Jan. 9 19 13 42-27. Noon 14 55 8.54 47 2 Star's R. A.. 7h 36m 12s 28° 22′ 46′′ N. April 18 1 44 58 62. Noon 16 8.8.59 15 4 Star's R. A.. 13h 17m 19* 10° 22′ 43 S. April 17 1 41 2.07. Noon 16 13h 17m 19s 82.59 13 1 Star's R. A.. Mid. 16 8 7.59 14.9 Star's decl. Distance at 6 hours, 106° 16' 11"; at 9 hours, 104° 30′ 26'. Nov. 16. 15 39 44 50. Noon 16 2.1.58 50 ⚫9 Mid. 16 75.59 10.6 10° 22′ 43′′ S. Star's R. A.. 1h 58m 38s Ship mean time obtained from moon's altitude. Objects observed, moon and star. This differs very little from Rule LI., p. 217. 1. Get a Greenwich date. 2. Take out of the Nautical Almanac and correct for Greenwich date the following quantities:-Right ascension of mean sun; moon's right ascension and declination; semidiameter, and horizontal parallax. 3. Correct the star's altitude for index correction, dip, and thus get the apparent altitude: from the star's apparent altitude subtract refraction; the result is the true altitude, which take from 90° for the star's true zenith distance. Then proceed as in 4, 5, &c., p. 217. Rule LIV. Ship mean time from planet's altitude. Objects observed, moon and planet. 1. Get a Greenwich date. 2. Take out of the Nautical Almanac and correct for Greenwich date the following quantities:-Right ascension of mean sun; planet's right ascension and declination; planet's horizontal parallax (if great accuracy is required); moon's semidiameter and horizontal parallax. 3. Correct the planet's observed altitude for index correction and dip, and thus get the apparent altitude; from the apparent altitude subtract the refraction and add the parallax in altitude (usually neglected, being very small); the result is the planet's true altitude, which subtract from 90° to get the true zenith distance. 4. Correct the moon's altitude as in 4, p. 221. 5. Get the auxiliary angle A as in 5, p. 221. 6. To find ship mean time as in 6, p. 221, using planet's declination and right ascension instead of star's. 7. Then proceed as in arts. 8, 9, p. 211. EXAMPLE. September 24, 1849, at 7h 50m P.M., mean time nearly, in lat. 47° 50′ N., and long. by account 2° 30′ W., the following lunar observation was taken : The height of eye above the sea was 20 feet; required the The error of the chronometer on ship mean time is found a little before or after the lunar distance is taken. For this purpose the observer selects any heavenly body whose bearing is nearly east or west, so that the error in the altitude may produce the smallest error in the resulting hour angle (see Rule XLIII.) Then the time being noted by the same chronometer when the distance is taken, ship mean time is known at the same instant, by applying the error found by the above observation. Rule LV. Objects observed, moon and stars. 1. Get a Greenwich date. 2. Take from the Nautical Almanac and correct for the Greenwich date, the following quantities: sun's declination, equation of time and semidiameter, right ascension of mean sun. Moon's right ascension and declination, moon's semidiameter and horizontal parallax. 3. To find the sun's hour angle. To the time shown by chronometer at the observation apply the error of chronometer with its proper sign, and thus get ship mean time; to this apply equation of time, the result is ship apparent time, and also the sun's hour angle. 4. To calculate the sun's altitude. Under the latitude* put down the sun's declination, take the sum if the names be unlike, but the difference if the names be alike; call the result v; add together constant log. 6301030, log. cos. latitude, log. cos. sun's declination, and log. haversines sun's hour angle, reject 30 in the index, and look out the result as a logarithm, and take its natural number to the nearest unit. Add together this natural number and the versine of the quantity v found above: the sum is the versine of the sun's true zenith distance, which find in the tables and subtract from 90°: the result is the sun's true altitude. To find the sun's apparent altitude. To the true altitude just found, add correction in altitude (for parallax and refraction) the result will be the sun's apparent altitude very nearly.† * When great accuracy is required, the latitude and horizontal parallax should be corrected for the spheroidal figure of the earth. In strictness the table for correction of altitude ought to have been entered with the apparent altitude, instead of the true, to get the correction in altitude; but the above is sufficiently correct. |