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thosphere, Earth's rigid outer layer. The great bulk of Earth _osphere. We can define the geosphere as the mass of solid )lten rock that extends more than 6000 km from Earth's llface to its center. The lithosphere, the top 100 km of the l' n~, is the most rigid (unbending) part of the geosphere. Oi­ )lnrations in mines have taken humans to a depth of less f i ll. T he deepest drill hole is about 12 km deep. Everything W :lhn tl t the geosphere at depths greater than 12 km comes dil"l'l" 1. ev idence, such as the increasing temperatures with hv passage of seismic (earthquake) waves, examination of ' l'!-o, and from the determination of Earth's properties, such 'll sity. Therefore, scientists have directly explored or pene­ 'ldy abo ut one-fifth of 1 percent of the distance to Earth's :TERMINED?
p sailors far out on the ocean determine their position so
I safely return to port? With no familiar landmarks, such
, cities, and geographic features, they cannot describe their
in terms of surface features the way people usuaHy do on
ng ClgO, explorers solved this problem by establishing a co­
. system that covers the whole Earth.
~)J1 a sheet of graph paper is a type of coordinate system.
on the paper has a unique address expressed in terms
along the x and y axes. Many cities are laid out in a
Ill.: system. Much of New York City has numbered av­
,n! run north-south and numbered streets that run east­
l)wing the street address of a building can help a person
OCiltC' it on a map or in the city itself. However, in Earth's
'pl:d ilIT3S there are no roads or street signs, and there is
) 111.1 rk the oceans' surface. Because of this, explorers used
'l'rvmions of the sun and stars to find their position on
CHAPTER
2:
EARTH'S DIMENSIONS AND NAVIGATION •
41
North Pole
90'
FIGURE 2-6. Latitude is
your angular distance
north or south of the
equator. For example,
this diagram shows that
the angle between the
equator and central
north Africa is about 20°.
So the latitude of this
part of Africa is 20oN. It
is the latitude of Mexico
and India as well.
East
West
South Pole
coordinates. This system is based on the spin (rotation) of Earth
on its axis. The axis is an imaginary line that passes through
Earth's North and South poles. Halfway between the poles is the
equator, an imaginary line that circles Earth. The first terrestrial
coordinate value is latitude. As shown in Figure 2-6, latitude is
the angular distance north or south of the equator.
Lines of equal latitude are called parallels because they run
east-west and are parallel to each other. The equator is the reJer­
ence line at latitude 0°. Both north and south of the equator, lat­
itude increases to a maximum of 90° at the poles. Parallels can be
drawn at any interval of latitude from the equator (0°) to the North
and South poles (90 0 N and S).
illl
)L'r S
I rJ;l(l'.
system estahlished hy carly sailors :tll d n:plLw­
lh ' ~ sys! ~' m nr I;ltit I Ilk .11 11 I 1 ~1 11g i t llde, cllf n l turcs ll'ial
.l , )rd ll 1i11l.'
DIVISIONS OF ANGLES Just as meters can be divided into cen­
timeters and millimeters, degrees of angle can be divided into
smaller units. Each degree is made up of 60 minutes (60') of
angle. So, 23~ degrees is 23 degrees and 30 minutes (23°30').
1
Furthermore, one minute of angle (6 0 of a degree) can further be
divided into 60 seconds (60"). On Earth's surface, an accuracy
of a second of latitude or longitude would establish your loca­
tion to within a rectangle that measures about 30 m on each
side. Visit the following Web site to convert street addresses tol
(Will latitudcllongitude in one step: htlp:llwww.stl-"Vemorse.org/
jOf lll a tloll.p.lJp
Nurtil l'ul"
West
South Pole
IGUR:.. 1 7. Longitude
is your angular distance
east or west of the
prime meridian. At 0°
longitude, the prime
meridian runs through
London, England,
western Africa and the
South Atlantic Ocean . All
lines of equal longitude
run from the North Pole
to the South Pole.
The second terrestrial coordinate value is longitude. As shown
in Figure 2-7, longitude measures angular distance east and west.
