Are night lights on earth captured by satellites from outer space a good way to measure inequality?
Economists Praveen Chakravarty and Vivek Dehejia certainly believe so. They acquired images grabbed by satellites from the US Air Force Defence Meteorological Satellite Programme. These satellites circle the earth 14 times a day and record lights from the earth's surface at night with sensors. They superimposed a map depicting India's districts on their images, allowing them to develop a unique data set of luminosity values, by district and over time.
Using data generated by the night lights, they studied of 387 of 640 districts in 12 states. These districts account for 85% of India's population and 80% of its GDP. Some 87% of parliamentary seats are in these districts. Using the novel methodology, the economists documented income divergence in India.
Most of India is dark at night because there is little economic activity going on. But the delicate tracery of lights as seen from space also showed that the states are becoming more unequal between and within them.
Some 380 districts in 12 states were on average just a fifth as bright as the big cities of Mumbai and Bangalore.
Also, 90% of all the districts are just a third as bright in the night as the top 10% of all districts. And the ratio has worsened between 1992 - a year after India embraced economic reforms - and 2013.
Over time researchers have used night lights captured from space to find out how cities expand, and track land-use changes. They have monitored light intensity to estimate use of energy during festive seasons. Night lights have been used to track conditions in conflict-affected areas, and help in disaster response. They have have helped explain seasonal fluctuations in infectious diseases, mapped carbon emissions and light pollution, among other things.