How does a ship find its way across thousands of miles of ocean and yet arrive at the harbour mouth as unerringly as a car steered home to its garage?
A nautical chart looks much like a road map, except that the roads are sea lanes’, with lightships, buoys, shoals and sandbanks instead of towns, railways, church towers and hilltops. And, of course, the lines of latitude and longitude are
Before leaving port, the ship’s navigator marks out his route on the chart. This chart is generally on what is known as Mercator’s Projection’, which means all the lines of longitude, or meridians, run parallel, wheareas in reality, of course, they meet at the Poles. The compass points to Magnetic North, which is a variable number of miles away irom 7rue North, so that in working out the course the navigator must allow for this variation. This may be ten or more degrees-and it is found on the ‘compass rose, a compass diagram printed on the chart at frequent intervals, or published in nautical tables.
For a course due East-90°-with a variation of 10° East the Magnetic course would be 80°. But before setting this for the helmsman, the navigator checks his deviation card-a
note of any errors in the compass when it was last tested. Suppose the deviation is 2° West, then that would have to be added to the figure of 80° to give the final compass course that the helmsman will follow. But the helm alone won’t keep a ship on its course, so at sea the navigator uses the chronometer and sextant to check at regular intervals. The time of the ship’s clocks is changed as it travels East or West of the Greenwich Meridian (0° longitude), and the difference between the ship’s clocks (showing local time) and the chronometer (showing Greenwich time) tells the navigator his longitude. This is a simple calculation: there are 360° of longitude, and the earth turns once on its axis in every 24 hours; 360 divided by 24 gives us 15-which means one hour of ‘sun’s progress equals 15. When it is noon at Greenwich, it is 1 p.m. if you are 15° East, and 11 a.m, if you are 15° West.
Latitude is checked by a sextant. This is a device for measuring angles, and as we know from generations of navigation just where the sun ought to be at a given latitude on any particular occasion, and where each of the principal stars should appear
at night, a check of the angle between the sun, moon or stars and the horizon provides a figure which need only be looked
up in a standard book of tables carried by every ship.
This, of course, is the method out at sea, where no landmarks are available to help in navigation. Along the coast the task of the officer on the bridge is much easier, for he can take bearings of lighthouses, church spires, prominent hilltops and other easily recognisable points marked on his chart.
Where the bearings intersect is the ship’s position.
Radio is another important aid to the navigator. He can take bearings from radio beacons-fixed points which send out radio signals in the same way that a lighthouse transmits intermittent beams of light. And, most important of all, he can use radar, a British invention which makes it possible for the navigator to know where ships, wrecks, buoys and other objects are in relation to his own vessel. The radar set sends out ultra-short-wave impulses in all directions; any of these striking an object sends back an echo to the set, and this appears as a bright dot on the monitor screen.