Page 186 - NAVAL SCIENCE 3 TEXTBOOK

Introduction to Navigation

Navigation is a fundamental nautical science. Like seamanship, a perpendicular to its axis. The word "equator" implies "equal parts."
knowledge of navigation is basic to operating on water. Navigation It lies exactl), halliva), between the north and south poles. The top
enables the mariner to locate his or her position und to get from half of the globe is called the Northel'll Helllisphere (northern "half-
one place to another. sphere"), and the lower half is the SOl/them Hemisphere.

T he Terrestrial S phere Great C ircles
To discliss navigation and nautical charts, we mllst first under- Meridians and the equator are called great circles because they
stand the Earth. In navigation, the Earth is called the globe or ter- each divide the globe into 1'.\'0 halves. Any circle drm\'n around the
restrial sphere (this latter term comes from the Latin word term, Earth so as to divide the world into equal parts, or hemispheres, is
which means "earth"). Our planet is spherical. Actually, our Earth called a great circle.
is a little flattened at the poles instead of being perfectly spherical. The concept of the grcat circle is vcry important, so it should
But this can be disregarded in Illost cases in navigation. be clearly understood. A great circle is any circlc formed on the
There are se\'eral reference points for locating objects 011 the Earth's surface by passing a plane through its center, dividing it
Earth. The lIorth pole and sOlllh pole are located at the ends of into 1'.\'0 halves,
the axis on which the Earth rotates. The imaginary lines running Look at the globe. You will note that all meridians are great cir-
through the poles and around the Earth arc c.lUcd lIIeririialls. The)' cles, Of all the lines going around the globe from east to west, how-
divide the surface of the Earth into sections much as yotl might cut ever, only the equator is a great circle. The other lines are called
an orange for eas), peeling. pnmllels, since they go around the globe parallel to, and north and
The imaginary line that fUllS arollnd the center of the Earth, sOllth of, the equator. They <.\fe all smaller circles than that made by
cutting every meridian in half and dividing the Earth into top the equator. Of the parallels, onl)' the equator cuts the globe into
and bottom halves, is called the equator. It is formed by passing 1'.\'0 hemispheres.
an imaginary plane horizontally through the center of the Earth, You can also see that a great circle does not have to be a merid-
ian or the equatorial parallel. A great circle is (/1/)' circle whose plane
passes through the Earth's center, no matter what direction,
Pol' \,Vhat is the significance of the great circle in navigation? Just
I this: the shortest distance beh\'een two points on the Earth (or an)'
sphere) lies along the path of a great circle passing through those
two points, This path, or segment of <.1 great circle, is an arc Oil the
Earth's rounded surface. It has to be an are, because a straight line
beh\'een 1'.\'0 points would go under the surface!

C ircular Measurement
You already have learned in your math classes that a circle's circfllI/-
jere/Ice (the distance around) contains 360 degrees (0), Regardless
of the size of the circle, whether it is the size of a ping-pong ball
01' of the globe, the circumference has 360 degrees, Each degree
contains 60 minutes ('), and each minute contains 60 seconds (It),
The globe with meridians of longitude and parallels of latitude. Any J'v1cilsurcment along a meridiem 01' parallel is expressed in terms of
line that cuts the globe into equal parts is a great circle. A random
great circle is shown cutting the meridians. The other great circles in degrees, minutes, and seconds of arc (the CUJye of the circle),
this drawing are the meridians and the equator.

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