Since the beginning of
our voyage, we have been using GPS (Global Positioning System) to determine our
position. This has allowed us all to concentrate more on learning to sail. Now
that we are all more confident we have been relying on it less and less.
Navigation is the
process of finding your position relative to your destination and then, after
plotting the shortest, easiest and safest route to that destination,
calculating the information needed to follow that route.
The hardest part of
this process used to be the first, finding your position. However, with the
advent of satellite§ navigation (GPS) this has become simply a mater of
pressing the right buttons. Before the days of synthetic hydrocarbons[1]
and silicon chips, navigators had to use other means. While on long sea passages,
out of sight of land and any landmarks, the only way to calculate your position
was to use the sun and the stars. These navigators had charts to tell them the
relative positions of land, relative of course to the position of all the other
land. The problem was to pinpoint their position on the oceans. This is why
they created a numerical system to indicate any point on the globe.
Upon the surface of
the world the mariners superimposed an imaginary grid, similar to those that
can be seen in textbooks and maths examinations. The difference between this
grid and the graphs in exams is that graphs are drawn on flat paper, where as
the world is not flat (as all enlightened people should know), but round like a
sphere. It is, in actuality, not a perfect sphere, as it bulges out at the
equator and is slightly flattened at the poles, both caused by the effects of the
centrifugal force caused by the Earth’s rotation. This is how the navigators
solved the problem of an approximately spherical Earth. They took the equator, the
line that circles the Earth equidistant from both poles, as the starting point
for the horizontal lines. Then they drew lines parallel to the equator, these are
called lines of latitude. Therefore the farther away the circular lines of
latitude were from the equator the smaller their circumferences became, decreasing
in size until they reached the poles.
The latitude lines are labelled 0˚ at the equator up to 90˚N at the North Pole and 90˚S at the South Pole. They are labelled with a degrees sign because they are equal to the angle that is made when a line is drawn from the equator to the centre of the Earth, to the point on Earth’s surface where you are standing. That is why latitude is measured in degrees. So every different angle has its own line circling the earth. If a more precise measurement is required, degrees are split into 60 minutes, which are again split into 60 seconds. However seconds are rarely used anymore, instead decimals of the minutes are used.
The latitude lines are labelled 0˚ at the equator up to 90˚N at the North Pole and 90˚S at the South Pole. They are labelled with a degrees sign because they are equal to the angle that is made when a line is drawn from the equator to the centre of the Earth, to the point on Earth’s surface where you are standing. That is why latitude is measured in degrees. So every different angle has its own line circling the earth. If a more precise measurement is required, degrees are split into 60 minutes, which are again split into 60 seconds. However seconds are rarely used anymore, instead decimals of the minutes are used.
While on our voyage we
have kept a log every hour noting down, among other things, our GPS position. It
was exiting to watch as our latitude slowly decreased, first showing 49˚54´N (Lizard
Point) then 42˚11´N (Bayona, north west Spain) then 37˚09´N (Cape Saint
Vincent, south west Portugal) and now down to our current latitude 33˚03´N (Porto
Santo, Madeira Archipelago).
The vertical grid (longitude),
to indicate how far around the Earth you are, is more complicated as there is
no starting point like the equator. So an arbitrarily line to start counting
from had to be chosen. These vertical lines run straight from the North Pole to
the South Pole and back again on the other side, and they are called lines of
longitude. This means that if you were to cut the earth in two along one of these
lines of longitude you would always get two equal halves. Any line that you can
do this with is called a great circle, or meridian. The only line of latitude
that is a great circle is the equator.
The choice of the longitudinal
great circle from which to start counting, the prime meridian, was a source of
political argument. England chose Greenwich, France chose Paris and several
other countries nominated there own prime meridians. Eventually it was decided
that Greenwich would be the international prime meridian, however the French continued
to use Paris for some time afterwards.
In contrast to our steadily
decreasing latitude, our longitude has gone from west to east and back again.
Starting at 5˚10´W (Lizard Point) then west to 11˚08´W (Bay of Biscay) turning
east along the Algarve coast as far as the Guadiana river 7˚27´W and now
currently 16˚07´W (Porto Santo).
[1] Synthetic
Hydrocarbons: Any synthetic substance that is composed of chains of hydrogen
and carbon i.e. plastic, glass fibre, synthetic fabric, Ex
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