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If we follow a constant rhumb line track, it would eventually lead to the pole (depending on your hemisphere) , so then it may not be possible to fly between any two points on a rhumb line track?

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Looking at the picture it gives the impression that I would reach the north pole before I would be able to reach my destination

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    $\begingroup$ 1. why not? 2. this is more of a geometry question, than aviation. $\endgroup$
    – Federico ♦
    Dec 9, 2021 at 20:33
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    $\begingroup$ I do not think a Rhumb line that follows a Latitude will ever lead to a Pole. The most basic example is circling the equator by going Due East/West. $\endgroup$
    – abelenky
    Dec 9, 2021 at 21:42
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    $\begingroup$ Sure you could. You could even fly a Fibonacci spiral to your destination if you like, but why? $\endgroup$
    – Michael Hall
    Dec 10, 2021 at 2:04
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    $\begingroup$ What the picture demonstrates is that if you choose a direction to fly without regard to where you want to go, you will likely not reach your desired destination. $\endgroup$
    – David K
    Dec 10, 2021 at 14:31
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    $\begingroup$ If you flew the rhumb line you depicted, you would fly through each and every point along the track. So why do you say it isn't possible to fly through two points along it? And if you are talking about two different points not on this particularly rhumb line, then simply draw a new rhumb line that connects them. You haven't done a very good job at explaining the basis of your confusion, and I don't know why this question has 6 upvotes... $\endgroup$
    – Michael Hall
    Dec 10, 2021 at 15:51

3 Answers 3

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A rhumb line is a line which crosses all meridians at the same angle. Such a route is of constant true heading. This property is the reason rhumb line navigation was invented, and Mercator maps created.

On a Mercator map you just draw the straight line between the two points. This means you can always draw a route between two arbitrary places. E.g. let's connect:

  • Rio de Janeiro
  • Nome, a small city in Alaska
  • An abitrary point at coordinates lat = 85°, lon = 0°.
  • Hawaii
  • Stonehenge

Dotted lines show how the route completes pole-to-pole (full rhumb lines)

World map with the named points joined with straight lines

On most projections, rhumb lines appear as curves spiraling between poles, e.g. on this orthographic projection:

Same map as above with a different projection, showing the previously straight lines as curves

Why rhumb line navigation in the first place?

16th century navigators knew perfectly the shortest route was a great circle. But following a great circle requires constant and precise heading adjustments.

To simplify navigation, they split a great circle route into segments, and navigated each segment as a rhumb line, that is a route within the same rhumb, a rhumb being 1/32th of compass rose (a 32-point rose was often part of the map). They adjusted the heading only at the beginning of the next segment, possibly after they confirmed their position using a sextant.

They used maps just invented by Gerardus Mercator for the purpose of facilitating drawing rhumb lines and determining headings.

Aircraft with computerized navigation systems and autopilots are able to adjust the heading as required. They restored the interest in pure great circle navigation.

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    $\begingroup$ @SachinChaudhary The latter. If you start close to the equator and head nearly due east (say, 89°), you will circle the Earth multiple times before getting close to the pole. (Drawn on a map in the Mercator projection, your route will comprise multiple parallel lines stretching all the way across the map). $\endgroup$
    – TooTea
    Dec 10, 2021 at 7:26
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    $\begingroup$ As long as your points of departure and destination are on the same hemisphere, you can connect any such points with a rhumb line. It's just a matter of selecting a suitable angle with regard to the longitudes. $\endgroup$
    – Jpe61
    Dec 10, 2021 at 13:31
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    $\begingroup$ @Jpe61 -- all the posted answers seem to show that the departure and destination do not need to be in the same hemisphere $\endgroup$
    – quiet flyer
    Dec 10, 2021 at 15:01
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    $\begingroup$ @mins Except for rhumb lines going straight east ($\beta=90^\circ$) or west ($\beta=-90^\circ$). They'll never reach a pole since they maintain latitude. The term loxodrome explicitly excludes these special cases. $\endgroup$
    – Bianfable
    Dec 10, 2021 at 18:58
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    $\begingroup$ @SachinChaudhary: “Looking at the picture it gives the impression that I would reach the north pole before I would be able to reach my destination”. Why do you say so? Which is your destination? The curve in your image touches a lot of points (infinitely many!). If your destination isn't among them, you have chosen a wrong angle beta; just chose the correct one, according to what this answer explains. $\endgroup$
    – DaG
    Dec 10, 2021 at 23:18
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Yes, it's possible to fly from any point on the Earth from any other point on the Earth via a Rhumb line. The destination pole depends on the course you select, not the hemisphere you happen to start in. Just because a Rhumb Line leads to a pole doesn't mean it leads to the nearest pole. A track of, say, 135 is always going to terminate at the South pole, even if you start in the Northern hemisphere.

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If you fly an orthodromic course, the rhumb changes continuously during the flight unless the points of departure and arrival are in the equator or in any great circle that passes through the poles...

It is, however, always possible to fly a constant-rhumb course –a loxodrome– between any two points on the planet.

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