But One Gets Latitude At Twelve O'clock,
Noon, By A Meridian Observation.
It is clear that in order to work
up my eight o'clock chronometer sight I must have my eight o'clock
Of course, if the Snark were sailing due west at six
knots per hour, for the intervening four hours her latitude would
not change. But if she were sailing due south, her latitude would
change to the tune of twenty-four miles. In which case a simple
addition or subtraction would convert the twelve o'clock latitude
into eight o'clock latitude. But suppose the Snark were sailing
southwest. Then the traverse tables must be consulted.
This is the illustration. At eight A.M. I took my chronometer
sight. At the same moment the distance recorded on the log was
noted. At twelve M., when the sight for latitude was taken. I
again noted the log, which showed me that since eight o'clock the
Snark had run 24 miles. Her true course had been west 0.75 south.
I entered Table I, in the distance column, on the page for 0.75
point courses, and stopped at 24, the number of miles run.
Opposite, in the next two columns, I found that the Snark had made
3.5 miles of southing or latitude, and that she had made 23.7 miles
of westing. To find my eight o'clock' latitude was easy. I had but
to subtract 3.5 miles from my noon latitude. All the elements being
present, I worked up my longitude.
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