Unlike latitude, there is no natural or logical place for longitude
measurements to begin. English explorers established their refer­
ence line at the Royal Observatory in Greenwich (GREN-itch),
England. Since England dominated world exploration and map­
making in the 16th century, a north-south line through Greenwich
became the world standard for measurements of longitude. Today,
the Greenwich meridian, also known as the prime meridian, has
become the reference line from which longitude is measured, as
you can see in Figure 2-7.
Lines of equal longitude are called meridians. Meridians all run
from the North Pole to the South Pole. The prime meridian has a
longitude of 0°. Longitude increases to the east and west to a max­
imum of 180°, a line that runs down the middle of the Pacific
Ocean. Meridians are not parallel because they meet at the North
and South Poles. As shown in Figure 2-8, the Eastern and West­
ern Hemispheres are the two halves of Earth bounded by the
prime meridian and the north-south line of 180° longitude.
Unfortunately, some people think of latitude and longitude as
only lines. For example, they confuse latitude, the angular distance
from the equator, with the lines on a map that show constant lat­
itude. If your only purpose is to read the coordinates on a map,
this is not a problem. However, if you want to understand what
Western
Hemisphere
1(, URE 2 ·8. The
prime meridian (0°
longitude) and the
International
dateline (roughly
180°) make a circle
that divides Earth
Into the Eastern
and Western
Hemispheres.
80'
o·
Meridians of longilud
latitude and longitude are and how t
a deeper understanding.
On a computer, Google Earth !t:l,
to view satellite imagery, maps, terra'
plore galaxies in the sky. Visit the 1
fun: http://eartlz.google.conzl
Finding Latitude
Earlier in this chapter you read th
North Star, were used to show ilia
servations can also be used to tell [­
the equator. Earth rotates on its axi
that Earth's axis should be pointed
the direction in which Earth's a
26,000-year cycle. In our lifetime t
pens to line up with a relatively bl
North Star. (The alignment is not
less than 1 degree from the proj ec­
tor can also use the Sun to determi
44 shows a sextant being used in c
An observer at the North Pole s
the night sky. The angle from the hI
90°. That observer is also located
<D MODELS
CHAPTER
North Pole
2:
EARTH'S DIMENSIONS AND NAVIGATION •
43
180'
180'150" 120' 90"
~':lEast
FIGURE 2-7. Longitude
is your angular distance
east or west of the
prime meridian. At 0°
longitude, the prime
meridian runs through
London, England ,
western Africa and the
South Atlantic Ocean. All
lines of equal longitude
run from the North Pole
to the South Pole.
~'u nd terrestrial coordinate value is longitude. As shown
l-7 , lon gin~ de measures angular d ~stance east and west.
it ude, there is no natural or logical place for longitude
ents to begin. English explorers established their refer­
i.lt the Royal Observatory in Greenwich (GREN-itch),
Since England dominated world exploration and map­
the 16th century, a north-south line through Greenwich
l: world standard for measurements of longitude. Today,
wid } m eridian, also known as the prime meridian, has
lC reference line from which longitude is measured, as
'\.: in Figure 2-7.
r equal longitude are called meridians. Meridians all run
(wth Pole to the South Pole. The prime meridian has a
)f O°. Longitude increases to the east and west to a max­
I HO°, a line that runs down the middle of the Pacific
.'ridia ns are no! parallel because they meet at the North
Poles. As shown in Figure 2-8, the Eastern and West­
.phcrcs are the two halves of Earth bounded by the
idi'lll and the north-south line of 180 0 longitude.
11 11 1ely, some people think of latitude and longitude as
For exa mple, they confuse btitude, the angular distance
lu ah lr, with the lines 1)11 ;( 1l1.1p th at show constcUlt lat­
lI tl" l)l1l y pUrpl)s(' is I I' l'I'ad t )l(' nJ(wdin;Jtes n n a map.
