Newsletter 7th Issue LASER.qxp:Layout 1 - Marlow Navigation

7th Issue
Dec 2010
Marine Story
The Problem of Longitude
Life before SAT NAV
So what was the problem and how was it
solved
Go onto any ship’s bridge these days and
you’ll have the ship’s position in latitude
and longitude to an accuracy of a few
metres. Thanks to satellite navigation and
global positioning systems, the ability to
determine position on the surface of the
earth is a simple readout from a satellite
receiver.
Latitude was easily determined.
Measuring the maximum elevation of the
sun with a sextant as it crossed your
meridian at noon allowed the early
navigators to obtain their latitude.
Because lines of latitude are parallel, the
concept of sailing ‘east’ or ‘west’ along a
line of latitude round the earth was easy.
But it was not always like this. Back in the
1700’s, the problem of ‘finding longitude’
was stretching the minds of everyone as
more and more sailing ships set out to
explore new territories, to conquer lands
and bring commodities from foreign
countries. There was no practical way to
determine longitude and despite the best
navigational charts and compasses, Vasco
da Gama, Ferdinand Magellan and Sir
Francis Drake were all getting to their
destinations by good luck or the Grace of
God! Even Captain William Bligh on the
‘Bounty’ and the circumnavigator Captain
James Cook took various tools and
devices to sea to test them but without
result.
Determining longitude was more tricky.
In those days, the navigator needed to
know the precise local time aboard the
ship and also the time at the home port at
the very same moment. The two clocks
enabled the navigator to convert hourly
difference into geographical separation.
The earth takes approximately 24 hours
to rotate once through 360 degrees, 1
hour marks 15 degrees of spin on the
surface. Every day the navigator reset his
ship’s clock to local noon when the sun
reaches the highest point in the sky. Then
he would consult his ‘port’ clock - every
hour’s difference between the two clocks
translated into 15 degrees of longitude.
There were no astronomical tables or
almanacs in those days of course!
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