We are frequently informed that the high speeds of modern aircraft have rendered celestial navigation in the air an infinitely more arduous and difficult task than that performed on surface craft. Having made our observations in the air, we must reduce them to position lines with all possible dispatch if they are to be of value. To attain this end, tabular forms, in abundance, for the simple and rapid reduction of observations have been devised and are flooding the market. Many are acquainted with these devices and can use them with assurance when no necessity for a hurried solution exists. However, when the pace must be quite rapid, experienced and novice alike stumble into simple, exasperating arithmetical mistakes which become numerous when fatigue mounts, and from which extrication becomes increasingly difficult. It is largely for this reason that such a splendid aid as celestial air navigation is skeptically regarded and is less popular than its worth justly merits.
Still further efforts to simplify and speed up calculations have resulted in Various arrangements of semi-permanent star curves. However, the system which does not, because of its inherent nature, contain one or more very objectionable faults is yet to be discovered. Flexibility and absence of all bounds of limitation are so demanded in celestial work that the logarithmic reductions of observations Persist, justly too, and will persist for some time in the simplified tabular form. I'd any navigators have taken to precomputing their own altitude and azimuth curves for a certain flight, the reason given for this being that the otherwise Necessary celestial computation in the air ls thus eliminated. If the navigator stops to consider such a statement, he will soon realize that it should be qualified. It is not so much the air work he wishes to avoid, for the time available to accomplish it is abundant; what he wishes to avoid is the necessity of hurried solutions-after-observation.
Many of the nautical tables used in marine practice are justly avoided in air navigation. In fact, “junked in toto” might better describe the treatment they have received and, in the majority of cases, merited. However, such tables are based upon the very fundamental principles of navigation, many of which can and have been adopted for air use. One of these slighted nautical tables, the writer has had occasion to use several times in air navigation and has found that when using it, he is able to “apply the brakes,” avoid the usual giddy, mistake-making pace, and, it follows, work deliberately and leisurely, still obtaining a fix in an amazingly short interval after observing. Since, with its use, the navigator may perform his computations with an assurance born of leisure, there is little necessity for predrawing altitude and azimuth curves for his particular flight.
Let the navigator set a time of observation sufficiently in advance to comfortably reduce his observation, or observations, by his usual method. The navigator’s only limitation in time is that he afford himself the opportunity to perform his work deliberately and without haste. Reduce the observation leisurely, plotting the azimuth line on the chart and recording the computed altitude. This completed, the corrections for index error, refraction, and others necessary, are applied to the computed altitude with reversed sign. With the dead-reckoning latitude and azimuth (quadrantal) enter the pictured table and pick out the change in altitude per minute of time (the altitude change is tabulated in minutes of arc).
Within a few minutes of the pre-selected time, let the navigator make his observations. Now let us note the few steps of work necessary before laying down the position line or lines. Determine the time, in minutes and fractions, by which the actual observation time differs from the pre-selected time of observation, multiplying it by the “altitude change” taken from the table. Apply this product to the computed altitude (already corrected for index error, refraction, etc.) as the rule directs, for the “adjusted” altitude (which likewise contains the corrections). Comparing the “adjusted” altitude with the actual sextant reading, we obtain the intercept and may immediately lay down our line of position. In short, by applying the product of the time discrepancy and the “altitude change” to the computed altitude (corrected for errors), we “adjust” it. The sextant reading may then be compared with the “adjusted” altitude, furnishing the intercept necessary to lay down the position line. Thus, by use of the change in altitude table, the hurried after-observation-reductions (mistake- breeders at the best) are supplanted by leisurely computations performed before observation, when the navigator can conveniently spare the time. If the navigator will limit his observations to a period within ten minutes of the time used in pre-computation (allowing a total of 20 minutes for observing) he will find that the table is satisfactory for use in all practical cases.
The altitude correction chart referred to herein has been taken from Nories’ Nautical Tables. For an interesting example of the use of “altitude correction” the reader is referred to Seaplane Solo in which Francis Chichester describes his hair-raising solo flight across the Tasman Sea. The time-azimuth-altitude table mentioned by the author is presumably that from Nories.
The fact that a table designed for marine practice may be used so advantageously in the air should encourage us to consider every possibility of all such tables before relegating them to the rubbish heap.
Keep the faculty of effort alive in you by a little gratuitous exercise every day. That is, by systematically ascetic or heroic in little unnecessary points, do every day or two something for no other reason than that you would rather not do it, so that when the hour of dire need draws nigh, it may find you not unnerved to stand the test. Asceticism of this sort is like the insurance which a man pays on his house and goods. The tax does him no good at the time, and possibly may never bring him a return. But if the fire does come, his having paid it will be his salvation from ruin. So with the man who has daily insured himself to habits of concentrated attention, energetic volitation, and self-denial in unnecessary things. He will stand like a tower when everything rocks around him, and when his softer fellow-mortals are winnowed like chaff in the blast.— James.