« ForrigeFortsett »
This Latitude differing only 2 Miles from that used in the Com-
putation, it may be depended upon as the true Latitude.
Refračtion to be subtracted 2 3 Correótion for Refračtion 28 18 16 38 Dip of the Horizon subtracted 4. 4. App. Alt. o 28 14 16 34 Sun's Semi-diameter added o 16 o 16 Correót Altitudes of Sun’s Centre 28 30 16 50
In the Table of Difference of Latitude and Departure, to the Course 3 ; Points, and Distance 22 Miles, the Difference of Latitude is 17 Miles, while the Ship sails from the Sun. - D d 2 Where
2 58 20 16 50 2.8959 -- Log. Ratio o, 18627 Ela. T. 2 26 40 Diff. N. S. 18322. Its Log. 4,26297
And as it differs but three Miles from the Latitude by Account, it may be taken as the true Latitude.
This Method of finding the Latitude is of excellent Use, since there are so many Circumstances at Sea, which deny the Opportunity of having the Sun's Meridian Altitude ; and as the knowing the true Latitude is of the greatest Consequence, especially in coming into the English Channel, &c. where there are frequent öß of Clouds, every Seaman ought to be ready at determining his Latitude, by this Method, whenever an Opportunity offers, lest he should not see the Sun upon the Meridian.
Note. The nearer to Noon the Observations are taken, the better; rovided the elapsed Time be not much less than Half the Interval of ro when they are both taken on the same Side of Noon, nor much greater than Once and Half the greater Interval, when taken on different Sides of Noon.
To find the Latitude by the Meridian Altitude of the Moon.
N Page 6th of the Month in the Nautical Almanack, find the I Time of the Moon's passing over the Meridian of Greenwich. Turn the Longitude into Time, by Table XVIII. and add it to the above Time, if it be West, but subtračt it, if it be East: The Sum, or Difference, will be nearly the Time of her Passage over the Meridian of the Place of Observation; which call Reduced Time. In Page 7th of the Month in the Almanack, find the Moon's Semidiameter and Horizontal Parallax, at the Reduced Time. Take the Difference between the Moon's Semidiameter and Dip, and add it to the observed Altitude, if the lower Limb was observed, but but subtract it, if the upper Limb was observed: The Sum or Difference will be the apparent Altitude of her Center. From the Proportional Logarithm of the Moon's Horizontal Parallax, found in Table XIX. increasing its Index by 10, subtract the Log. Co-sine of the Moon's apparent Alt. the Remainder will be the Prop. Log. of the Moon's Parallax in Altitude, from which take her Refračtion, the Difference will be a Correótion, which being added to the apparent Altitude, will give the true Altitude of her Center: Hence the Zenith Distance, to which apply her Declination, and you will have the Latitude. Note. The Moon's Declination is set down in Page the 6th of the Month for every Noon and Midnight in the Nautical Almanack. Therefore find the Declination for the nearest Noon and Midnight both before and after the reduced Time, and take the Difference. Then as 12 Hours : is to the Difference in 12 Hours :: so is the reduced Time : to a proportional Part; which being added to, or subtracted from the Declination the Noon or Midnight before the reduced Time, according as it is increasing or decreasing, will give the Declination at the Time and Place of Observation.
To find the Latitude by the Meridian Altitude of a Planet.
In Page 4th of the Month in the Nautical Almanack are given the Declinations and Times of the Planets Passage over the Meridian of Greenwich every 6 Days.
Reduce the Longitude into Time, and add it to, or subtrađt it from, the Times of their Passages over the Meridian of Greenwich, according as their Longitude is East or West: The Sum or Difference will be the Time they pass the Meridian of the Place of Obfervation: Correct the observed Altitude for the Dip and Refračtion,
with this correct Altitude and Declination find the Latitude; as for