Celestial Air Navigation Text

Office of the Chief of the Air Corps

Year: 1937

Randolph Field Printing Office - March 1, 1937 - 3500

Celestial Air Navigation Text, Office of the Chief of the Air Corps,
1937, Randolph Field Printing Office - March 1, 1937 - 3500 CELESTIAL AIR NAVIGAT10N TEXT Prepared under the direction of the Chief of the Air Corps 1937 TABLE OF CONTENTS Chapter Page I. Definitions and the fundamentals of celestial navigation . . . . . . . . . . .1 II. Position circles. The astronomical triangle and its reduction . . . . . . .11 III. Time and the nautical almanac . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 IV. The sextant and the errors of observation . . . . . . . . . . . . . . . . . . . 41 V. Position lines and their use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 VI. Precomputed curves; simultaneous star altitude curves; change in altitude tables; other reductions. . . . . . . . . . . . . . . . . . . . . . 59 VII. Star identification and the Rude Star Finder . . . . . . . . . . . . . . . . . 71 VIII. Latitude and azimuth by Polaris; compass swinging by celestial azimuths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 IX. Flight Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 FOREWORD The following celestial navigation text is patterned after the course in this subject given by the 19th Bombardment Group. Since previous extensive training in the astronomy of navigation is seldom met with, liberties have been taken in the matter of terminology when such have been thought desirable. It is believed that the subject matter is so compiled as to eliminate the necessity of the usual index. At the end of the first few chapters several usual questions are propounded and answered. Sample observations using sun, moon star and planet, will be found after the last chapter. Earlier will be found examples of the problems of computing sidereal time and of obtaining latitude and azimuth from Polaris. All of these will be found useful references for the student when difficulties are encountered in such problems. The subject of time is most difficult for the beginning student in navigation. It is presented in an entirely original manner in this text. Every effort has been made to remove many of the usual obstacles. Let this not be interpreted to mean that the subject, as here presented, will be mastered easily. Regardless of the method of presentation, the subject of time will be found difficult. Increasing demands for greater accuracy in aircraft sextant observations will doubtless result in the development of satisfactory averaging devices for existing bubble and pendulum sextants and the development of gyroscopic sextants, both of which will yield a sufficiently accurate single observation. This is the greatest need in celestial air navigation at present. Several mechanical computers are on the market. It is argued that these will reduce the number of arithmetical mistakes made in hurried reductions common to air navigation. Such devices do not eliminate the necessity of using the almanac. However, the incorporation, in a computer, of certain almanac data is certainly within the realm of possibility. On the other hand, years of navigation practice have been responsible for the development of many excellent tables and booklets that will be difficult of replacement by mechanical means. Regardless of the path the navigator chooses to follow in his selection of equipment, tables, computers, etc., nothing will be found to have the general utility and the value of a thorough understanding of the fundamentals of navigational astronomy. Such a knowledge can come only from study and practice. The purpose of this text is to aid the beginner in the former. CHAPTER I DEFINITIONS AND THE FUNDAMENTALS OF CELESTIAL NAVIGATION. An understanding of several terms used, frequently in celestial navigation is necessary before the general principles upon which the art' is based may be studied. A knowledge of the terms used in D. R. navigation is assumed, and only those needing special clarification or emphasis will be touched upon in this chapter. The definitions - and usages of the necessary D. R. terms are fully covered in Chapters I to IV, Hydrographic Office Publication #9 (The American Practical Navigator, Bowditch), which publication should be a part of every navigator's equipment. THE EARTH. The earth is an oblate spheroid, being a nearly spherical body slightly flattened at the poles; its equatorial axis measures about 7,927 statute miles and its polar axis, around which it rotates, about 7,900 statute miles. For the purpose of navigation the earth is regarded as a true sphere equal in area to the area of the surface of the earth. No material error results from this assumption and the "mean sphere" will be assumed in all future references to the earth. NAUTICAL MILE. The nautical mile is one minute of great circle arc on the earth's surface. It measures 6,080 feet and subtends one minute of angle at the center of the earth. Since latitude is measured along great circles on the earth's surface passing through the geographical poles, one minute of latitude therefore equals one nautical mile. Unless otherwise stated the term mile hereafter will imply the nautical mile. CELESTIAL SPHERE. The celestial sphere is an imaginary spherical surface of infinite radius which has as its center the observer's eye or the center of the earth, the two being coincident because of the celestial sphere's magnitude. To an observer on the earth's surface the heavenly bodies, interspersed in space at various distances from the earth, appear projected upon the celestial sphere at points where the lines joining them with the observer's eye intersect the sphere. An observer conceived to be at the earth's center and looking through its solid. substance not only would see the heavenly bodies projected onto the celestial sphere but in addition would see the imaginary points and circles on the earth's surface (the geographical poles, meridians, equator, and parallels of latitude) projected onto it. The imaginary terrestrial points and reference circles thus projected to the celestial sphere constitute the celestial reference markings. The only purpose the celestial sphere serves is that it enables a substitution of spherical triangles to be made for solid angles in the necessary trigonometrical solutions. HORIZON. The horizon generally used in aerial navigation is determined by a bubble, ball, or pendulum in the observer's instrument. The horizon these devices determine is a plane passing through the observer's eye, perpendicular to the vertical at his position. This horizon will be assumed in future references unless otherwise qualified. The visible horizon is that line appearing to an observer at sea to mark the intersection of earth and sky. ALTITUDE. Altitude is the least angular elevation of a body above the horizon at any instant. ZENITH. The zenith of an observer on the earth's surface is the point of the celestial sphere vertically overhead. ZENITH DISTANCE. Zenith distance of a body is its angular distance from the observer's zenith. It is the complement of the body's altitude. AZIMUTH. The azimuth of a celestial body is its bearing measured from the true north or south point. It is measured 180 degrees to the east or west. GEOGRAPHICAL POSITION OF A BODY. The geographical position of a celestial body is a point on earth's surface which is exactly under the given heavenly body at any one instant. An observer at the geographical position would find the corresponding body exactly at his zenith. FIGURE I. FIGURE 2. FIGURE 3. Figure 4 shows a position circle drawn from a measured altitude and
known geographical position. The azimuth angles A and A', at two points on
the circle, are indicated, and it should be carefully
noted that these angles differ in magnitude. From this it maybe seen that
the azimuth angle an observer measures changes as he travels around the
position circle. Consequently, having measured the azimuth, a point on the
position circle could be found at which the radius makes with the meridian
at that point, angle equalling the measured azimuth. The intersection of
the radius and position circle would then mark the observer's position.
Theoretically this is quite correct. It is not practical, however, for the
simple reason that the azimuth cannot be measured to the degree of
accuracy required to permit such a procedure. Since it is quite difficult
for beginning students of navigation to grasp this fact, an explanation of
it will be valuable, and is warranted at this time. FIGURE 5 FIGURE 6 |