Teacher Background
Navigation
Most of us are familiar with some form of navigation. If we drive, in fact we are
navigating over a system of roads; essentially a grid pattern superimposed on a flat
surface. Navigation is of the utmost importance in oceanography. In the absence of roads
ocean navigation necessarily makes use of latitude and longitude; essentially a grid
pattern arranged to fit over the sphere of the earth.
Lines of latitude run east-west, while lines of longitude run north-south, but that is not
their only difference.
Latitude
Imagine the Earth was a transparent sphere (actually
the shape is slightly oval; because of the Earth's
rotation, its equator bulges out a little). Through the
transparent Earth (drawing) we can see its equatorial
plane, and its middle the point is O, the center of the
Earth.
To specify the latitude of some point P on the surface,
draw the radius OP to that point. Then the elevation
angle of that point above the equator is its latitude λ-northern latitude if north of the equator, southern (or
negative) latitude if south of it.
On a globe of the Earth, lines of latitude are circles of different
size. The longest is the equator, whose latitude is zero, while at
the poles--at latitudes 90° north and 90° south (or -90°) the
circles shrink to a point.
Longitude
On the globe, lines of constant longitude ("meridians") extend
from pole to pole, like the segment boundaries on a peeled
orange.Every meridian must cross the equator. Since the equator
is a circle, we can divide it--like any circle--into 360 degrees, and the longitude φ of a
point is then the marked value of that division where its meridian meets the equator.
http://www-istp.gsfc.nasa.gov/stargaze/Slatlong.htm
To summarize the above information:
Latitude
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starts at zero degrees at the equator
goes to 90 degrees at each pole
is determined by measuring angle between lines with points at the surface of the earth at
the equator, at the center of the earth, and at the surface of the earth at the given line of
latitude.
lines produce circles about the earth at evenly spaced intervals, and the circles get
smaller toward the poles
Longitude
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lines pass through the poles and converge at the poles
therefore they are all great circles
What is a great circle?
o It is a circle about the circumference of a sphere which:
o if prescribing a plane, cuts the sphere exactly in half.
o prescribes the a line which, when traveled, is the shortest distance between two
points over the surface of a sphere.
How is longitude determined?
o It is counted degrees east or west of the prime meridian (That line of longitude
which passes through Greenwich, England)
o therefore, maximum longitude is180 degrees angular distance from prime
meridian.
o Is determined in fashion similar to latitude.
Degrees of latitude are equally spaced, but the slight flattening at the poles causes the length of a
degree of latitude to vary from 110.57 km (68.70 mi) at the equator to 111.70 km (69.41 mi) at the
poles. At the equator, meridians of longitude 1 degree apart are separated by a distance of
111.32 km (69.17 mi); at the poles, meridians converge. Each degree of latitude and longitude is
divided into 60 minutes
Sources:
http://www-ocean.tamu.edu/~dkobilka/navigation.html
http://encarta.msn.com/encyclopedia_761563211/Latitude_and_Longitude.html
Mercator Projections of the World
In the Mercator projection (c. 1569), Greenland, which has 0.8 million square miles, is shown as
being equal to Africa, which has 11.6 million square miles.
A good discussion of this can be found at the Diversophy site, including the following execerpt:
"The Mercator projection creates increasing distortions of size as you move away from the
equator. As you get closer to the poles the distortion becomes severe. Cartographers refer to the
inability to compare size on a Mercator projection as "the Greenland Problem." Greenland
appears to be the same size as Africa, yet Africa's land mass is actually fourteen times larger
(see figure below right). Because the Mercator distorts size so much at the poles it is common to
crop Antarctica off the map. This practice results in the Northern Hemisphere appearing much
larger than it really is. Typically, the cropping technique results in a map showing the equator
about 60% of the way down the map, diminishing the size and importance of the developing
countries.
This was convenient, psychologically and practically, through the eras of colonial domination
when most of the world powers were European. It suited them to maintain an image of the world
with Europe at the center and looking much larger than it really was. Was this conscious or
deliberate? Probably not, as most map users probably never realized the Eurocentric bias
inherent in their world view. When there are so many other projections to chose from, why is it
that today the Mercator projection is still such a widely recognized image used to represent the
globe? The answer may be simply convention or habit. The inertia of habit is a powerful force.
Source: http://www.public.asu.edu/~aarios/resourcebank/maps/page10.html
For a good discussion of the shortcomings of the Mercator Projection and the Peters Map of the
World, go to http://geography.about.com/library/weekly/aa030201c.htm
There really isn’t a good reason to even bring up the Peters Map since you will not be using it in
class, but it is another example of a popular world map. (It was even used in an episode of “The
West Wing” on television.)
For a good description of the Robinson Projection of the World, go to
http://www.cnr.colostate.edu/class_info/nr502/lg2/projection_descriptions/robinson.html
Nautical miles and Distances
Nautical miles are good old navy terms. The nautical mile was based on the circumference of the
earth at the equator. Since the earth is 360 degrees of means there are
360 60 min   21,600 "minutes" of longitude around the earth.
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1
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This was taken as the basis for the nautical mile; thus, by definition, 1 minute of longitude at the
equator is equal to 1 nautical mile. This means at the equator, one degree of longitude = 60 nm =
69.05 miles. To convert nautical miles to miles, use http://209.182.50.103/length/nautical-milesto-miles.htm
(Source: http://www.onlineconversion.com/faq_07.htm)
Alternative measures for nautical miles (nm), and measures of latitude and longitude are:
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the distance for one degree of latitude = 60 nm  69.05 miles , therefore one
minute of latitude = 1nm  1.151 miles everywhere on earth
the distance for one degree of longitude at any latitude = 60 nmcoslatitude .
Examples: The distance for one degree of longitude at the equator is
60 nmcos0  60 nm1  60 nm .
The distance for one degree of longitude at 45N latitude is
60 nmcos45  60 nm   ___ nm ,
Source: http://www-ocean.tamu.edu/~dkobilka/navigation.html