At 5:16 pm Eastern time on this date 50 years ago, about 30 minutes after sundown, a fast-moving ripple of deeper darkness spread over much of the northeastern United States and a part of Canada. A misset relay in an Ontario power station began a cascade of power grid overloads and disconnects across 80,000 square miles of the U.S. and Canada. Some 30 million people, including New York City’s roughly 7.8 million, were affected for up to 14 hours. Each year, more than 4 trillion kilowatts of electricity are generated by U.S. power companies. About 67 percent of that electricity is produced in the 1,400 plants burning fossil fuels. You can find more statistics on communities across the country by downloading the Census Bureau’s “dwellr” mobile application at <www.census.gov/mobile>.