Earth at Night: What NASA's Black Marble Map Actually Shows

Of all the satellite images humans have made of Earth, the one most likely to stop you in your tracks is the nighttime view: the lights of every city, road and shipping lane glowing against the dark. It's called Black Marble, and it's now available as a basemap on LEV. Before you switch to it, it's worth knowing exactly what you're looking at — because it isn't quite what most people assume.

It isn't live — and that's a feature, not a bug

The most common question about a city-lights map is "is this tonight?" The honest answer is no. Black Marble is a yearly composite, and that's the right way to make it.

If you tried to publish a single night's image of the whole Earth, you'd get a picture full of holes: clouds blocking entire countries, moonlight overpowering smaller lights, snow glare washing out the high latitudes, fires and lightning faking lights that don't exist. So instead, scientists collect months of observations, sift out every clean cloudless pixel they can find, run an algorithm that removes every non-human light source, and assemble the best of all those nights into one global picture. The result is the most honest map of where the lights actually are — which is why every famous "Earth at night" image you've ever seen is a composite, not a snapshot.

The view on LEV is NASA's 2016 global composite, the most recent fully-processed worldwide Black Marble dataset published in NASA's GIBS catalog. Patterns of human settlement change slowly enough that this image is still an excellent picture of the present; we'll update to a newer global composite when NASA releases one.

How the picture is made

Behind the beauty is a remarkable instrument. The VIIRS Day/Night Band on the Suomi NPP satellite is sensitive enough to spot the light from a single ship at sea or a town in the desert. But raw nighttime imagery is messy — full of bright objects that have nothing to do with humans. The Black Marble algorithm cleans up:

• Moonlight reflecting off snow, cloud and water
• Auroras lighting up the polar sky
• Lightning flashes during storms
• Wildfires and gas flares
• Atmospheric airglow — a faint natural luminescence you can't see by eye

What survives those filters is, as closely as a satellite can determine, the light humans put there.

How to read it

The Black Marble basemap is most rewarding at the zoomed-out scales where the human footprint reads as a pattern. A few things to look for:

• The two great megalopolises. The east coast of the United States from Boston to Washington, and northeast Asia from Tokyo to the Pearl River Delta, are arguably the brightest human structures visible from space.
• The Nile. The river is drawn in lights — a perfectly traced bright thread through black desert.
• The lights of the sea. Squid-fishing fleets in the western Pacific and gas flares in the Persian Gulf show up as bright marine constellations. Don't mistake them for cities.
• The Korean DMZ. A continuous bright sweep on the south side, near-total darkness on the north — the world's most famous brightness boundary.

When tonight does matter: blackouts and disasters

Although the Black Marble basemap is a yearly composite, the same VIIRS instrument is what NASA uses for post-disaster blackout assessments. After a major hurricane, earthquake or war, agencies compare the latest night-lights image to the long-term baseline to estimate which areas have lost power and roughly how many people are affected. So while LEV's city-lights view doesn't update overnight, the science behind it is one of the most powerful tools we have for seeing the human impact of an event — and the live disaster-alerts layer and outbreak tracker help connect the dots between an event and the places affected.

A small honest caveat

Two things to know when reading the map:

• Light isn't development, and lit isn't always good. Some places shine bright with energy waste; some unlit places are perfectly thriving towns that just use lights carefully. Pretty pictures can mislead if you read them moralistically. Read the lights as information, not virtue.
• The boundary between sky and city can be blurry. Long-distance haze over major cities makes them look bigger from orbit than they are. Distant suburbs blur into urban cores.

Switch the basemap to city lights from the style toggle in the bottom-left corner — and if you want the cleanest way to compare night-lights to where people actually live, pair it with the live layers for fires, earthquakes and outbreaks. Where the lights are, much of human life is.