378 FORMS OF ENERGY Celestial navigation is the same for both avigation and marine navigation, except that in avigation the observations and computations are made less accurately, due to the unsteady and more rapid motion of the airplane. Celestial navigation involves the following steps: 1. Determine the angle of elevation of the sun or stars selected with the sextant. 2. Note the exact time of the observation. 3. Compute the line of position from the sextant observation and the time it was made. 4. Plot the line of position on a chart. 5. Determine a fix (i.e., the position) by plotting a line of position from a second star. The line of position is a short section of a circle drawn with the radius starting at a point directly under a given star at the time of observation, as obtained from an almanac, and extending a distance of 90° minus the angle of elevation above the horizon observed with the sextant. This section of the circle is drawn for a short distance near the position where the airplane is thought to be. The intersection of this circle with the line of position obtained from another celestial object gives the position of the airplane. 2. Navigation by Terrestrial Observation (Pilotage) Is Applicable Only over Familiar Terrain. Terrestrial observation is applicable only for short flights over familiar terrain or well-marked airways. It is not economical to follow highways or railroads because they seldom take the shortest path between two points. A pilot who is depending upon terrestrial observation would be lost if night overtook him or the visibility should become poor, due to a sudden change in weather. Terrestrial navigation has been made easier by the erection of airway beacon lights, which may be recognized and followed by night with the use of charts (maps) of the civil airways. By day, these beacon stations may be recognized by the large numbers painted on the power sheds. Airport hangars generally have large identification marks or names on them. Detailed aeronautical charts, revised every five years or less, show the important topographical information of a region, such as rivers and lakes; the cultural features, such as cities, highways, and railroads; the relief, i.e., the ridges, valleys, canyons, bluffs, and mountains with their altitudes; and such aeronautical data as airports, army fields, beacons, and radio stations.
AVIGATION 379 Aeronautical charts are similar to relief maps in the manner in which they show differences in altitude. Relief is shown by contour lines, areas of equal altitude between the contour lines being shown in the same color, while areas of different altitude are shown in different colors. A contour represents an imaginary line on the ground, every point of which is at the same altitude. With a little practice one can easily read a contour map or chart. Figure 168 shows how a contour is constructed. For cross-country flying the course is laid out on the chart and carefully studied by the pilot before taking off. By 1941 the United States Government had over 25,000 miles of civil airways, with rotating beacons at intervals of about 15 miles and Fig. 168. How contour maps are made. with intermediate or emergency landing fields at 50-mile intervals. Traffic is controlled by means of radio communication on these civil airways to avoid collisions. 3. Navigation by Dead Reckoning Depends upon the Use of Five Fundamental Instruments. Navigation by dead reckoning depends upon the determination of positions by means of calculating their direction and distance from a known position, from the course, the direction, speed of the wind, and the cruising air speed. With the above information the ground speed and track are easily obtained by simple geometric plotting. Dead reckoning is used when other methods of navigation are not possible, and it is usually quite accurate. Five fundamental instruments are essential for navigation by dead reckoning. (i) A clock or watch. (2) An altimeter. An altimeter is an aneroid barometer that registers atmospheric pressure on a scale which is calibrated to read in feet above sea level. The altimeter has to be set just before leaving the ground in accordance with the atmospheric pressure at that time and the altitude of the airport. Inasmuch as an increase in temperature causes air to expand, the altimeter will give readings which are too low to the extent of about 2 per cent for each 10° F. rise in temperature