WHAT'S THE DIFFERENCE BETWEEN LATITUDE AND LONGITUDE?
R/V Melville,
15° 48.1' N, 14° 45.5' E - Course: 213, Speed: 9.9 knots
As we steam this pattern of survey lines
(looks much like mowing your yard) mapping the ocean floor, it is clear such
navigational precision was not possible even twenty years ago. The early European sailors who visited the Marianas
were certainly navigationally challenged.
In order to navigate across an ocean a ship's navigator must know latitude
and longitude.
I hope everyone
knows the difference between latitude and longitude. An exercise you might want to consider is to take a grapefruit (or
orange) and mark lines of latitude and longitude on it with a marker.Make
the lines equiangular. An easy way to do this is to cut the fruit in
half and use a protractor. Do not forget
to put in the international date line. Assuming
you can do all this, put in a dot for Greenwich, England. Why is Greenwich important? You might as well locate yourself as well.
Throw the R/V Melville in for practice (present position at top of
page).
This exercise should be an easy
one. However, without the latitude/longitude
framework, you would be out of luck. You can see why this system is so important. It has not always been easy. Latitude was historically not the difficult
measurement. Longitude was. The determination of longitude is a matter that
requires good timing. Do you know
why?
Early attempts to determine longitude
were based on the assumption that it could be determined astronomically.
Unfortunately, that process could take several hours of calculation. British clockmaker John Harrison was convinced the better method
lay in having an accurate clock aboard the ship. He was right. In 1764, Harrison's
clock was put to the test on a voyage to Barbados. Accurate determination of longitude was now possible. How?
The secret is in knowing the difference
in time between a specific fixed location and your location.The specific fixed
location to know is Greenwich,
England.
Greenwich is the prime meridian. The
prime meridian divides the earth into eastern and western halves just as the
equator divides the earth into northern and southern halves. Why Greenwich? Because England was the world sea power at the time, and they got
to decide. Greenwich, England is located
at 51° 29' N (latitude), 0°00'W (longitude). If you sail west of this location, your longitude is a higher number
W. If you sail east, it is a higher
number .
Now for the
technique. If
you know the difference in time between your present location and the time
in Greenwich (called GMT or Zulu by the military), you can work out longitude. The earth rotates at 1° of longitude in four
minutes. That is 15° per hour. One complete revolution requires approximately
24 hours, one day. The last information
you need is a table that tells you what time it is in Greenwich at sunrise
for each day of the year. (The tables
also give other times such as sunset, twilight, moonrise, etc.).
All you have to do now is observe and record the time of sunrise in
your location. A bit of simple math and you have your answer.
Therefore, if the sun rises two hours later at your location that in Greenwich you at ____° (E or W) longitude. Simple.