According to wiki: “The Mercator projection was designed for use in marine navigation because of its unique property of representing any course of constant bearing as a straight segment. Such a course, known as a rhumb (or, mathematically, a loxodrome) is preferred in marine navigation because ships can sail in a constant compass direction, reducing the difficult, error-prone course corrections that otherwise would be needed frequently when sailing a different course.”
But also according to wiki: “A rhumb line can be contrasted with a great circle, which is the path of shortest distance between two points on the surface of a sphere. On a great circle, the bearing to the destination point does not remain constant. If one were to drive a car along a great circle one would hold the steering wheel fixed, but to follow a rhumb line one would have to turn the wheel, turning it more sharply as the poles are approached. In other words, a great circle is locally “straight” with zero geodesic curvature, whereas a rhumb line has non-zero geodesic curvature.”
Isn’t this contradictory? Maybe I’m not getting what constant bearing means. But why would sailors prefer to continue turning in a direction, as opposed to going constantly straight for the duration of the trip? Doesn’t it make more sense to do the latter
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A great circle is the best path. And with modern navigation systems these are easy to follow. The problem is that because of the wind, waves, tides, etc. the ship is constantly turning in random directions and you need to be constantly adjusting the rudder to compensate. In order to know which way to turn the rudder you need to measure your bearing. And this is done with a compass.
So the navigator tells the helmsman the bearing he needs to keep, which then follows a rhumb line. A few hours later the navigator might have redone their calculations and issue a new course to the helmsman. So you end up with a track made up of multiple rhumb lines approximating a great circle line.
So it is basically a limitation of the navigational instruments. Modern computer navigation computers are much better at this though and can recalculate the optimal route every second. And the helmsman is usually replaced by an autopilot. So modern ships follow the great circle lines much closer then they did in the past. However there are still a need to take manual control, either by manual navigation or even manual control over the rudder at times. And in these cases it is much easier for the humans to use constant bearing rhumb lines rather then great circle lines.
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