Latitude
and Longitude
The Grid
What
if you had to tell visitors how to find their way to a bank? You might tell
them, "Well, it's at the corner of B Street and Y Avenue."
The
pattern of streets makes a grid, which you can use to locate particular places.
Each line on the grid (in this case, each street) has its own name, so you can
specify a point by naming one vertical line and one horizontal line on the
grid.
Early
sailors and explorers had the same problem as your visitor: they needed to find
their way. However, they were not exploring a city with a convenient grid or
streets; they were sailing across vast oceans, seas of empty spaces, so how
could they name where they were and get where they wanted to go?
One
of the first and best solutions to this problem, one that we still use today,
was to create an imaginary grid
and lay it over the map of the world.
THEN 
The
Robinson Projection, designed by the National Geographic Society
Maps
have been made by humans for millenia. One of the oldest
surviving maps of a town was found on the walls in a Stone Age settlement in
what is now Turkey. It was made about
6,000 years ago. It shows
houses and religious buildings and roads to other towns. (See The History of
Maps) In about 200 BC, the ancient Greek geographer Eratosthenes was one of the
first to create a map overlaid by a grid.
The Key to
Exploration
Today
scientists, geographers, and sailors still use a grid system very similar to
the one invented by Eratosthenes.Each point on the grid has a number
and corresponds to one particular point on the Earth. The vertical lines are
called meridians of longitude, whereas the horizontal lines are called parallels of latitude.
Latitude
gives the location of a place north or south of the equator, whereas longitude
locates a place east or wet. To give numbers to the points on this grid,
everyone had to agree on a zero, or staring point.
For latitude, the great circle of the equator makes a perfect
starting point, so it was designated the prime parallel, and latitude is
expressed in measurements of 0 degrees at the equator to 90 degrees north or
south to the poles.
For longitude, no convenient zero point (prime meridian) could be found, so much
confusion resulted, as map makers chose conflicting starting points. In 1884,
an international prime meridian was agreed upon, passing through the Greenwich
Observatory in England. Longitude was and is measured from 0 degrees at the
prime meridian to 180 degrees
at the International Date Line.
(In
regions around the equator, the distance between two meridians one degree apart
is approximately 111.1 km. As the
traveler approaches one of the poles, this distance shrinks dramatically. If an observer is standing on the North
Pole, he could draw a circle in the snow that has a circumference of 1 meter.
It would contain 360 degrees of meridians. At that latitude near the pole, the
physical distance between two meridians one degree apart is 1m/360 or about 3
mm. The physical distance between meridians strongly depends on the latitude of
the observer.)
Each
degree of latitude and longitude is divided into 60 minutes, and each minute
into 60 seconds. This allows us to
assign an exact position to any point on the Earth.
By
taking readings of the stars and the Sun, sailors can find their latitude and
longitude, and find their position on the sea. Scientists, explorers, and
geographers use them to make accurate maps of the locations and features of
lands that they are investigating.
Why
do scientist care about mapping?
Today mapmakers and scientists use longitude and latitude to map out changes in
the environment, in the location and extent of biomes, of animal populations,
and patterns of weather and pollution. An entire school of geography, called
biogeography, has grown up around the idea of mapping living systems.
