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conversion is easily done with the following ratio:
1.6.2. Closely related to the concept of distance is speed, which determines the rate of change of
position. Speed is usually expressed in miles per hour, this being either statute miles per hour or NM per
hour. If the measure of distance is NM, it is customary to speak of speed in terms of knots. Thus, a speed
of 200 knots and a speed of 200 NM per hour are the same thing. It is incorrect to say 200 knots per hour
unless referring to acceleration.
1.7. Direction. Remember, direction is the position of one point in space relative to another without
reference to the distance between them. The time-honored point system for specifying a direction as
north, north-northwest, northwest, west-northwest, west, etc., is not adequate for modern navigation. It
has been replaced for most purposes by a numerical system. The numerical system (Figure 1.7) divides
the horizon into 360o, starting with north as 000o and continuing clockwise through east 090o, south
180o, west 270o, and back to north.
1.7.1. The circle, called a compass rose, represents the horizon divided into 360o. The nearly vertical
lines in the illustration are meridians drawn as straight lines with the meridian of position A passing
through 000o and 180o of the compass rose. Position B lies at a true direction of 062o from A, and
position C is at a true direction of 220o from A.
26 AFPAM11-216 1 MARCH 2001
Figure 1.7. Numerical System Is Used in Air Navigation.
1.7.2. Since determination of direction is one of the most important parts of the navigator's work, the
various terms involved should be clearly understood. Generally, in navigation, unless otherwise stated,
directions are called true directions.
1.7.3. Course is the intended horizontal direction of travel. Heading is the horizontal direction in which
an aircraft is pointed. Heading is the actual orientation of the longitudinal axis of the aircraft at any
instant, while course is the direction intended to be made good. Track is the actual horizontal direction
made by the aircraft over the earth.
Figure 1.8. Measuring True Bearing From True North.
AFPAM11-216 1 MARCH 2001 27
1.7.4. Bearing is the horizontal direction of one terrestrial point from another. As illustrated in Figure
1.8, the direction of the island from the aircraft is marked by a visual bearing called the line of sight
(LOS). Bearings are usually expressed in terms of one of two reference directions: (1) true north (TN) or
(2) the direction in which the aircraft is pointed. If TN is the reference direction, the bearing is called a
true bearing (TB). If the reference direction is the heading of the aircraft, the bearing is called a relative
bearing (RB) as shown in Figure 1.9.
Figure 1.9. Measuring Relative Bearing From Aircraft Heading.
1.8. Great Circle and Rhumb Line Direction. The direction of the great circle, shown in Figure 1.10,
makes an angle of about 40o with the meridian near Washington, DC , about 85o with the meridian near
Iceland, and a still greater angle with the meridian near Moscow. In other words, the direction of the
great circle is constantly changing as progress is made along the route, and is different at every point
along the great circle. Flying such a route requires constant change of direction and would be difficult to
fly under ordinary conditions. Still, it is the most desirable route because it is the shortest distance
between any two points.
Figure 1.10. Great Circle.
28 AFPAM11-216 1 MARCH 2001
1.8.1. A line that makes the same angle with each meridian is called a rhumb line. An aircraft holding a
constant true heading would be flying a rhumb line. Flying this sort of path results in a greater distance
traveled, but it is easier to steer. If continued, a rhumb line spirals toward the poles in a constant true
direction but never reaches them. The spiral formed is called a loxodrome or loxodromic curve as shown
in Figure 1.11.
Figure 1.11. A Rhumb Line or Loxodrome.
1.8.2. Between two points on the earth, the great circle is shorter than the rhumb line, but the difference
is negligible for short distances (except in high latitudes) or if the line approximates a meridian or the
equator.
Section 1C— Time
1.9. Introduction. In celestial navigation, navigators determine the aircraft's position by observing the
celestial bodies. The apparent position of these bodies changes with time. Therefore, determining the
aircraft's position relies on timing the observation exactly. We measure time by the rotation of the earth
and the resulting apparent motions of the celestial bodies. This chapter considers several different
systems of measurement, each with a special use. Before you learn the various kinds of time, you must
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