NAVIGATION BY ASTRONOMICAL OBSERVATION
In navigation by astronomical observation, the position of the aëroplane or airship is found by observing the height above the horizon of either the sun or another heavenly body, such as a star that is easy of recognition. The method depends upon the known fact that at any given instant the sun is vertically above some definite point on the earth's surface. This point can be calculated from the time of the observation and the declination and equation of time, as tabulated in the nautical almanac.
In the case of stars, the right ascension of the sun and of the star also enter into the calculation. The method of carrying out such calculations is too involved for the scope of this volume, and the reader is referred to many of the excellent text books published on the subject of navigation.
Since the navigator knows, from the time of his observation, the point on the earth's surface over which is the heavenly body in question, it is clear that around this point circles on the surface of the earth may be described. From any point in any one circle the heavenly body will appear to have the same altitude or elevation above the horizon. A single observation of the altitude of any one heavenly body shows, therefore, only that the observer may be at any point on such a circle of equal altitude—otherwise known as a Sumner circle. But it does not fix that point.
A second simultaneous observation, of a different heavenly body, will give a different circle, corresponding to the position of the second body. The intersection of these two circles determines the point of observation.
This fact constitutes a reliable basis for fixing one's position during a clear night, when many stars are visible and choice of suitable heavenly bodies may be made. During the day, however, the light of the sun prevents other heavenly bodies from being seen, so that only a single observation is possible.
If the aëroplane or airship were not moving, then two successive observations of the sun, with an interval of an hour or more between them, would give the intersecting circles and fix the position. But the aircraft being in motion, it is necessary to combine the method of "Dead Reckoning" with the use of the Sumner circles, as found by observation of the sun's altitude.
In order to avoid drawing the entire circle, a small portion only of it is shown on the chart—so small that it may be regarded as a straight line. Such a small section of the Sumner circle is known as a "position line."
The desired track is laid out on the chart, and the "Dead Reckoning" position for the time of the solar observation is indicated on it. The track should be intersected at this point by the position line, the observation thus forming a check upon the "Dead Reckoning."