Out of the Blue and Into the Black
New Views of the Earth at Night
The night is nowhere near as dark as most of us think. In fact, the Earth is never really dark. And we don’t have to be in the dark about what is happening at night anymore either. —Steven Miller, atmospheric scientist, Colorado State University
The night side of Earth twinkles with light. The first thing to stand out is the cities. “Nothing tells us more about the spread of humans across the Earth than city lights,” asserts Chris Elvidge, a NOAA scientist who has studied them for 20 years.
Away from human settlements, light still shines. Wildfires and volcanoes rage. Oil and gas wells burn like candles. Auroras dance across the polar skies. Moonlight and starlight reflect off the water, snow, clouds, and deserts. Even the air and ocean sometimes glow.
A handful of scientists have observed earthly night lights over the past four decades with military satellites and astronaut photography. But in 2012, the view became significantly clearer. The Suomi National Polar-orbiting Partnership (NPP) satellite — launched in October 2011 by NASA, the National Oceanic and Atmospheric Administration (NOAA), and the Department of Defense — carries a low-light sensor that can distinguish night lights with six times better spatial resolution and 250 times better resolution of lighting levels (dynamic range) than before. Also, because Suomi NPP is a civilian science satellite, data is available to scientists within minutes to hours of acquisition.
The Visible Infrared Imaging Radiometer Suite (VIIRS) on Suomi NPP can observe dim light down to the scale of an isolated highway lamp or fishing boat. It can even detect faint, nocturnal atmospheric light — known as airglow — and observe clouds lit by it. Through the use of its “day-night band,” VIIRS can make the first quantitative measurements of light emissions and reflections, distinguishing the intensity and the sources of night light. The sum of these measurements gives us a global view of the human footprint on the Earth.
“These lights have always been there, but we’ve never had an ability to take full advantage of them,” Miller says. “Now we finally have a way of doing that.”