Earth Science
October 15, 2015
SCIENCE NEWS
Understanding Locations on Earth
The diagram to the left
shows both parallels
and meridians on a
globe.
Lines of latitude are
parallel to each other
and increase to 90ºN
and 90ºS of the equator.
Lines of longitude meet
at both the north and
south poles and range
from 0º to 180º east and
west.
A coordinate system is used to determine locations on earth that includes latitude and longitude.
Latitude measures degrees north and south of the equator, and longitude measures degrees east
and west of the prime meridian. The equator runs around the earth and divides it into two equal
hemispheres, north and south. The prime meridian (0º) runs from the north pole to the south pole
and passes through Greenwich, England. The line of longitude that runs opposite of the prime
meridian is known as the international date line and is where each new day begins. It takes 24 hours for the earth to rotate 360º on its axis, therefore it can be calculated that the earth
rotates 15º/hr on its axis. Time zones have been determined to allow for each location to have noon
when the sun is highest in the sky, or when the sun is at solar noon. Time is determined using the
prime meridian. Time decreases, or is earlier, west of the prime meridian. Locations east of the
prime meridian are later, or have an increase of time, all the way up to 180º. Remember, time is
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Earth Science
October 15, 2015
based on solar noon, so we can always visualize a specific time at a local location if we know that
the earth rotates, in a counter clockwise motion, as viewed from above the north pole.
To calculate the time at a local longitude, the time at
the prime meridian, (Greenwich Mean Time) is useful.
Use the 15º/hr rule to determine the time. If you are
located at 75ºW and it is 5:00pm at Greenwich,
England (GMT), your time is five hours earlier, or
12:00 noon. Time is the same when two locations are located on
the same line of longitude.
The above diagram
shows the lines of
longitude with respect
to the prime meridian,
which runs through
Greenwich.
The diagram to the
right shows the
earth’s counterclockwise rotation
with half of the earth
being in daylight and
the other half in
darkness.
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