A Guide to the Local Marine Board Examinations of Masters and Mates

LATITUDE BY MERIDIAN ALTITUDE OF

THE SUN.

FIND THE GREENWICH DATE.--Write down the day at the head of the question, and 0h 0m. 08.. Turn the Longitude into time by Table 19, or by multiplying it by 4 and dividing by 60; and

West, add;
East, subtract.

When the long. is East, to be subtracted, borrow 24 hours, and make the date one day less.

TO CORRECT THE DECLINATION.-Write down the sun's declination from page 1 of the month for the Greenwich day and put the declination for the following day under it.

Declinations same name, subtract the less from the greater. Declinations contrary names, add them together.

Name the result the Daily Variation.

Under the Daily Variation place the hours and minutes of the Greenwich date. Take the logs. for the Daily Variation and Greenwich date from Table 33, and add them together. The sum of these logs. found in the same Table 33, gives the correction to be applied to the Declination for the Greenwich day.

Declination increasing, add the correction;

Declination decreasing, subtract.

ANOTHER METHOD FOR CORRECTING THE DECLINATION.— Take out the Declination from page 1 for the Greenwich date, and the Difference for 1 hour. Multiply the "Diff. for 1 hour,' or Hourly Diff., by the Hours and tenths of the Greenwich date; the result is the correction ;

Declination increasing, add;

Declination decreasing, subtract.

To find the tenths, divide the minutes of the Greenwich date by 6, or take sixes out of the minutes. After multiplying the Hourly diff. by the Greenwich time, mark off from the right as many decimal figures as there are decimals in the Hourly diff. and multiplier.

Note.-When the Declinations are contrary names, call the Decl. decreasing; and if the Correction is greater than the Decl., subtract the Decl. from the Correction, and name the result contrary to the Decl.

FIND THE TRUE ALTITUDE, by applying to the observed;-
INDEX ERROR, add if + (plus), subtract if—(minus);
DIP for height of eye, Table 5, always subtract;
SUN'S CORRECTION (Refraction and Parallax), Table 18,
taken out for the altitude, always subtract;

SUN'S SEMIDIAMETER, page 2 of month, Nautical Almanac,
for given day; Lower limb, add; Upper limb, subtract;
The result is the true altitude of Sun's centre.

TO FIND THE ZENITH DISTANCE.-Subtract the True Altitude from 90° 0' 0".

TO NAME THE ZENITH DISTANCE.-When the Sun is bearing North, name the zenith distance South; or bearing South, name zenith distance North.

TO FIND THE LATITUDE.-Under the zenith Distance place the Corrected Declination; and if of the same name, add; contrary names subtract.

Declination, and Contrary names, subtract.
Zenith Distance, S

Name the result, Latitude, the same as the greater.

MERCATOR'S SAILING.

TO FIND THE DIFFERENCE OF LATITUDE.—

Latitudes contrary names add them together.

Latitudes same name, subtract the less from the greater.

TO FIND THE MERIDIONAL DIFFERENCE OF LATITUDE.Take out the meridional parts from Table 3 for each of the latitudes. Latitudes contrary names add the meridional parts together; but, if the latitudes have the same name, subtract the less from the greater.

TO FIND THE DIFFERENCE OF LONGITUDE.—

Longitudes contrary names add them together.

Longitudes same name, subtract the less from the greater. If the longitudes have contrary names, and their sum exceeds 180° 0', subtract it from 360° 0', and the result is the diff. long. The diff. lat. and diff. long. are reduced to miles, by multiplying the degrees by 60, and adding in the minutes.

TO FIND THE COURSE.-From the log. of the diff. long., Table 24, 10 added to the index, subtract the log. of the meridional diff. lat.; the remainder is the log. tangent of Course, Table 25.

TO FIND THE DISTANCE.-To the log. Secant Course, Table 25, add log. diff. lat. Table 24; subtract 10 from the index, and the number corresponding to the sum of the two logs. is the Distance.

To find Course.
Log. Diff. long.
Log. Mer. diff. lat.
Course... tangent.

subtract.

To find Distance.
Secant Course
Log. Diff. lat.

Distance...log.

add.

TO NAME THE COURSE.-Name the course according to the ship making Northing or Southing; and Easting or West

ing:

From North lat. to greater North, Course North.

From North lat. to less North, Course South.

From North lat. to South, Course South.

From South lat. to greater South, Course South.
From South lat. to less South, Course North.
From South lat. to North, Course North.

From East long. to greater East, Course East.
From East long. to less East, Course West.
From East long. to West, Course West.

From West long. to greater West, Course West.
From West long. to less West, Course East.
From West long. to East, Course East.

Note. This rule for naming the Course East or West does not apply when the sum of the Longitudes is greater than 180° 0'. In such a case, Name the Course the same as the Long. left.

Required the Course and Distance from A to B, by calculation on Mercator's principle.