<p>An isogonic line is a point of constant magnetic declination, <em>i.e.</em> a line where the angle between geographic north and magnetic north is some particular value.  (Or, equivalently, geographic south &amp; magnetic south.)  You're asking about the points in the diagram at which several isogonic lines meet.  If we imagine taking a compass and walking around such a point, the <em>difference</em> between geographic south and magnetic south would change rapidly as we cross many different isogonic lines, each with its own declination.</p>
<p>One of the points on your map (the lower right one) is the Magnetic South Pole.  If we imagine walking in a circle this point, our compass would always point towards the center of the circle, and so magnetic south would be changing rapidly.  However, the direction of geographic south would stay pretty much the same so long as we didn't walk in too big of a circle.  Thus, at this point the difference between geographic south and magnetic south changes rapidly because the direction of <em>magnetic south</em> changes rapidly.</p>
<p>The other point on your map is the Geographic South Pole.  If we imagine walking in a circle about this point, our compass would always point pretty much in the same direction (towards the Magnetic South Pole.)  However, the direction of geographic south would change rapidly as we walked around the south pole. Thus, at this point the difference between geographic south and magnetic south changes rapidly because the direction of <em>geographic south</em> changes rapidly.</p>
<p>Note that <a href="https://maps.ngdc.noaa.gov/viewers/historical_declination/" rel="noreferrer">the same thing happens in the Arctic Ocean</a> as well.</p>
<p><a href="https://i.sstatic.net/ZP4rZ.jpg" rel="noreferrer"><img src="https://i.sstatic.net/ZP4rZ.jpg" alt="enter image description here" /></a></p>

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2021-08-09T15:00:59.747Z
2021-08-09T15:41:04.543Z
2021-08-09T15:41:04.543Z
Michael Seifert
<p>While reading about sundials, I've seen maps depicting isogonic lines (those that connect points with the same magnetic declination values). Pray tell, what's happening in Antarctica? Why does it appear to have two South Poles?</p>
<p><a href="https://www.youtube.com/watch?v=j7LqqvTmnVM&amp;ab_channel=SteveJBlack" rel="noreferrer">https://www.youtube.com/watch?v=j7LqqvTmnVM&amp;ab_channel=SteveJBlack</a></p>
<p><a href="https://i.sstatic.net/mmvg7.jpg" rel="noreferrer"><img src="https://i.sstatic.net/mmvg7.jpg" alt="enter image description here" /></a></p>

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2021-08-10T11:15:47.377Z
@MichaelSeifert Great answer, thanks very much! How do I accept it?
C Camerini
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2021-08-10T13:09:34.893Z
You should be able to click a grey tick mark just beneath the up/down arrows for the answer "score".  For example: https://meta.stackexchange.com/a/86979/195335
AJP
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2021-08-10T13:24:44.857Z
thank you all. I've been using StackOverflow for years as a developer. what a great community Stack Exchange is.
C Camerini
<geography>
What happens with the isogonic lines at the South Pole?
C Camerini
# What happens with the isogonic lines at the South Pole?

(Asked by: username_0 on 2021-08-09)

While reading about sundials, I've seen maps depicting isogonic lines (those that connect points with the same magnetic declination values). Pray tell, what's happening in Antarctica? Why does it appear to have two South Poles?

<https://www.youtube.com/watch?v=j7LqqvTmnVM&ab_channel=SteveJBlack>

[![enter image description here](https://i.sstatic.net/mmvg7.jpg)](https://i.sstatic.net/mmvg7.jpg)

**Tags:** geography

> **Comments:**
> **username_0**: @username_2 Great answer, thanks very much! How do I accept it?  
> **username_1**: You should be able to click a grey tick mark just beneath the up/down arrows for the answer "score".  For example: https://meta.stackexchange.com/a/86979/195335  
> **username_0**: thank you all. I've been using StackOverflow for years as a developer. what a great community Stack Exchange is.  

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## 1 answers

### Answer by username_2 on 2021-08-09. Score: 14 | Accepted answer

An isogonic line is a point of constant magnetic declination, *i.e.* a line where the angle between geographic north and magnetic north is some particular value. (Or, equivalently, geographic south & magnetic south.) You're asking about the points in the diagram at which several isogonic lines meet. If we imagine taking a compass and walking around such a point, the *difference* between geographic south and magnetic south would change rapidly as we cross many different isogonic lines, each with its own declination.

One of the points on your map (the lower right one) is the Magnetic South Pole. If we imagine walking in a circle this point, our compass would always point towards the center of the circle, and so magnetic south would be changing rapidly. However, the direction of geographic south would stay pretty much the same so long as we didn't walk in too big of a circle. Thus, at this point the difference between geographic south and magnetic south changes rapidly because the direction of *magnetic south* changes rapidly.

The other point on your map is the Geographic South Pole. If we imagine walking in a circle about this point, our compass would always point pretty much in the same direction (towards the Magnetic South Pole.) However, the direction of geographic south would change rapidly as we walked around the south pole. Thus, at this point the difference between geographic south and magnetic south changes rapidly because the direction of *geographic south* changes rapidly.

Note that [the same thing happens in the Arctic Ocean](https://maps.ngdc.noaa.gov/viewers/historical_declination/) as well.

[![enter image description here](https://i.sstatic.net/ZP4rZ.jpg)](https://i.sstatic.net/ZP4rZ.jpg)


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