28 seamanship secrets
of a minute or in degrees, minutes, and seconds. One degree of latitude equals
60 minutes; one minute of latitude equals 60 seconds.
1 degree (°) of latitude = 60 minutes
1 minute (') of latitude = 60 seconds (")
On charts showing degrees, minutes, and seconds, it’s oft en convenient to change
the seconds into tenths of a minute. Simply divide the number of seconds by six.
Round oddball seconds to the closest number divisible by six.
Convert 42°25'18"N into degrees, minutes, and tenths.
Q Since 18 seconds is exactly divisible by six, we simply divide it by six to convert
it to tenths of a minute: 18"/6 = 0.3'
Q Our converted latitude is 42°25.3'N
Convert 13°49'26"N into degrees, minutes, and tenths.
Q Since 26 seconds is not exactly divisible by six, round it to the closest number
divisible by six, which is 24, and then divide by six: 24/6 = 0.4'
Q Our converted latitude is 13°49.4'N
LONGITUDE
Now let’s go back to our globe, this time dividing it vertically into two equal halves.
Imagine that we do this by lowering a giant guillotine, but let’s be careful: the blade
must pass through both the North and the South poles. Th e resulting cut will also pass
through the earth’s center and—because this system was invented in England—we
must orient the blade so that it also passes through the city of Greenwich, England,
on the earth’s surface. Th at half of the circle that passes through Greenwich is called
the Greenwich meridian (or prime meridian) and labeled 0 degrees. Th e remaining
half of the circle on the opposite side of the globe from Greenwich is labeled
180 degrees. Measurement of longitude begins at the Greenwich meridian.
Now let’s use our blade again and make additional slices on each side of the
Greenwich meridian. Each of these circles—called meridians—represents one
degree of longitude. Traveling west from the Greenwich meridian, we will pass
1 degree west, then 2 degrees west, and so on to 180 degrees. If we travel east from
the Greenwich meridian, we will pass 1 degree east, then 2 degrees east, and continue
all the way around the globe to 180 degrees.
The 180th meridian coincides more or less with the international date line
(though the latter is distorted by bumps and jogs so as not to divide certain
Pacific island nations into multiple time zones).
Measure Longitude on the Chart’s Top or Bottom Margin Scale
Use either the top or bottom scale of the chart to measure longitude. Like latitude,
longitude is broken down into degrees, minutes, and tenths of minutes or into
degrees, minutes, and seconds. Use the same methods shown above to convert
seconds to tenths of a minute of longitude.