Longitudes and Latitudes
Diederik Willemsen www.sailingissues.com
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
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.
A little History
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 Ptolemy, the
second century CE.
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.
Circles parallel to the equator, ranging from 0° to 90° N or
S. Only the equator is a great circle.
half-Circles converging at the poles, ranging from 0° to
180° E or W. Each pair of meridians forms a great circle.
0° or the Greenwich meridian which divides the Western and
The intersection of a sphere and a plane that passes through
the sphere's centre.
The intersection of a sphere and a plane that does not pass
though the sphere's centre.
By convention 24 zones, each 15° longitude wide. Hence, noon
at Greenwich gives midnight at 180° E.
Greenwich Mean Time or UTC or Zulu, and is the time in
Greenwich. Antonym: Local time. Berlin's local time = GMT +
The 180° meridian which extends from or is opposite to the
prime meridian. Here, not only the hour changes when
crossing the meridian, but also the date.
Position property defined by the number of degrees North or
South of the equator, varies from 0° to 90°.
Position property defined by the number of degrees East or
West of the prime meridian, varies from 0° to 180°.
Latitudes first and Longitudes second. For example: Nijmegen
(NED) = 51° 50',1 N , 05° 52',0 E.
One nm is one minute (') on the vertical scale on the chart.
1' equals 1852 metres.
miles per hour.