lower limb was 55° 6'; the zenith north of the moon and the height of the eye 16 feet: required the latitude. Here the time of observation at the ship is not given, it must therefore be calculated, and we have these data for this purpose, viz., that the ship is 40° W. of Greenwich, and that the moon is on its meridian. The following process therefore immediately suggests itself. The moon passed the merid. of Greenwich Feb.19 (Naut.Alm.) 6h 56m 0 Hence 1h 3m is the moon's retardation in 251 3m, and, by proportion using for the longitude 40°W., its value in time 2h 40m, we have, 25h 3m: 1h 3m :: 2h 40m: 0h 6m 42s; that is, the moon is retarded 6m 42 in passing from the meridian of Greenwich to that of the ship, and therefore, instead of the apparent time at the ship being 6" 56m, as it necessarily would be if there were no retardation, it will be 6m 42s later. Hence Having thus got the apparent time at Greenwich when the observation was made, we may, by a reference to the Nautical Almanac and a subsequent proportion, find the moon's declination at that time: thus Moon's declination at Greenwich, Feb.19 at noon 26° 38' 17" Feb. 19 at midn. 26 54 39 16 22. Change of declination in 12 hours .. 12h: 9h 42m 42s : : 16′ 22′′ : 13′ 15′′; hence 13' 15" is the amount of the change of declination, from noon to 9h 43m, on the supposition, however, that the motion of the moon in declination may be considered as equable during the twelve hours. But on account of the irregular motion of the moon, this supposition introduces a sensible error, which may however be corrected by means of the table of "Equation of Second Differences," given in the Nautical Almanac, and explained by Dr. Maskelyne's accompanying "Explanation." The correct change of declination is thus found to be 14' 16". But from the year 1833, the declination of the moon will be given in the Nautical Almanac to every three hours, and the change for any shorter interval may then be obtained with the requisite accuracy by proportion, as above. Taking in the present case 14' 16" for the correct change, we have Declination for preceding noon 26°38' 17" N. 14 16 Before we can find the proper correction for parallax, we must deduce the apparent altitude of the centre, Observed altitude of 'Os L. L. Semidiameter (Naut. Alm.) Apparent alt. of 's centre 55° 6' 0" 3 50 16 13 13 55 18 36 cos. 9.755217 3572log, 3.552911 2033log. 3.308128 Hor.par.in seconds at 9443m (Naut. Alm.) therefore the correction for parallax is 33' 53". Having thus reduced all the corrections to the time of observation, we readily obtain the true altitude, and thence the latitude as follows: These examples will, no doubt, be found sufficient to put the student in possession of the method of applying the various corrections to the observed meridian altitude of a celestial object, in order to deduce from it the latitude of the ship. But it should be remarked, that in most works on Nautical Astronomy, subsidiary tables are inserted for the purpose of abridging some of the foregoing corrective operations; such tables, therefore, offer very acceptable aid to the practical navigator. The most esteemed works of this kind are Dr. Mackay's "Treatise on the Theory and Practice of finding the Longitude at Sea"; the "Nautical Tables" of J. De Mendoza Rios, Mr. Riddle's book on Navigation and Nautical Astronomy, and the work of Dr. Bowditch. It should also be observed here, that in the preceding examples the celestial object is supposed to be on the meridian above the pole; that is, to be higher than the elevated pole. But, if a meridian altitude be taken below the pole, which may be done if the object is circumpolar, or so near to the elevated pole as to perform its apparent daily revolution about it without passing below the horizon, then the latitude of the place will be equal to the sum of the true altitude, and the codeclination or polar distance of the object; for this sum will obviously measure the elevation of the pole above the horizon, which is equal to the latitude. 112. To determine the latitude at sea, by means of two altitudes of the sun, and the time between the observations. In the preceding article we have shown how to determine the latitude of the ship by the meridian altitude of the sun, or of any other heavenly body, whose declination may be found. But, as already remarked, the object we wish to observe may be obscured when it comes to the meridian, and this may happen for many days together, although it may be frequently visible at other times of the day. As therefore the opportunity for a meridian observation cannot be depended upon, it becomes an important problem to determine the latitude at sea, by observations made out of the meridian; and considerable attention has accordingly been paid, by scientific persons, to the method of finding the latitude by "double altitudes," and various tables have been computed to facilitate the operation. But the direct method, by spherical trigonometry, though rather long, involving three spherical triangles, will be more readily remembered, and more easily applied by persons familiar with the rules and formulas of Trigonometry, than any indirect or approximate process; we shall therefore explain the direct method. the interval between the observations; and the quantity sought is the colatitude zp. Now, in the triangle Pss', we have given two sides and the included angle to find the third side ss', and one of the remaining angles, say the angle rss'. In the triangle zss' we have given the three sides to find the angle s'sz; having then the angles PSS', s'sz, the angle zsp equal to their difference, becomes known, so that we have, lastly, two sides and the included angle in the triangle zsp, to find the third side zp. Before the application of the trigonometrical process, the observed altitudes must, of course, be reduced to the true altitudes, as in the preceding examples. Moreover, as the ship most probably sails during the interval of the observation, an additional reduction becomes necessary; the first altitude must be reduced to what it would have been if taken at the place where the second was taken this correction will be known if we know the number of minutes or miles which the ship has made, directly towards or directly from the sun, since leaving the place where the first observation was made, To find this, take the angle included between the ship's course and the sun's bearing, at the first observation; and considering this angle as a course, and the distance sailed as the corresponding distance, find by the traverse table, or by the operation of plane sailing, the difference of latitude, which will be the amount of the approach to, or departure from, the sun. This must be added to the first altitude if the angle is less than 90°, because the ship will have approached towards the sun; but it must be subtracted when the angle exceeds 90°. If the angle is 90°, no correction for the ship's change of place will be necessary, since in that case sailing in a perpendicular to the direction of the sun, she maintains always the same distance from him, Where great accuracy is aimed at, account should be taken of the ship's change of longitude during the interval of the observations; when converted into time it must be added to the interval of time between the observations when the ship has sailed eastward, and subtracted when she has sailed westward. This correction is very easily applied. Having thus mentioned the necessary preparative corrections, we shall now give an example of the trigonometrical operation. EXAMPLES. Let the two zenith distances corrected be (see last fig.)zs= 73° 54' 13", zs' 47° 45' 51", the corresponding declinations 8° 18' and 8° 15' north, and the interval of time three hours; to determine the latitude, |
