<p>Yes, very much so.</p>
<p>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.</p>
<p><a href="https://i.sstatic.net/s1o7J.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/s1o7J.png" alt="this image of the globe" /></a></p>
<p>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)</p>
<p>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).</p>
<p>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.</p>

CC BY-SA 4.0
2021-02-05T13:42:49.150Z
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.
Alchimista
CC BY-SA 4.0
2021-02-05T14:36:00.170Z
I disagree. Discussing equatorial vs. polar circumference is insignificant against the flaw in the principle of taking an *arbitrary* latitude circle.
planetmaker
CC BY-SA 4.0
2021-02-06T09:30:28.770Z
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.
Alchimista
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2021-02-08T04:52:12.270Z
Thx @djohnm very much indeed
planetmaker
CC BY-SA 4.0
2021-02-08T14:53:36.510Z
@Alchimista Yes, the flattening of the Earth is important, but using a sphere as the [figure of the Earth](https://en.wikipedia.org/wiki/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%.
PM 2Ring
CC BY-SA 4.0
2021-02-08T14:55:58.737Z
@Alchimista (cont) The [WGS-84 reference ellipsoid](https://en.wikipedia.org/wiki/World_Geodetic_System#) used by the current GPS system uses the flattening parameter $1/f = 298.257223563$ exactly. [Ellipsoid geodesy is fun](https://math.stackexchange.com/a/1340899/207316), but I'd hate to do it without a computer. ;)
PM 2Ring
CC BY-SA 4.0
2021-02-08T16:26:47.550Z
@PM 2Ring nice to have the data. But I wasn't against the use of a sphere nor about the simple answer. It is true! Just pointing out that one would have to define along what the distance has to be measured even when taking great circles. It is all, as for the fact that using any circle at different lat should be so obvious and dismissed even by a short comment.
Alchimista
CC BY-SA 4.0
2021-02-05T12:01:24.087Z
2021-02-08T04:51:15.470Z
2021-02-08T04:51:15.470Z
planetmaker
<p>Astronomy is new to me so my question might be stupid: I read that a <a href="https://en.wikipedia.org/wiki/Nautical_mile" rel="nofollow noreferrer">nautical mile</a> 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?</p>

CC BY-SA 4.0
2021-02-05T12:24:03.233Z
@Alchimista no, that's definitely not the main reason.
planetmaker
CC BY-SA 4.0
2021-02-05T13:39:15.030Z
@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..
Alchimista
CC BY-SA 4.0
2021-02-06T09:35:05.513Z
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).
Alchimista
<terminology><definition>
Defining a nautical mile
Ambica Govind
# Defining a nautical mile

(Asked by: username_0 on 2021-02-05)

Astronomy is new to me so my question might be stupid: I read that a [nautical mile](https://en.wikipedia.org/wiki/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?

**Tags:** terminology, definition
> **Comments:**
> **username_1**: @Alchimista no, that's definitely not the main reason.  
> **username_2**: @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..  
> **username_2**: 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).  

---

## 1 answers

### Answer by username_1 on 2021-02-05. Score: 1 | Accepted answer

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](https://i.sstatic.net/s1o7J.png)](https://i.sstatic.net/s1o7J.png)

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.

> **Comments:**
> **username_2**: 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.  
> **username_1**: I disagree. Discussing equatorial vs. polar circumference is insignificant against the flaw in the principle of taking an *arbitrary* latitude circle.  
> **username_2**: 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.  
> **username_1**: Thx @djohnm very much indeed  
> **username_3**: @Alchimista Yes, the flattening of the Earth is important, but using a sphere as the [figure of the Earth](https://en.wikipedia.org/wiki/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%.  
> **username_3**: @Alchimista (cont) The [WGS-84 reference ellipsoid](https://en.wikipedia.org/wiki/World_Geodetic_System#) used by the current GPS system uses the flattening parameter $1/f = 298.257223563$ exactly. [Ellipsoid geodesy is fun](https://math.stackexchange.com/a/1340899/207316), but I'd hate to do it without a computer. ;)  
> **username_2**: @PM 2Ring nice to have the data. But I wasn't against the use of a sphere nor about the simple answer. It is true! Just pointing out that one would have to define along what the distance has to be measured even when taking great circles. It is all, as for the fact that using any circle at different lat should be so obvious and dismissed even by a short comment.  

---

