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Comment by enriquto | Hacker News Reader

Maybe I studied too much topology in my youth, but the title of this article strikes me as nonsensical... How do you walk around an island? You can swim around an island, or walk around a lake. But you cannot really walk around an island (or, if you can, it seems very easy to do: just go to the island and turn around yourself).

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He visited every port on all 4 coasts of the island, roughly covering the perimeter. We understand this to mean in common speech that he walked "around" the island even though he didn't truly circumnavigate it. We know what the title means.

Some islands, you can definitely walk around, even in the mathematical sense. Examples are https://en.wikipedia.org/wiki/Lake_island and https://en.wikipedia.org/wiki/Surrounded_Islands (if you timed your visit at just the right time).

One could also argue that wading through the sea in knee-deep water would be walking around the island in the strict mathematical sense. That’s doable for some islands.

And of course, the true nitpicking topologist realizes that, topologically, walking in a circle with radius 1m anywhere outside Australia is the same as walking in a circle with radius of almost 20,000 km that encompasses Australia, and thus is ‘walking around Australia’. I just did that. Took me a few seconds, at most ;-)

To "walk around" means to roam or wander. Most people understand this use of the phrase.

But in this case, the man seems to have actually walked the perimeter of the island.

To "walk around" means to roam or wander. Most people understand this use of the phrase.

Except they clearly didn't mean that the guy roamed Australia, because that's not particularly noteworthy. He "packed his backpack and walked around the entire continent of Australia by the coastline". The grandparent is pedantically correct that well actually you cannot walk around island. Must be fun at parties...

What are you on about? He walked around the island - walked the length of the perimeter. Just like swimming around it but on the land. What's confusing about that?

It doesn't mean anything unless you specify the maximum allowed distance to the coast (or the average distance to the coast along your ride, or the median distance to the coast). Notice that you probably want to define the distance of each point of the coast to some point in your traject, and not the other way round (which would make your endeavor trivial).

Really, it seems a very complicated concept to define precisely, and it would entail a lot of seemingly arbitrary parameters.

You are confusing “it is complicated” with “I am over complicating it”.

But why do you need to define it? What are you trying to achieve? Any reasonable person already understand what it means to walk around an island.

> But why do you need to define i

Wait, isn't this Hacker news? I want to be given a definition and analyze it deeply.

For instance, looking at the wikipedia page it seems that the closest that De Brune got to Cape York Tip (the northernmost point of Australia) was about one thousand kilometers. If this "counts" as a complete tour, he might as well had walked consistently 1000km inland for a much shorter tour (but maybe harder, due to the desert?).

> I want to be given a definition and analyze it deeply.

Well that's anti-social and not welcome here. Please check the guidelines for the site for how to communicate.

https://news.ycombinator.com/newsguidelines.html

> Please respond to the strongest plausible interpretation of what someone says

> don't cross-examine

Dude, what the fuck.

Mathematics is pretty much on topic here. I made a valid point, just for fun, that giving a precise mathematical definition of "walking around an island" is a tricky issue. Of course everybody knows what does it mean roughly, but making that point, about a mathematical definition cannot be construed as anti-social. Unless you consider mathematics anti-social.

> Of course everybody knows what does it mean roughly

So you know this goes against

> Please respond to the strongest plausible interpretation of what someone says

But you've generated such a strong negative reaction that you've been flagged and the comment has been removed, so it's a lot of people saying it, not just my random opinion!

I think the following definition is well-defined and would be acceptable to the vast majority of people:

A minimal-length, non-intersecting loop for which every point on the coast is within d metres of at least one point on the loop.

The only parameter is d, which I think most people would happily set to 5 or 10 km for a country the size of Australia.

It may not be unique, but I think anything obeying this would count.

I like this definition. Not sure that the "non-intersecting" condition is necessary. I was thinking about a different one, given by the condition that the path never goes farther than d meters from the sea (while encircling the part of the land that is farther than d meters).

From the Wikipedia article someone else posted:

In September 1921 he began a walk around the perimeter of Australia, from Sydney to Sydney, anticlockwise.

https://en.wikipedia.org/wiki/Aidan_de_Brune

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