Having secured your morning sight, the next duty is to get the compass 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 compass azimuth. That will give you the L.A.T. with which to enter the Azimuth Tables to get the true bearing corresponding to the compass bearing recently observed. Apply the variation from the chart to get the magnetic bearing. The difference between this magnetic bearing and the compass 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 Saturday 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 Altitudes
You know the principles and methods governing sights of this character. To know your latitude 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 latitude 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 altitudes.
Sharply at noon you take your meridian altitude 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 latitude and longitude by observation, latitude and longitude by dead reckoning, deviation of the compass 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. latitude 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 latitude, 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 latitude 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 latitude 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 altitudes of different ones you can ascertain your position, both as to latitude 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 compass. 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.