A couple of months ago, someone in Cloudy Night's Light-Pollution Forum wondered what's the most light-polluted city on Earth. I guessed that it was somewhere in Asia. A different person said that he was sure that North America headed the list. To make a long story short, I was dead wrong and he was absolutely correct.

It occurred to me that if you measure "most light-polluted" by the area where the Milky Way is difficult or impossible to see, then the Light-Pollution Atlas could provide an objective answer. All I had to do was count the number of red and white squares within each metropolitan area, correct for the fact that places at high latitudes are stretched horizontally, and I would be done.

There are a few caveats. As I said in a recent blog, it's unclear how accurately the Light Pollution Atlas captures actual conditions on the ground. Also, many big cities are on seacoasts, and it was tricky to separate the white line representing the coastline from the white for light pollution. Finally, it's quite possible that the methodology behind the Light Pollution Atlas has some kind of regional bias. But it's hard to believe that any such bias could be big enough to explain the differences that I found.

Nine of the top ten light-polluting metropolitan areas are in North America. Sorting by white squares (rather than red and white combined), Tokyo just slides onto the list in the #10 spot. But Tokyo is by far the world's most populous metropolitan area, almost equal to New York and L.A. combined. No city in Europe even makes the top 20.

It's not surprising that cities in the less developed world don't make the list. Beijing, Shanghai, Delhi, Mumbai, and Kolkatta each have light-pollution blobs comparable to a U.S. city of two million, though each of those metropolises have roughly ten times that number of people. But in these parts of the world, energy is far more expensive relative to income, and people don't splash it around heedlessly. Moreover, most of the 120 million or so people living in the map quadrant at lower left are in villages that have essentially no outdoor lighting at all.

But Europe and Japan have standards of living comparable to the U.S. I estimate that the U.S quadrant is home to about 50 million, the European quadrant to about 80 million, and the Japanese quadrant to about 90 million. Yet according to this map, the amount of light pollution in Europe and Japan is far less than in the U.S. Why is this true? Can those areas be a model for better lighting practice in the U.S.? I'll explore those subjects in a future blog. Meanwhile, I would love to hear comments from people with on-the-ground astronomy experience both in the northeastern U.S. and in Japan and/or northwestern Europe.