The maps of the US lights at night show that there is a line running down the middle of the country. East of that line it is essentially solid light, with little islands here or there of darkness (often large state or national parks). West of that line, it is opposite. Mostly dark, with islands of light at major cities.
What is puzzling is the location of the line. As far as I can see comparing maps, there is no obvious geographical feature that would have halted dense development at that line.
The line runs from Winnipeg in Canada, down through Grand Forks and Fargo, then Sioux Falls and Sioux City, then Omaha and Lincoln, then Wichita, then Oklahoma City, then down to Dallas and San Antonio.
I wonder if that line was just where westward expansion had reached before the Gold Rush and the rapid increase in migration to the far west, and afterwards people who would have otherwise went slightly west were instead going far west, and so we ended up with a sparse area in between?
My best guess is that the line represents the upstream limit of navigability of the Mississippi River's western tributaries. West of this line, it makes less and less economic sense to settle (unless it's next to a railroad line) since it's so difficult to ship things to and from the outside world.
That's a very good observation. I grew up on the line (in Topeka). I believe it is roughly where Western expansion had reached at the time the transcontinental railroad was finished. I don't know if correlation equals causation there. It's also a line where water starts to be less available in Kansas and areas south.
What is puzzling is the location of the line. As far as I can see comparing maps, there is no obvious geographical feature that would have halted dense development at that line.
A good map showing this is the light pollution map at DarkSiteFinder.com: http://darksitefinder.com/maps/world.html
The line runs from Winnipeg in Canada, down through Grand Forks and Fargo, then Sioux Falls and Sioux City, then Omaha and Lincoln, then Wichita, then Oklahoma City, then down to Dallas and San Antonio.
I wonder if that line was just where westward expansion had reached before the Gold Rush and the rapid increase in migration to the far west, and afterwards people who would have otherwise went slightly west were instead going far west, and so we ended up with a sparse area in between?