Geoscience Reference
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Equator
Equator
(a)
(b)
(c)
Figure 2.3 Orientation of great and small circles. Together, these circles form Earth's grid system. (a) All the great circles
that are oriented north and south intersect at the poles. (b) Except for the great circle at the Equator, all circles oriented east
and west on Earth are small circles. (c) Overlay of great and small circles to form the Earth's grid system.
never meet. The largest of these parallel circles (the Equator)
lies halfway between the two poles and is a great circle because
its outline corresponds with the maximum circumference of
Earth. North and south of the Equator, however, the parallel
circles have smaller diameters and are known as small circles
(Figure 2.2b).
To visualize this sequence of small circles, imagine what
happens when you slice a lemon or pineapple. First, the slices
are generally parallel to one another as you work across the
fruit. Second, the circumference of each slice is progressively
smaller as you move away from the center of the fruit (the
great circle of the fruit). As with great circles, small circles
can be aligned in any direction on the globe. Geographers
use this network of north-south intersecting great circles
and east-west parallel small circles to create a grid on the
Earth's surface (Figure 2.3) that allows them to determine any
location within about 30 m (100 ft). This system is known as
the latitude/longitude coordinate system. Let's see how this
system works.
at its maximum east-west circumference, otherwise known
as the Equator (Figure 2.4a). The portion of Earth north of
the Equator is called the Northern Hemisphere , whereas
the area south of the Equator is the Southern Hemisphere .
Latitude is determined using simple geometry by measuring
the angle of any point on the Earth's surface north or south of
the Equator. This calculation is accomplished by projecting
two lines from the center of Earth to its surface. One of these
lines always extends from the center of Earth to the surface
along the Equator. The other line extends to the north or south
from the center of Earth directly to the location in question,
depending on whether it is in the Northern or Southern
Hemisphere, respectively (Figure 2.4b). We can then calculate
the latitude of the location from the geometric arc between the
two intersecting lines.
In this system the Equator has a value of 0° latitude.
Latitudes close to the Equator have small geometric arcs from
the Earth's center and, therefore, have relatively low-latitude
designations such as 5° or 10° N or S (see Figure 2.4a).
Localities progressively farther north (N) or south (S) of the
Equator have progressively greater angles from the Earth's cen-
ter and, therefore, have progressively larger latitude designa-
tions. The maximum angle is perpendicular (90°) to the center
of the equatorial great circle. In the Northern Hemisphere, this
angle corresponds to the North Pole (90° N), whereas it corre-
lates with the South Pole (90° S) in the Southern Hemisphere.
Latitude
Latitude is the portion of the grid system that uses the
parallel set of circles with outlines extending east and west.
Using these circles, latitude measures location north and
south of a geographical reference. The natural reference for
latitude is the great circle that bisects the middle of Earth
Small circles Circles that intersect the Earth's surface and
that do not pass through the center of the planet.
Equator The great circle that lies halfway between the North
and South Poles.
Latitude The part of the Earth's grid system that determines
location north and south of the Equator.
The half of Earth that lies north of the
Northern Hemisphere
Equator.
The half of Earth that lies south of
Southern Hemisphere
the Equator.
 
 
 
 
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