professional wrestler and historian. You can reach him by his website, or via email. |
I am a Business Analyst, and a professional wrestler. This blog is about my journey into writing and being a new author. I also include topics such as my acting career and everyday living. If you have any comments, please leave one.
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!
ReplyDeleteThis 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!
ReplyDeleteWilliam - 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.
ReplyDelete