HE Aircraft Sextant is utilized in navigating an airplane by center of the body and the center of the bubble are aligned Tcelestial means. Its function is to determine the position of horizontally. When the two images are thus arranged, the the aircraft with respect to the earth by measuring the angular- astigmatizer may be inserted into the optical system by pressing altitude of the sun, moon, stars or planets. Besides the sextant, a a switch handle. The astigmatizer elongates the image of the celestial navigator requires such additional equipment as the celestial body, and the resulting symmetrical arrangement of current Air Almanac, an accurate watch, and tables for the the images facilitates estimating the center of the bubble, performance of necessary computations. The art of navigating makes for precision accuracy in the use of the Aircraft Sextant. by observation of heavenly bodies is largely dependent upon When the two images are thus brought into relation, the the skill with which the aircraft sextant is used. combined reading of the two scales is the angular- altitude of Its theoretical principle is based on the optical fact that when the heavenly body. The average of numerous such sightings is a ray of light is twice reflected in the same plane, the angle determined, time is recorded, and the position of the aircraft is between the first and last directions is twice the angle between arrived at by reference to the Current Air Almanac together the reflecting surfaces. As in direct observations, altitudes up to with the necessary computations. 90° must be measured, it follows that an angle of 45° between The averaging device is comprised of a trigger-operated the two reflectors would be sufficient to measure any altitude pencil and a ratchet. The pencil makes a vertical mark on the so encountered. As an arc of 45° is one-eighth of a circle, surface, recording numerous observations from which an instruments of this type having a reflector capable of being average is taken for the plotting of position. 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 This article was originally published in the April, 1944, issue of be very inaccurate if taken from the air, their employment in Air Tech magazine, vol 4, no 3, p 41. aircraft is precluded, hence an instrument capable of measuring Photo credited to Eclipse-Pioneer Div of Bendix Aviation Corp. 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

1. Rotatable prism 8. Bubble chamber 2. Sector 9. Diaphragm chamber 3. Shaft 10. Knurled knob 4. Worm 11. Astigmatizer 5. Knob 14. Pencil 6. Graduated scale 15. Ratchet 7. Scale 16. Surface