THE AIRCRAFT SEXTANT

The Aircraft Sextant is utilized in navigating an airplane by celestial means. Its function is to determine the position of the aircraft with respect to the earth by measuring the angular-altitude of the sun, moon, stars or planets. Besides the sextant, a celestial navigator requires such additional equipment as the current Air Almanac, an accurate watch, and tables for the performance of necessary computations. The art of navigating by observation of heavenly bodies is largely dependent upon the skill with which the aircraft sextant is used.

Its theoretical principle is based on the optical fact that when a ray of light is twice reflected in the same plane, the angle between the first and last directions is twice the angle between the reflecting surfaces. As in direct observations, altitudes up to 90° must be measured, it follows that an angle of 45° between the two reflectors would be sufficient to measure any altitude so encountered. As an arc of 45° is one-eighth of a circle, instruments of this type having a reflector capable of being rotated through an arc of 45° are called octants. One having an arc of 60° is called a sextant, and one with an arc of 90°, a quadrant.

As double altitudes and horizontal angle observations would be very inaccurate if taken from the air, their employment in aircraft is precluded, hence an instrument capable of measuring angles up to 90° is adequate for aerial use ― in other words, an octant. The additional angles available with the sextant, while of no particular value from a practical standpoint, are not objectionable. The quadrant, however, is too bulky and awkward to handle for aerial use. Therefore, either octants or sextants may be used from aircraft. For years, practically all instruments used for this purpose by mariners have been sextants, and navigators have become so accustomed to calling such instruments sextants that the term is loosely applied to all instruments for measuring altitudes of heavenly bodies, whether in fact they are sextants, octants, or quadrants.

Operation of the Aircraft Sextant

Among the basic parts of the instrument is a rotatable prism which is connected to the sector by a shaft. Teeth of the sector mesh with a worm which is operated by the knob. A graduated scale is attached to the sector. This scale, read through a window of the instrument housing, carries a graduation line for each five degrees. On the periphery of the knob is another scale. It is marked off into five principal divisions, each representing one degree. These major graduations are further subdivided into thirty parts, one for each two minutes of arc. Thus, by turning the knob, the prism is rotated to the required angle for the sighting of the heavenly body, and coincidentally, the two scales are moved to record this angle.

Other principal parts of. the instrument include the telescope system, comprised of various prisms and lenses as noted in the sketch; also the artificial horizon which consists of a bubble chamber plus the diaphragm chamber. These two are component parts of a single metal housing. The bubble chamber, with glass top and bottom, form a part of the optical system. In operation, a bubble is formed by turning the knurled knob clockwise. The bubble thus formed represents an artificial horizon. Next, a celestial body is sighted and by turning the knob its image is brought alongside the bubble so that the center of the body and the center of the bubble are aligned horizontally. When the two images are thus arranged, the astigmatizer may be inserted into the optical system by pressing a switch handle. The astigmatizer elongates the image of the celestial body, and the resulting symmetrical arrangement of the images facilitates estimating the center of the bubble, makes for precision accuracy in the use of the Aircraft Sextant.

When the two images are thus brought into relation, the combined reading of the two scales is the angular-altitude of the heavenly body. The average of numerous such sightings is determined, time is recorded, and the position of the aircraft is arrived at by reference to the Current Air Almanac together with the necessary computations.

The averaging device is comprised of a trigger-operated pencil and a ratchet. The pencil makes a vertical mark on the surface, recording numerous observations from which an average is taken for the plotting of position.

This article was originally published in the April, 1944, issue of Air Tech magazine, vol 4, no 3, p 41.
Photo credited to Eclipse-Pioneer Div of Bendix Aviation Corp
The PDF of this article includes a photo of an aircraft sextant and a phantom drawing showing the interior arrangement.