Lectures in Navigation Part 21

Test each s.e.xtant for index error and record the result where you can refer to it easily.

See that all charts of the harbor out of which you are to steam are corrected to date and are familiar to you, both as to sailing directions and buoys, and also as to lights and other aids to navigation.

Examine, in detail, the steering engine and steering apparatus. In case of its disarrangement your intimate knowledge of it may be most valuable.

See that the patent log and sounding machine are in good order. See that the lead lines are well soaked in water, stretched, and properly marked.

See that the lighting system in the chart room and the navigator's room is such that when any door is not tightly closed the lights in the room are extinguished. Likewise, when the doors are closed, see that the lights will light and without repeated slamming of the doors.

If possible, provide yourself with a flashlight set back in a metal tube so that the rays of the light are not diffused but can be focussed only on one spot at a time.

See that your charts are arranged neatly in the drawers provided for them in the chart room. If, as is usual, the charts must be folded to get them in the drawers, mark them legibly on the outside and in the same place on each chart. Put in the top drawers those charts you know you will use most frequently. This will save endless time and confusion.

Be sure you have a full complement of necessary instruments, including s.e.xtants, a stadimeter, binoculars, watches, stop watch, dividers, parallel rulers, pencils, work books; also all necessary books, such as smooth and deck log books, several volumes of Bowditch, Nautical Almanacs, Azimuth Tables, Pilot books, Light and Buoy lists, Star Identification Tables, etc. You will be repaid a thousand times for whatever effort you expend to have your navigational equipment complete to the smallest detail. The shortage, for instance, of a pair of dividers would be an unending annoyance to you. This is also true of almost any other item mentioned above. Prepare yourself, then, while you are in port and have plenty of opportunity to secure the equipment you desire.

_(b) While at Sea_

The least amount of work required of a navigator in time of peace would include (1) a morning sight for longitude, (2) a noon sight for lat.i.tude, (3) an afternoon sight for longitude, (4) an A.M. azimuth to check the deviation of the compa.s.s, and (5) the dead reckoning for the day's run from noon to noon.

Navigating in war time requires more work than this. If possible, the ship's position must be known accurately at any time of day or night for, in case of an emergency, the lives of all on board may be imperilled by inaccurate knowledge of your whereabouts. This means that more sights must be taken and more celestial bodies observed. While every navigator has his own idea as to the proper amount of work to do in a day, it would seem as though the following would cover the minimum amount of work necessary under present conditions:

1. An A.M. sight of the sun for longitude.

2. An azimuth of the sun for checking the deviation of the compa.s.s, taken right after the A.M. sun sight.

3. The watch time of Local Apparent Noon.

4. Ex-meridian and meridian alt.i.tudes of the sun for lat.i.tude.

5. A P.M. sight of the sun for longitude.

6. An evening twilight sight of three or four stars, preferably one in each quadrant. If these alt.i.tudes are taken correctly your position can be found to the dot.

7. A morning twilight sight for a fix or, at least, for lat.i.tude by Polaris.

8. The dead reckoning from noon to noon.

9. Distance run during the last 24 hours, from noon to noon

10. Distance to destination.

11. Set and drift of the current.

_1. The A.M. Sun Sight_

In order to make this a valuable sight for longitude it should be taken when the sun is on or as near as possible to the prime vertical. As the sun, in North lat.i.tudes, pa.s.ses the prime vertical before sunrise in the winter, the following remarks do not hold for that season. In winter the only rule to follow is to observe the sun as soon as it is 10 or more above the horizon. In summer find out from the Azimuth Tables the local apparent time when the sun will bear 90. Estimate, as closely as possible, the longitude you will be in the next morning when the local apparent time is as just found in the Azimuth Tables. This can be done by calculating the dead reckoning from the previous sight, or, what is even simpler, laying the distance off on the plotting chart. With this information find the W.T. corresponding to the L.A.T. mentioned above by some such formula as this: L.A.T. Lo. = G.A.T. Eq. T. (sign reversed) = G.M.T. C.C.(sign reversed) = C.T. - (C-W) = W.T. This will not be absolutely accurate, for the longitude you are in is only approximate, but it will be close enough for good results. This resulting W.T. will be the time to take the A.M. sight. About fifteen minutes before that time compare your watch with your chronometer to get the C-W. Also bring up the C.C. to date and make a note of it so that as much as possible of this detail work is accomplished before the sight is taken. Next, take your s.e.xtant and test it for index error. This should be done regularly before each series of sights as it is impossible to tell what may have happened when the s.e.xtant is lying idle, except by the above test. Now, with your s.e.xtant, watch and notebook, go to the place from which you have decided to take your observations and, at the proper watch time, start taking your alt.i.tudes. It is always advisable to take a number of sights, closely following each other, so that an error in one may be corrected somewhat by the others. Take at least three sights in close succession. At the same time have the log read and enter it in your notebook. An equally good method in fair weather is to secure the distance run from the revolutions of the propeller.

Having taken your sights, go to the standard compa.s.s and get a bearing of the sun, at the same time noting in your book the W.T. of the bearing and the compa.s.s heading of the ship. You are now ready to go below into the chart room and work out your position. What method shall you use?

That depends upon your preference. You have missed the point of the previous lectures, however, if you forget that the New Navigation is based upon the Marc St. Hilaire Method, and this is undoubtedly the method your captain will prefer you to use if he is an Annapolis graduate. In this connection let me remind you again of the one fact, the oversight of which discourages so many beginners with the Marc St.

Hilaire Method. The most probable fix, which you get by one sight only, is not actually a fix at all. Nor does any other method give you an accurate fix under like conditions. What the most probable fix is, and all it claims to be, is a point through which the required Sumner line is to be drawn. If your D.R. position happens to be only one mile away from the most probable fix, that is no a.s.surance that the most probable fix is near the actual position of the ship. You may be 25 miles away from it. But the important information gained is that, though you may be 25 miles away, you know on what line you are, and when this line is later crossed with another line of position that fix will be accurate.

