Advanced Pilot Training: TM 1-205 Air Navigation - Section III Time, Direction, Bearing

21. Time.-Time is of primary importance to the navigator. By its use he can calculate the distance traveled along his course or in celestial navigation he can calculate his longitude. There are three different kinds of time:

a. Mean or civil time.-Mean time is based on the travel of an imaginary or fictitious sun. This is the time as kept by watches or chronometers, and the time we use in our time zones. At the same instant the civil times of two points of different longitude are never the same. Mean or civil time is not to be confused with standard or zone time which is based on the civil time of certain selected meridians. Standard time is described in paragraph 24.

b. Apparent or solar time.-Solar time is time based on the travel of the real sun. The real sun is also known as the true sun or apparent sun. The time read from a sun dial is apparent time. Solar time is used extensively in celestial navigation.

c. Sidereal or star time.-This time is used in celestial -navigation when determining the relationship between time and the position of the stars. It is based on the travel of the stars.

22. Day.-The time interval between two successive transits of a heavenly body is a day. The civil day, solar day, and sidereal day are -not of the same duration. The difference between these various kinds of days is described in TM 1-206. The civil day is the only one of particular interest to the pilot and is the interval between two successive transits of the mean sun, not the real sun. The interval between transits is divided into 24 hours, each of 60 minutes.

23. Time and longitude.-a. There is a definite relationship between time and longitude-24 hours of time is equal to 360° of arc of longitude. Thus each hour of time equals 151 of longitude. Other subdivisions are shown as follows:

Using the above relationship, if the longitude and time of one place are known and the longitude of a second place is known, the time of the second place may be determined.

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25. The 24-hour system.-This system of keeping time eliminates the use of the abbreviations A. M. and P. M. The values for A. M. time are unchanged except that four figures are always used: 8:00 A. M. becomes 0800 hour; 3:15 A. M. becomes 0315 hour; and 11:38 A. M. becomes 1138 hour. The value of P. M. time is increased by 1200, hence 1:15 P. M., 7:42 P. M., and 11:19 P. M. become 1315 hour, 1942 hour, and 2319 hour, respectively. The use of this system decreases the chances of making errors by eliminating the A. M. and P. M. abbreviations, and for this reason it has been adopted for use in Air Corps operations.

26. Direction and bearing.-a. Marine method.-The mariner divides the circle into 32 equal parts called "points," each point equaling 11015'. To sail two points east of north means, therefore, to sail 22'30'east of north, or north northeast (N.N.E.), etc. This system is not used in air navigation and is becoming outdated in surface navigation.

b. Second system.-In a second system somewhat similar to the above, the circle is divided into four equal parts, indicating the four cardinal directions, N, E, S, and W. The direction from A to B, in gure 18, is called "North 301 East"; A to F is "South 60' East"; A to C is "South 300 West"; A to D is "North 601 West." Each direction in this system is referred to as north (or south) so many degrees (or west). This way of designating direction has a special use in calculating courses and distances. These calculations are de scribed in section IX, chapter 2.

c. Air navigation system.- (1) The system of measuring and naming directions used in air navigation is easier to use than either of the above. It consists of designating directions in relation to north by measuring them clockwise from north through any are up to 3600. In this system it is not necessary to refer to north, east, south, or west, as the numerical value shows the exact direction. Thus in figure 18, AN=3601 or 01, AB=30", AE=90', AF=1200, AS=1800, AC=210', AW=2700, and AD=3001. To measure the direction from a point A to a point B, the angle between the meridian which passes through A and the line connecting A and B is measured clockwise from north. If this angle equals 30', it may be said that(a) The bearing from A to B is 300.

(b) B bears 30° from A, or A bears 210° from B.

(c) The direction from A to B is 30°

(d) The true course from A to B is 30°.

(2) Another example of the difference between a direction (or course) and a bearing (or azimuth) may be seen by referring to figure 19.

(a) Angle a is the course angle from A to B.

(b) Angle b is the course angle from B to A.

(c) Angle Z is the bearing or azimuth of B as measured at the point A.

(d) Angle Z' is the bearing or azimuth of A as measured at the point B.

27. Directions on a map.-a. How measured.-(1) When it is desired to find the course or direction between two points on a map a connecting line is drawn between these points. Then, if the map is based on the Lambert projection,  the angle between the connecting line and the meridian which is nearest to midway between the points is measured. Thus, in figure 19  (Lambert projection) the course angle between A and B is angle a rather than angle Z or angle b rather than angle Z'. For usual flights the use of an ordinary protractor to measure the angle will give sufficient accuracy.

(2) It should be explained that when the course is measured on the meridian nearest halfway, a plane following that course will not exactly follow the straight line on the chart but will slightly depart therefrom near the middle of the route. However, when courses are measured as recommended in the following paragraphs, the departure is so slight that it may be considered that the plane does track the straight line throughout its entire length.

b. Long courses.- (1) When the two points are separated by not more than 3° or 4° of longitude, the true course may be measured on the meridian nearest halfway as described above and as illustrated in figure 19. The entire distance is then flown as one cours

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(2) When the difference of longitude between the two points is more than 3° or 4°, the straight line on the chart should be divided into sections crossing not more than 3° or 4° of longitude each. The true course to be flown for each section is then measured on the middle meridian of that section.

(3) Example- (a) Figure 20 illustrates the method of determining the series of true courses to be flown between St. Louis and Minot. The distance is 862.7 statute miles. The difference of longitude is nearly 12° which is too great to be flown satisfactorily in one course. The route is therefore divided into three sections crossing approximately 4° of longitude each. The true course to be flown throughout the total length of each section is measured on the middle meridian of that section, and the course is changed in flight as the end of each succeeding section is reached.

(b) For the flight from St. Louis to Minot only two regional charts are required. It is a simple matter to join these two charts and draw the straight line between the two places. When using the sectional series, six charts are necessary, and it is inconvenient to join so many charts; in this case, therefore, the route should first be plotted on one of the small-scale planning charts and then transferred to the regional and sectional charts.

c. Computing course and distance.-The course and distance between two points may be computed mathematically with great accuracy. The methods are described in section IX, chapter 2.