magnetic variation (MAGVAR) at that location.
around the surface, what path does the line
follow? You must qualify the term distance used
Compass heading differs from magnetic heading
by the amount of magnetic deviation. Compass
in navigation to show how to measure the
heading differs from true heading by the amount
distance. The shortest distance on the earth's
of compass error (deviation variation).
s u r f a c e from NAS San Diego to Sydney,
Australia, is 6,530 miles. However, via Honolulu
BEARING
and Guam, a frequently used route, it is 8,602
miles. You can express the length of a chosen line
Bearing is the horizontal direction of one
in various units, such as miles, kilometers, or
yards.
terrestrial point from another. Bearings can be
expressed by two terms--true north or the direc-
TIME
tion in which the aircraft is pointing. If true north
is the reference direction, the bearing is a true
Time has many definitions. The two defini-
bearing. If the reference direction is the heading
tions used with navigation are as follows: (1) the
of the aircraft, the bearing is a relative bearing.
hour of the day and (2) an elapsed interval. The
If you get a bearing by radio, it is a radio
first appoints a definite instant, as takeoff time
bearing; if visual, it is a visual bearing. Thus, the
is 0214. The second definition appoints an
direction between two objects on (or near) the
interval, such as time of flight, 2 hours 15 minutes.
surface of the earth can be described concisely by
saying: THE (RADIO, VISUAL) BEARING OF
POLES
A FROM B IS X (RELATIVE, TRUE).
The earth's geographic poles are the extremities
DISTANCE
of the earth's axis of rotation. Look at figure 7-1.
Here, Pn, E, Ps and W represent the surface of
Distance is the separation between two points.
the earth at sea level. Line PnPs is the axis of
rotation. The earth's rotation is such that all
To measure distance, you measure the length of
points in the hemisphere, PnWPs, approach the
a line joining the two points. This seems
understandable enough. However, suppose that
reader. Those points in the opposite hemisphere
will recede from the reader. The extremities of the
the two points are on opposite sides of a baseball.
How do you draw the line? Does it run through
axis, points Pn and PSare the north and south
poles, respectively. A man on the surface of the
the center of the ball, or around the surface? If
earth, facing in the direction of rotation, has the
North Pole on his left. East will be in front of
him, the South Pole on his right, and west behind
him.
The earth has some of the properties of a bar
magnet. The magnetic poles are the regions near
Figure 7-1.-Schematic representation of earth showing axis
Figure 7-2.-The equator is a great circle whose plane is
of rotation and equator.
perpendicular to the polar axis.
