Eventually, during the Sung dynasty (1000 CE) many trading ships were then able to sail as far as Saudi Arabia using compasses for marine navigation. Between 1405 and 1433, Emperor Chu Ti's Treasure Fleet of the Dragon Throne ruled the entire South Pacific and the Indian Ocean, a territory that ranges from Korea and Japan to the Eastern coast of Africa.
At this time Western mariners were still rather ignorant of the navigational use of the magnet. Petrus Perigrinus van Maricourt
wrote a first treatise on the magnet itself: “De Magnete” (1269). And though its nautical use was already mentioned in 1187 by the English monk Alexander Neckham, the use onboard only came about around the 13th and 14th century in the Mediterranean Sea.
Much later, in 1545, Pedro de Medina (Sevilla 1493-1567) wrote the Spanish standard work “Arte de Navegar” on marine compass navigation. This masterpiece was first translated in Dutch (1580) and was -O Irony- used by Jacob van Heemskerk when the Dutch destroyed the Spanish fleet near Gibraltar in 1607. The drawback was of course Van Heemskerk's own death during this victory.
Magnetic observations made by explorers in subsequent decades showed however that these suggestions were true. But it took until the early nineteenth century, to pinpoint the magnetic north pole somewhere in Arctic Canada (78° N , 104° W). From then on the angle between the true North and the Magnetic North could be precisely corrected for. This correction angle is called magnetic variation or declination.
It is believed that the Earth's magnetic field is produced by electrical currents that originate in the hot, liquid, outer core of the rotating Earth. The flow of electric currents in this core is continually changing, so the magnetic field produced by those currents also changes. This means that at the surface of the Earth, both the strength and direction of the magnetic field will vary over the years. This gradual change is called the secular variation of the magnetic field. Therefore, variation changes not only with the location of a vessel on the earth but also varies in time.
The correction for magnetic variation for your location is shown on the nearest! nautical chart's compass rose. In this example we find a variation of 4° 15' W in 2009, with an indicated annual correction of 0° 08' E. Hence, in 2011 this variation is estimated to be 3° 59', almost 4° West. This means that if we sail 90° on the chart (the true course), the compass would read 94°.
Another example: let's say the compass rose gives a variation of 2° 50' E in 2007, with a correction of 0° 04' E per year. In 2009 this variation is estimated to be 2° 58', almost 3° East.
Now, if we sail 90° on the chart, the compass would read 87°.
These overlayed compass roses show the difference between true north and magnetic north when the magnetic variation is 10° West.
From the image we find: tc = cc + var
in which “cc” and “tc” stand for “compass course” and “true course”, respectively.
To convert a true course into a compass course we need first assign a “-” to a Western and a “+” to a Eastern variation. Note that this makes sense! because of the clockwise direction of the compass rose. Here, the inner circle is turned 10° anticlockwise, hence -10°.
Now, use the same but re-written equation:
cc = tc - var
235° = 225° - (-10°)
So, to sail a true course of 225°, the helmsman has to steer a compass course of 235°.
To convert a compass course into a true course we can use the original equation. If we have steered a compass course of 200°, we have to plot a true course of 203° in the chart if the variation is 3° East or a true course of 190° if the variation is 10° West.
Furthermore, the deviation changes with the ship's heading, resulting in a deviation table as shown below. The vertical axis states the correction in degrees West or East, where East is again positive.
When a compass is newly installed it often shows larger deviations than this and needs compensation by carefully placing small magnets around the compass. It is the remaining error that is shown in your deviation table.
You can check your table every now and then by placing your boat in the line of a pair of leading lights and turning her 360 degrees.
Converting a compass course into a true course, we can still use our equation but we need to add the correction for deviation:
cc + var + dev = tc
Magnetic courses are used for three reasons:
To summarise, we have three types of “north” (true, magnetic and compass north) like we have three types of courses: tc, mc and cc. All these are related by deviation and variation.
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