"Two sights make a fix" is the whole matter in a nutsh.e.l.l.

_2. The Compa.s.s Error_

Having secured your morning sight, the next duty is to get the compa.s.s error. From your morning sight computation you know the watch time corresponding to the L.A.T. of the same sight. Find the difference between the two and apply this difference to the watch time of the compa.s.s azimuth. That will give you the L.A.T. with which to enter the Azimuth Tables to get the true bearing corresponding to the compa.s.s bearing recently observed. Apply the variation from the chart to get the magnetic bearing. The difference between this magnetic bearing and the compa.s.s bearing will be the required deviation, which you should compare with your Deviation Table. If there is a marked difference, and you are sure of your figures, use the new deviation in computing courses on this heading of the ship.

_3. The Watch Time of Local Apparent Noon_

You are now ready to figure the watch time of local apparent noon.

Unless you have a decided preference to the contrary, do this by the method explained in the Sat.u.r.day Lecture, Week VI. Do not forget that in subtracting the L.A.T. of the morning sight from 24 hours to get the total time to noon, in case the ship were stationary, you do _not_ use the L.A.T. of the D.R. position, but the L.A.T. found by subtracting from G.A.T. the longitude of the most probable fix. This will give you the L.A.T., based on the longitude of the most probable fix, which will be slightly different from the L.A.T. based on the D.R. longitude. When you have secured the watch time of local apparent noon, subtract 30 minutes from it and notify the quartermaster that at that time by your watch the deck clocks are to be set to 11.30 A.M. If this change of time is very great (providing you are on an almost easterly or westerly course), it is wise to have the clocks set back in the night watches to allow for most of the time you figure you will lose. This will not work such a hardship or such an advantage to the officers and men who have the forenoon watch and will also be easier for the cooks. The clocks can then be slightly but accurately changed at 11.30 A.M., as mentioned above.

_4. Ex-Meridian and Meridian Alt.i.tudes_

You know the principles and methods governing sights of this character.

To know your lat.i.tude exactly at noon is usually required when you are steaming in convoy, for at that time your position signals are hoisted, and it is a matter of pride with the navigator not only to have his position exact but promptly. If your A.M. sight was taken when the sun was on or near the prime vertical, a change in lat.i.tude at noon will make no change in longitude. Hence you can figure your longitude at noon just as soon as you have secured the corrected time from the A.M. sight to noon (which you have done right after working the A.M. sight). You will have your longitude, then, before you go on the bridge to observe for ex-meridian and meridian alt.i.tudes.

Sharply at noon you take your meridian alt.i.tude and tell a messenger to notify the captain that it is noon at the ship. The captain then orders eight bells struck, and you are ready to hand in your noon report, consisting of lat.i.tude and longitude by observation, lat.i.tude and longitude by dead reckoning, deviation of the compa.s.s on the ship's head at 8 A.M., distance made good since the preceding noon, distance to destination, set and drift of current (Note:--When steaming in convoy this is unnecessary and usually omitted), and any other pertinent remarks. If the sun was not taken on or near the prime vertical at the time of the A.M. sight, take out your longitude factor for the coming noon position and calculate your D.R. lat.i.tude at noon. By correcting the longitude of the A.M. sight, run to noon, with the difference of longitude, readily found at noon with the longitude factor and the error in lat.i.tude, you will have the correct noon longitude to hand in, with only a moment's delay. It will be very hard, however, to get all this information in on time without the use of lat.i.tude constants. There is no room for a discussion of these constants here, but they are easy to work and you should learn how to use them. The information is in Bowditch Art. 325, p. 128, and Art. 405, p. 181.

_5. The P.M. Sun Sight_

This is another longitude sight and so any previous remarks about sights of this character are applicable here. If the day is fine you need not work out this sight until after evening twilight, for a fix then by stars will give both lat.i.tude and longitude, whereas your afternoon sun sight will only give you a longitude. This P.M. sun sight is a good check sight, to be used or not, according as to whether other earlier or later sights have been obtained.

_6. The Evening Twilight Sight_

The beauty of using stars is that by almost simultaneous alt.i.tudes of different ones you can ascertain your position, both as to lat.i.tude and longitude. In the North Atlantic during the summer months Vega, Deneb or Altair in the East, Antares or Deneb Kaitos in the South, Arcturus in the West, and Polaris, Mizar, or Kochab in the North form an ideal combination which includes every quadrant of the compa.s.s. In the winter months, Capella, Castor or Pollux in the East, Sirius or any star in Orion's belt in the South, Deneb in the West, and Polaris in the North are equally as good.

_7. The Morning Twilight Sight_

In clear weather this should be primarily a sight for lat.i.tude, since the A.M. sun sight for longitude will follow it. A lat.i.tude by Polaris, and at the same time some star in one of the southern quadrants, as a check, will give admirable results.

_8. The Dead Reckoning from Noon to Noon_

If there is no change of course in the late forenoon, as is usually the case, the dead reckoning for the day's run can be figured any time before noon so that it will be all ready to hand in to the captain with the other noon data. It is much easier to lay this off on the chart than to go to the trouble of calculating it by Table 2, Bowditch. On the other hand, such a calculation checks the chart work and should be worked out if you wish to make "a.s.surance doubly sure."

_9. Distance Run During the Last 24 Hours_

Here, again, an answer by chart and an answer by figures is a good thing to secure. As you become accustomed to your work you will find the answer by chart infinitely easier and quicker to get. It is just as accurate, too, if you lay the distances off carefully with the dividers and parallel rulers.