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Astronomy is new to me so my question might be stupid: I read that a nautical mile is defined as one minute of latitude along any line of longitude. What if it was the other way round, i.e. one minute of longitude along any line of latitude? Would it make any difference?

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    $\begingroup$ @Alchimista no, that's definitely not the main reason. $\endgroup$ Commented Feb 5, 2021 at 12:24
  • $\begingroup$ @planetmaker well is very obvious that circle at different lats have different radiis too. I mean that there is in principle a difference even if equator is chosen. I spoke of two radiis, not any number.. $\endgroup$
    – Alchimista
    Commented Feb 5, 2021 at 13:39
  • $\begingroup$ I don't understand why my comment was deleted. It could have neglect the obvious asked by the question but it was pointing to the fact that Earth isn't a perfect ball (rather obvious too, but at least not as much). $\endgroup$
    – Alchimista
    Commented Feb 6, 2021 at 9:35

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Yes, very much so.

The lines or circle of constant longitude always form a great arc which intersects with the poles; thus it always has the same length. A circle of constant latitude varies in circumference: the largest is at the equator while it has intermediate length at intermediate latitudes and no length anymore when you reach the pole.

this image of the globe

See e.g. that image from wikipedia and follow a circle in longitude, thus from North to South. And compare the different sizes of different circles for different lattitudes (thus the circles which are parallel to the equator)

Mind though that the nautical mile is no longer defined via the fraction of the polar circumference of Earth. It is defined as exactly 1852m in length, and thus can in principle be determined as accurately as one can measure time (1852 / 299 792 458 seconds).

In earlier times it was a MUCH easier task to determine a difference in latitude as it directly relates to the (culmination) height of celestial objects above the horizon. It was VERY DIFFICULT to impossible to determine difference in longitude unless you had a very accurate clock and an almanach of the rise and set times of celestial objects.

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  • $\begingroup$ I think that this answer would benefit of mentioning the real shape of Earth. That would makes a difference even measuring along great arcs. Not that the answer is incorrect because of this, of course. $\endgroup$
    – Alchimista
    Commented Feb 5, 2021 at 13:42
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    $\begingroup$ I disagree. Discussing equatorial vs. polar circumference is insignificant against the flaw in the principle of taking an arbitrary latitude circle. $\endgroup$ Commented Feb 5, 2021 at 14:36
  • $\begingroup$ Yes of course as the radius at the pole is zero any other consideration is negligible. I should have downvoted the question instead of commenting your answer. $\endgroup$
    – Alchimista
    Commented Feb 6, 2021 at 9:30
  • $\begingroup$ Thx @djohnm very much indeed $\endgroup$ Commented Feb 8, 2021 at 4:52
  • $\begingroup$ @Alchimista Yes, the flattening of the Earth is important, but using a sphere as the figure of the Earth is a good first approximation, and quite adequate to address the OP's question regarding the size of a minute of longitude vs a minute of latitude. The polar semiaxis is only ~21 km shorter than the equatorial semiaxis, a difference of ~0.3%. $\endgroup$
    – PM 2Ring
    Commented Feb 8, 2021 at 14:53

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