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How are the cardinal points (North-East-South-West) defined? On equatorial regions it is somewhat obvious. But what about the higher latitudes? If the pole star is up above the sky instead of horizon, how do we define North? What is the case on the geographic poles?

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Geographic Meridian

Every point on Earth is on some meridian:

A (geographical) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian.

From where you are standing, North is the direction on the horizon along this meridian or "line" of longitude that extends to the Earth's geographic North pole, and South is the direction on the horizon 180 degrees opposite, towards the South pole.

East and west are the perpendicular directions, which are also found extending lines of latitude from where you are standing towards the horizon.

Locally you can think of the region under your feet as flat, and so North, South, East and West all form right angles even far from the equator.

Close to the poles, none of that changes mathematically, and exactly on the poles those terms suddenly no longer have any meaning.

Celestial Meridian

meridian:

In astronomy, the meridian is the great circle passing through the celestial poles, as well as the zenith and nadir of an observer's location.

Consequently, it contains also the north and south points on the horizon, and it is perpendicular to the celestial equator and horizon. A celestial meridian is coplanar with the analogous terrestrial meridian projected onto the celestial sphere. Hence, the number of celestial meridians is also infinite.

Meridian_on_celestial_sphere.png

Source

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    $\begingroup$ For completeness, the North and South poles are where the Earth's axis of rotation intersects the surface. The breakdown of the cardinal directions at the poles is an example of a coordinate singularity. $\endgroup$ – mistertribs Feb 10 at 16:40
  • $\begingroup$ @mistertribs you are welcome to make an edit; I have a hunch you can say it "more correctly" [sic] than I can ;-) $\endgroup$ – uhoh Feb 10 at 16:43

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