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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The way I read it, it has to do with “knowing where you’re going”. Yes, a great-circle would be shortest en you could go ‘fixed steer’, but what if you can’t be sure your steering is properly fixated? Fixating the rudder on a ship will not prevent it from drifting and turning (as opposed to rubber tires on concrete). Thus, you would prefer a method where you can constantly check and correct your course in a simple way. Doing this by keeping the same direction on a compass, gives you the ‘rhumb line’ as your traveled path.
disclaimer: I have no knowledge of the subject, just interpreting the text you provided.
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.
Trying to keep it simple, so experts plz don’t nitpick
On a great circle route, your heading (I’ll neglect heading vs course and magnetic variation and stuff to keep this simple) is constantly changing over the route. Computers make this easy, as they’re constantly calculating. It’s harder to calculate (and execute) by hand.
A rhumb line is much simpler. Sail this heading, and keep sailing it. You can break it up into a series of rhumb lines if you want. Down near the equator, the difference between great circle and rhumb line distances is minimal…often on the order of a few percent or less…and that’s where they did a whole lot of sailing back in the early days. The differences really only start to become significant (and can become quite significant) when higher latitudes are involved.
All these explanations about constant compass heading are correct, but miss an important point.
To sail a great circle, you have To adjust your heading as you move. Your required heading is a function of where you are.
Before the era of accurate clocks (roughly 1800) ships at sea rarely had an accurate idea of their longitude. This makes it difficult to know the correct heading adjustment.
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