Thursday, May 22, 2008

Celestial Navigation without a Sextant

Items needed: The Nautical Almanac, Sight Reduction Tables (H.O. 249, Vol. II or III), accurate time, plotting paper, graph paper, scratch paper, pencil, dividers, and plastic plotter.

A. Note body (sun) (while the concept applies to the moon, stars, and planets as well, this examination will be of the body most com­monly used; when applying this procedure to the moon, use the" Altitude Correction Ta­bles-Moon," found on the inside back cover of the N.A.), date, deduced latitude and long­itude (DR position). Select upper or lower portion (limb) of the sun for a one-time effort. To increase your chances of success with the sun, there is the possibility of taking both up­per and lower limb readings. This is fine, but keep the mathematical calculations separate, as the main correction for the semidiameter of the sun is added for the lower limb and subtracted for the upper limb.) Determine height of eye above water.

B. Try to judge the instant at which the se­lected limb crosses the horizon (when it ap­pears in the case of a rising sun, or disappears when setting) and note the exact watch time. In the case of the moon, to some degree, and the stars and planets, in particular, it is recom­mended that you use binoculars to increase light-gathering ability and to improve the esti­mate of when the body crossed the horizon. Use of this technique for stars and planets is generally limited to clear nights with substan­tial moonlight providing a clearly defined horizon. At dawn and dusk, when these bodies become more visible than during the day, the ambient light usually impairs your ability to pick up the bodies as they rise or be certain of their departure when they set.
There is, however, the possibility of "bob­bing" a limb of the moon or the planets and stars at any time during the night. The technique may be used for either a selected body disappearing over the horizon to the west or one scheduled to appear in the east. For a disappearing body, a low position in the boat is desirable for an appearing body, position yourself as high as possible. Either way, by standing upright and stooping, it is possible for a few moments to make the body appear and disappear. As this occurs, note the time and judge your average height of eye to ob­tain the "dip" factor.

C. Since you have no series of readings, you cannot plot them on graph paper. Conse­quently, you cannot be entirely confident that the timing was correct and was not overly in­fluenced by the motion of the vessel. Uncer­tainty of 20 seconds as to when the limb or the body actually appeared when rising, or disappeared when setting, produces an un­certainty of approximately five nautical miles in your calculated line of position (LOP). However, if you are without a sextant, ac­curacy of plus or minus five miles might well be considered a magnificent success.

D. Even though no sextant was actually used, the calculations for producing the observed altitude (Ho) are the same. The difference is that the sextant altitude is zero, since the ob­servation was made when the limb or body was, in effect, brought down to the horizon. Obviously, there is no sextant error correc­tion. Therefore, the calculation is worked as folIows:

1. Take observed read­ing (0°00.0').
2. Subtract dip for height of eye found inside front cover of the Nautical Almanac.
3. The result is apparent altitude. (Ha)
4. From page A3 of the Nautical Almanac, under "Sun," se­lect the month period and upper or lower limb correction. (Alt. Corr.
5. Due to the low Ha, it is desirable to correct for the effects on the atmosphere of baro­metric pressure and temperature. See page A4, "Altitude Correction Tables Additional Corrections," in the Nautical Almanac. Lacking exact baro­metric and tempera­ture information, it is worthwhile making a guess in order to ob­tain the best adjust­ment possible.
6. The result is the ob­served altitude of the sun. (Ho)

E. One additional point must be made, about the determination and construc­tion of the intercept for the LOP. Due to re­fraction, the limb of the sun (or other body) is always below the actual horizon. The effect of atmosphere is to permit observation when otherwise the limb would have disappeared. As a result, there may be some positive and negative numbers in your calculations. In such instances, the corrections must be deter­mined algebraically. For negative altitudes, a negative correction is numerically added. Normally, with the Ho being greater than Hc, the intercept would be toward the sun (or other body). Since the intercept is negative, however, the LOP is placed away from the sun. Also algebraically, a positive correction must be numerically subtracted.

In both instances, you can appreciate that the rule "Ho more than Hc, place LOP toward the body" still applies. It just takes some reassur­ance that a negative Ho with a larger number than the negative Hc is actually just "more negative" and therefore smaller, or that a negative Ho must pass all the way up through zero to be compared with a positive Hc. The result is addition. Again, since a negative Ho is obviously smaller than a positive Hc, the in­tercept must be away from the assumed posi­tion (AP), with the intercept being the sum.