;1 pl'lIh k rn. Ilpw l'VI 'I , It yut( W: l! H tll 1I11dt: rslil nd 'Vh,tl
100"
100'
Eastern
Hemisphere
Western
Hemisphere
80'
.JQE 2-8. The
I ,(line meridian (0°
Icmgitude) and the
Illtornational
d ltellne (roughly
1BOO) make a circle
lildt divides Earth
Illto the Eastern
md Western
Ilumispheres.
80'
\
\
" V
\
Prime
Meridian
o·
Meridians of longitude
latitude and longitude are and how they are determined, you need
a deeper understanding.
On a computer, Google Earth lets you "fly" anywhere on Earth
to view satellite imagery, maps, terrain, 3-D buildings and even ex­
plore galaxies in the sky. Visit the following Web site to join the
fun: Itttp://eartlz.googie.CQ11I/
ing Latitude
Earlier in this chapter you read that observations of Polaris, the
North Star, were used to show that Earth is a sphere. Those ob­
servations can also be used to tell how far north a person is from
the equator, Earth rotates on its axis. There is no scientific reason
that Earth's axis should be pointed to any particular star. In fa ct,
the direction in which Earth's axis points moves through a
26,OOO-year cycle. In our lifetime the north-south axis just hap­
pens to line up with a relatively bright star called Polaris, or the
North Star. (The alignment is not perfect, since Polaris is a little
less than 1 degree from the projection of Earth's axis.) A naviga­
tor can also use the Sun to determine latitude. Figure 2- 9 on page
44 shows a sextant being used in celestial navigation.
An observer at the North Pole sees Polaris directly overhead in
the night sky. The angle from th e horizon up to Polaris is therefore
<)(\" , T h;l t L)bsc("VI'J" is al !oln IlK:l1ed 90'" Ilnrti1 nf the equator. As the
Slllllh
nr Ih l."
l'lju ;I IPr Ihl' Nelllil :-,\
with ;\ star IlWP, a ll ,-,b~LI"V l'l" ,\ '" d d('TI
sky that is dircctly ;1buVL' thL' So ut h 1\
tion called the Southern Cl"l'lss. 111 ;I P
done in the Northern Hemisph ere. so
angle of that point in the starry sky ;,h,
used to sighting on Polaris, it did Ill'll
latitude in the Southern Hemisphcrt..'.
STUDENT ACTIVITY 2-4 _DETERMININ
FIGURE 2-9. Precision
instruments such as this
mariner's sextant are used to
determine latitude with
remarkable accuracy.
latitude of the observer decreases, the altitude of Polaris also de­
creases. At the equator, Polaris is visible right on the northern hori­
zon. Here, Polaris is 0° above the horizon, the latitude at Earth's
equator is 0° . Therefore, for any observer in the Northern Hemi­
sphere, latitude can be determined by observing the angle of Po­
laris above the horizon. The altitude of Polaris equals the latitude of the
observe1: Figure 2-10 illustrates how two stars in the Big Dipper can
be used to find Polaris.
FIGURE 2-10.
The "pointer stars"
in the Big Dipper
always line up with
the North Star
(Polaris). If you
rotate your book
through 90 0 , 180 0 ,
and 270 you will
see how the
alignment of these
two constellations
changes over a
24-hour period.
(Ignore the horizon.)
The same rotation
can be observed if
you view these
stars at the same
clock time each
night for one year.
1: MATHEMAnCAL
ANALYSIS 1. 2. 3
6: MODELS 2
6: MAGNITUDE AND
SCALE 3
au can construct an instrument h)
the following simple materials: <I
weight, and a sighting device such as ,I
is similar in principle to instruments 1I ~
of years. Figure 2-11 shows this naviga
If you use a standard protractor wb
zon, the string will fall along the ';
straight up, the string will line up wit!
need to subtract your angle readings II
Your latitude is equal to the angle of
horizon. Do your results agree with thi
tion on maps or other references?
0
,
FIGURE 2 11 . To make
a simple celestial
navigation instrument,
use protractor, a
drinking straw, and a
weight on a string.