It's been nearly 50 years since the crew of Apollo 8 redefined our image of the planet with the famous "Earthrise" photograph. In that time, satellites have taken millions of photos of every corner of the Earth to give us an increasingly detailed picture of the "blue marble."
In honor of Earth Day 2017, TIME partnered with scientists at the Environmental Defense Fund to create a never-before-seen animation of 21 years of nighttime imagery of the Earth. The images were the subject of a recent paper on how this luminosity correlates to almost every major indicator of a nation's well-being, from GDP to carbon emissions. Collected by EDF researcher Jeremy Proville from Department of Defense satellite footage, the images paint a composite picture of how the world has changed over two decades.
While it makes for a pretty picture, data on how much light a place emits at night is also valuable when it comes to geopolitics, thanks to the unadulterated democracy of studying light from space. Countries might report economic or industrial progress in different ways, using different methods of measurement or even cooking the books for better headlines, but the amount of light they emit is gauged the same way from every spot on Earth. Based on this trove of satellite data, Proville and fellow researchers were able to demonstrate that a nation's nocturnal glow is a powerful predictor of a wide variety of demographic, economic and industrial indicators.
Due to the staggering gigabytes of data that are publicly available from government satellites, most previous efforts to correlate nighttime lights with variables like a region's economy have been limited to a few years of data or a handful of countries. Using technology provided by Google, Proville and his co-authors were able to go deeper — crunching the numbers on 21 years of data at a granular level and examining the correlations between light and eight major variables, including population, emissions and poverty.
"We were able to confirm that those relationships exist and get even stronger when you expand the timescale and the number of countries," Proville said. While nighttime light emissions won't become the sole measuring stick for a nation's stature, Proville predicts that the metric will become an increasingly powerful way to detect misinformation in terms of how a country reports the status of its economy and population. "I think this is going to prove itself as an important way to vet things," he said. "We're getting such great imagery that we're going to be able to monitor and vet things in a way that's never been done, and it's going to be harder and harder for countries to lie about things."
For example, one can easily see the difference between North and South Korea just by the differential in light emitted from the two nations.
Useful images are not restricted to nighttime lights. Groups like Global Forest Watch use satellite photography for up-to-date information on deforestation. And while the use of orbiting cameras may not be new, the images are getting better and better as newer, more sophisticated satellites go online. The day may not be far off when data like a country's nighttime lights stands alongside its GDP as a measure of its global standing.