Great Circle Navigation

When sailing ships sail around the world, the aim of ocean navigation is to follow the shortest route whenever possible. Exceptions to the rule happen it is necessary of avoid iceberg regions, fog, and areas of bad weather and places like the doldrums. All the main shipping lines of tall ships follow great circle rountes for the shortes distance between tow points is on the Great circle route between them.

A great circle is any circle which passes completely round the globe and whose plane passes through the center of the earth. Thus, the equator is a great circle but other parallels of latitude are not.

But there are many deviations from this prescription. When old sailing ships were sailing from Europe to the Cape of Good Hope they would not sail straight down in a line towards the tip of Africa. Instead they would sail for South America to avoid the doldrums on the eastern side of the Atlantic Ocean alongside Africa. Ships would become becalmed for ages if they got into those areas. If they sailed for South America and Rio de Janeiro they could take advantage of the prevailing trade winds and make better time than if they sailed due south all the way down the Atlantic.

Another deviation was the Dutch sailing patterns from the Cape of Good Hope to The East Indies. The Dutch VOC sailing ships would sail due east from the Cape of Good Hope along the roaring forties for a predetermined distance, (say one thousand sea miles) and then head up the indian ocean straight towards Java and thereby taking advantage of the prevailing trade winds. This route was much quicker than going along the coast of Madagascar. The roaring forties would push the ships very quickly east and after they turned up the south-west trades would blow them straight to Java.

These sailing orders were the reason that so many Dutch sailing ships hit the west coast of Australia. They had gone too far east before turning and had bumped into Australia. A classic example is the tragic case of the Batavia which went too far and ended up smacking into the Abrolhos Islands with disastrous consequences for all concerned.

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