EXAMPLES. November 22, 1853, the true distance of Saturn from the moon was found to be 77° 52′ 45′′, required Greenwich mean Find mean time at Greenwich from each of the following observations :— (57.) November 24, 1853, when true distance of Aldebaran was 93° 38′ 45′′. Ans., 3h 57m 18s. (58.) Sept. 24, 1853, when true distance of Regulus was 58° 45' 8". Ans., 16h 3m 6s. (59.) May 27, 1853, when true distance of the sun was 110° 8' 50". Ans., 14h 2m 22s. To take out a planet's right ascension and declination. Proceed as in the similar rules for finding the sun's right ascension and declination (pp. 74, 81). The rules above given are sufficient to enable the student to acquire a knowledge of the principal contents of the Nautical Almanac. They will be continually referred to in the subsequent rules for finding the latitude and longitude. We have supposed in the above examples the motion of the heavenly body to be uniform in the interval between the Greenwich times taken out of the Nautical Almanac. This is seldom the case, although in most of the questions in Nautical Astronomy it may be so assumed without any practical error. When, however, very accurate results are required, a correction must be used, called the equation of second differences. The investigation of this equation, and examples of its application, must be postponed for the present. CHAPTER V. PRELIMINARY PROBLEMS AND RULES IN NAUTICAL ASTRONOMY. CORRECTIONS FOR PARALLAX, REFRACTION, CONTRACTION OF THE MOON'S SEMIDIAMETER, AND DIP. Given, mean solar time, and the equation of time, to find the apparent solar time; or, Given, apparent solar time, and the equation of time, to find mean solar time. Rule XVI. 1. Get a Greenwich date (p. 70.). 2. Correct the equation of time for this date (p. 77). 3. Apply the equation of time with its proper sign (as shown in the Nautical Almanac) to the given time. 4. The result is the time required. EXAMPLES. 1. April 27, 1846, at 9h 10m P.M., mean time, in long. 16° W., required apparent solar time. 2. June 22, 1852, at 5h 42m P.M., apparent solar time, in long. 100° 30' E., required mean solar time. (61.) Dec. 10, 1853, at 4h 42m P.M. apparent solar time, in long. 80° 45′ W., required mean solar time. Ans., 4h 35m 185.1. (62.) Feb. 23, 1848, at 10h 40m A.M. apparent solar time, in long. 1° 6' W., required mean solar time. Rule XVII. Given, mean solar time, to find sidereal time. 1. Get a Greenwich date (p. 70). 2. Correct the right ascension of the mean sun by the table (p. 83), or by proportional logarithms, or otherwise for the Greenwich date. 3. Add together the corrected right ascension of mean sun and mean time at the ship. 4. The sum (rejecting 24 hours if greater than 24 hours) will be sidereal time. EXAMPLES. Feb. 24, 1848, at 10h 40m 30s A.M. mean time, in long. 1° 6' W., required sidereal time. (63.) July 10, 1853, at 0h 42m 10s P.M. mean time, in long. 84° 42′ W., required sidereal time. Ans., 7h 56m 29s.7. (64.) Sept. 30, 1853, at 6h 42m 108 A.M. mean time, in long. 100° 42′ W., required sidereal time. Ans., 7h 18m 588.59. (65.) Dec. 8, 1853, at 10h 10m 428 P.M., mean time, in long. 18° 32′ E., required sidereal time. Given, apparent solar time, to find sidereal time. 1. Get a Greenwich date (p. 70.) 2. Correct the equation of time and also the right ascension of the mean sun for Greenwich date (pp. 77, 83). 3. Apply corrected equation of time to ship apparent time, and thus get ship mean time. Then, as in the last rule, 4. Add together ship mean time and right ascension of mean sun. 5. The sum (rejecting 24 hours if greater than 24 hours) will be sidereal time. EXAMPLES. May 24, 1853, at 6h 8m 40s A.M. apparent solar time, in long. 20° 20′ W., required sidereal time. |