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AUGMENTATION OF "SEMIDIAMETER ACCORDING TO HER INCREASE IN ALTIDUDE.
The moon's horizontal semidiameter is found in page 3 of each month in the "Nautical Almanac," for every day at mean noon and midnight at Greenwich; and the sun's in page 2, for every mean noon.
PARALLAX OF THE SUN ON THE FIRST DAY OF EACH MONTH, THE MEAN HORIZONTAL PARALLAX BEING 8.60".
The sun's horizontal parallax is also given for every ten days, in the "Nautical
Almanac," immediately before the ephemeris of the planets.
The sun's parallax in altitude, for every degree, is given in the last of "Dr. Pearson's Solar Tables;" vol. i. page 180.
REDUCTION OF the moon's EQUATORIAL HORIZONTAL PARALLAX, TO
THE HORIZONTAL PARALLAX IN ANY LATITUDE.
The moon's horizontal parallax, given in the third page of each month in the "Nautical Almanac" for noon and midnight, is the equatorial parallax for Greenwich mean noon and midnight; from thence it is to be deduced for the time and place of observation. The correction for latitude, on account of the spherical figure of the earth, is seldom thought of at sea, but can be made from the table above. Thus, supposing the hor. equat. par. to be 58'; the hor. par. in lat. 52° would be 58'-7.2"-57.52".8.
This reduced hor. par. is to be farther corrected for altitude by means of tables for that purpose (see Pearson, vol. 1, page 188 to 196: and Riddle, page 156* to 173); or by the following rule:-sin. hor. par. X cos alt. sine. par. in alt.
* Riddle's tables are for clearing the lunar distance, and the corrections are for both parallax and refraction,