Just How Wide is Hudson Bay, Really?

On July 27, 2009 · 2 Comments

Everything about Canada is larger than life. It’s difficult to wrap one’s mind around its incredible breadth and scale. I came across a tantalizing fact that I thought might help conceptualize its vastness but instead it’s a clever illusion, a little geo-slight-of-hand. In fact I think it’s more enjoyable as a mind-bender than as a trivia question.

People sometimes ask how I come up with ideas for Twelve Mile Circle. There are many ways actually. Often I simply jot down odd tidbits and detritus as I stumble across them on the web, in print or verbally. I pull them from the list as needed, either because I want to focus on a specific theme or geographic area, or as a cure for the occasional writer’s block. I probably have a couple of hundred topics waiting in reserve.

Somewhere I read or heard or otherwise noted on my list that Hudson Bay is so large that its eastern edge is due north of Washington, DC and its western edge is due north of Kansas City. That’s amazing, I thought when I first read it, and I placed it on list for a time when I could explore it further.

For those unfamiliar with the locations involved, Kansas City is about half way across the North American continent. It’s actually a little skewed to the east but "in the middle" more-or-less. Thus the Hudson Bay statement would lead many to conclude, subconsciously perhaps, that Hudson Bay must also be half a continent wide. It’s not, but what a great perceptional illusion.

I thought I would plot this all on a map. Ponder this for a moment:



View Width of Hudson Bay in a larger map

For the sake of simplicity I rounded to the following four latitude/longitude coordinates to create the box:

  • 59.0N, 76.9W = Hudson Bay East
  • 59.0N, 94.7W = Hudson Bay West
  • 39.0N, 76.9W = Washington
  • 39.0N, 94.7W = Kansas City

Nothing looks amiss. All the numbers align. The shaded area forms a perfect rectangle, with the edges of Hudson Bay directly north of Washington and Kansas City.

The trick would be much more obvious if we looked at a globe, because then we would notice with little effort that longitudes converge on two points, the north and south poles. There are many ways to represent the spherical Earth on a two dimensional surface, but it is difficult to do this without some type of distortion. Most of the online maps, Google Maps included, flatten the planet so that lines of longitude appear to run in parallel, conveniently ignoring that they actually converge. As a result the two blue lines lines on my map appear to be the same length but in reality they are not.

One can calculate the great-circle or orthodromic distance and determine the shortest line between two points on a sphere. In fact, there are a number of tools on the Intertubes that will do just that. Simply plug in the coordinates and let someone else worry about the math.

The results are striking. The latitudinal distance between the eastern and western longitudinal extremes of Hudson Bay is roughly 1,000 kilometres (630 miles). Between Washington and Kansas City, however, the distance between the same lines is a little more than 1,500 kilometres (950 miles).

Hudson Bay is still very wide. A thousand kilometres is no laughing matter. Nonetheless, the distance between Washington and Kansas City is fifty percent further. And that’s why I loved the statement.

On July 27, 2009 · 2 Comments

2 Responses to “Just How Wide is Hudson Bay, Really?”

  1. Saint Cad says:

    Looks like the sequel (based on the Mercator distortion also) will be “How Big is Greenland, really?”

Comments are closed.

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