Longitudes and
Latitudes
The earth can be regarded as a spherical object, which revolves around the sun in 365.25 days. Since we're dealing with a 3-dimensional shape we need coordinates of a different form than the usual x- and y-axes. Though adding an extra z-axes would make sense for submarines, we will most likely be found on the surface of this sphere while using another system of coordinates.
This system covers our planet with imaginary lines called Parallels & Meridians, see figure 1. All these lines together provide the grid which enables us to describe any position in Longitudes and Latitudes. Where to place the divide between the North and South hemispheres was obvious: the equator. But the division of the Eastern and Western hemispheres was the source of much political turmoil. As you probably know, Greenwich (Great Britain) won, placing for example The Netherlands on the Eastern and Ireland on the Western Hemisphere. It takes the earth 24 hours for a full rotation of 360 degrees. Thus, every hour we rotate 15 degrees longitude, see figure 2.
Also 12:00 Greenwich Mean Time (GMT) is 24:00 Local time
at the other side of the planet: 180 E or 180 W: the Date Line. Crossing
this meridian changes not only the hour but also the date.
The North pole has a latitude of 90 degrees N and the South Pole 90° S.
The Meridians cover twice this angle up to a max of 180°. So, Meridians
converge at the poles, whereas Parallels run parallel to each other (nomen
est omen) and never meet. All Meridians and the Equator (the biggest
parallel) form great circles, but the other
parallels form small circles. A great circle
divides the earth in two exact halves. In figure 3 the position of Boston
is shown using latitude and longitude. The position is 42° 21' 30" N , 71°
03' 37" W [degrees, minutes, seconds]. Most sailors will actually use 42°
21',5 N , 71° 03',6 W as a notation for this position. On small scaled
charts we want to be accurate within one minute or one nautical mile. On
larger scaled charts the accuracy is more likely to be within a tenth of a
mile (a cable).
If the earth were a perfect sphere all meridians would be as long as the
equator. Unfortunately, it is not and therefore we will use the equator
length as the circumference of our earth. This circle is divided into 360
degrees, or 360° x 60' = 21600' minutes. One minute of this great circle
is exactly the same as one nautical mile, or to put it shortly: 1 nm = 1'.
The length of the equator is 40000 odd kilometres. Hence, one nautical
mile is around 1852 meters long. With all this information we are now able
to describe any position in latitudes and longitudes. Moreover, we can
state the distance between two of those positions using nautical miles or
minutes. All we need now is a proper way to define speed. For that,
sailors use knots, the number of nautical miles an hour.
Mariners during the 15th century relied
on charts called "portolans" to assist them on their voyages. Portolan
comes from the Italian word portolani, which were medieval pilot
books. The portolans contained maps of coastlines, locations of harbors,
river mouths, and man-made features visible from the sea. They were
a compilation of centuries of seafarer observations. As sailors' skills
improved and the use of the compass was more widespread, portolans
improved in accuracy. Also Columbus used these portolans on his journeys.
Portuguese chartmakers added the meridian line, a point useful for
latitude sailing as well as for navigating solely by compass. A geographic
feature could now be located through the use of its distance in degrees of
latitude from a ship's point of departure. Note that the use of latitude
and longitude was understood since the time of
During the fifteenth century Portugal led the European world in sea
exploration. The golden age of discovery for Portugal lasted almost a
century until the Dutch eventually seized their trade routes from them.
As we move to the next chapter of this course we enter the sixteenth
century when the Mercator chart was invented.
|