An image of Earth at night as seen from space appeared on my laptop screen during an internet search. The captivating image with a vague title—“Global Map”—drew me in like an insect mesmerized by the warm glow of a streetlamp at night.
Initially, the map reminded me of flying at night under clear skies, with illuminated cities, suburbs, and roads below, often reaching toward the horizon, but it was obviously at a much bigger scale. Yet, despite the vast distance, it was still possible to identify many locales by their artificial lightprint. The bright nodes of major metropoles: Paris, Delhi, Johannesburg, Mexico City, and the dazzling sprawl of Cairo and illuminated “snake” of the Nile River. I was struck by the map’s simple yet arresting aesthetic: shades of dark blue and yellow, both contrasting and complementary.
What does it mean, I wondered, to observe and visualize Earth only by day, given that daytime is half of the planet’s time and place? This map showed there is another half to the story: night, frequently forgotten or ignored because of what I now call an implicit daytime bias.
Night is defined by the absence of sunlight, yet the map revealed a nocturnal world that was not entirely dark. Sure, large swaths of terrestrial Earth were still the deepest shades of navy blue, indicating some illumination by the moon but little artificial light. But part of what made the map so remarkable, even stunning, was the extent and intensity of illumination—so much artificial light that it was visible from space.
This map showed there is another half to the story: night, frequently forgotten or ignored because of what I now call an implicit daytime bias.
Some scientists have characterized such illumination as light pollution. One prominent scientist has defined light pollution simply as adverse artificial light at night, due to its detrimental scientific, ecological, human health, aesthetic, and other effects. However, not all scientists studying nocturnal artificial light agree on what exactly “adverse” means.
Other scientists offer a technical definition of light pollution: “an increased volume concentration of photons in the nocturnal environment above naturally expected values.” This useful but dry explanation fails to capture the affective power of the map—its illustration of human presence on the planet and the resulting proliferation of artificial light. Part of the map’s power is that we often think of night as obscuring what is readily visible by day. The hypervisibility of nocturnal illumination made by people undermines this assumption. As the map shows, night is particularly effective at illuminating artificial light.
This global map captured a snapshot of nocturnal Earth in 2012, but, as a historian, I started pondering what the map would have looked like if it had been made in, say, 1912 or 1812. In short, I realized that night has a history.
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Several years after unexpectedly glimpsing the nocturnal map, I decided to write a book about the history of artificial light at night and how scientists came to be concerned about it.
In Transforming Night, my journey begins with the first scientists who raised questions about nocturnal artificial light: astronomers. Many observatories had been built in or near city centers before extensive nighttime illumination. But as artificial light as well as air pollution and vibrations from new transit systems, like streetcars, increased in urban areas, many of these observatories closed, the buildings repurposed for non-observational work, and new scientific facilities constructed in more distant locales in an attempt to escape the problematic new qualities of urban nightscapes. Such was the pattern at Mexico’s Observatorio Astronómico Nacional, which was founded in Mexico City and then moved four times between 1867 and 1969.
The history of Mexico’s observatory suggests how night and associated darkness have changed. It illustrates three of the ways artificial light has transformed night. Human-made illumination has gotten both bigger and brighter, accelerating since the nineteenth century and especially since the early twenty-first century.
Scientists have identified a fourth way human-made illumination is altering night. Different lighting technologies produce differently colored light. The recent adoption of light-emitting diodes (LEDs) has made many cities’ lightprint whiter.
Data for this map was acquired over 9 days in April 2012 and 13 days in October 2012. It took satellite 312 orbits and 2.5 terabytes of data to get a clear shot of every parcel of Earth’s land surface and islands. (Download Map Views—NASA Earth Observatory and NOAA National Geophysical Data Center).
Some scientists and activists concerned about nocturnal artificial light have argued that modern nightscapes have become inverted: from point sources of illumination scattered across the sky amid an otherwise dark nightscape to bright lights dotting the Earth’s surface with luminous haze from those lights rising above. They have suggested that earthly lights have become terrestrial stars blocking celestial illumination, including actual stars.
In many parts of the world, the daily experiences of night are becoming less night-like. In dense, highly illuminated cities like Hong Kong, inhabitants spend their nights in what scientists call perpetual “artificial twilight”: a luminous hinterland where daylight is never entirely replaced by nocturnal darkness. This phenomenon raises the question, Is night without natural darkness still night?
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When I began research for my book, I wasn’t sure where the story would end. That changed with SpaceX.
On May 23, 2019, the now-public spaceflight, telecommunications, and AI company launched by Elon Musk deployed sixty Starlink test satellites into low-Earth orbit to provide low-cost internet to rural and remote communities. The next day, Marco Langbroek, a Dutch amateur astronomer, was one of the first people to publish a video showing the satellites traversing the night sky mere hours after launch. He described a “spectacular view” over northwestern Europe: “a ‘train’ of bright satellites, all moving close together in a line, moved across the sky.”
“Global Map” also demonstrates that we need to consider not only the history of nocturnal artificial illumination but also its future.
“It started with two faint flashing objects,” Langbroek explained. “Then, a few tens of seconds later, my jaw dropped as the ‘train’ entered the field of view.” By naked eye, the satellites appear as moving “stars” from the Earth’s surface, but in astronomical images, they appear as parallel streaks interfering with the observation of celestial phenomena. Given what he was seeing, “I could not help shouting ‘oaaaaah!’”—followed by a few expletives, Langbroek added.
At the time, I happened to be in Colorado doing fieldwork. I watched astronomy and light-pollution Twitter (now X) blow up online, as these two overlapping communities suddenly realized what was happening before their eyes, cameras, and telescopes.
They didn’t anticipate what has occurred since. More satellites were deployed between 2019 and mid-2024 than in the previous sixty years combined. As of June 15, 2026, over 16,000 active satellites are currently orbiting Earth (of those, 10,634 are Starlink satellites), but this figure includes only operational spacecraft. It doesn’t include any space debris, also called “space junk”—or the one million additional satellites SpaceX alone hopes to deploy.
Ground-based artificial illumination since the late nineteenth century had already altered nightscapes. Space-based light is now extending the geography of nocturnal artificial illumination into extraterrestrial territory, rapidly and dramatically expanding the spatial reach of where human-made light is located and ultimately its impacts on night across almost the entire planet, unless it is addressed—and quickly. Transformations of night due to both ground- and space-based light are momentous in the context of human history, given all cultures’ connections with nightscapes.
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Ultimately, I decided to end my book where it began—with the map that started it all, although I don’t look at it the same way anymore. “Global Map” is at once an image of artificial light at night, light pollution, and light poverty. Composed from data collected in 2011, it is a historical artifact, already out of date.
“Global Map” also demonstrates that we need to consider not only the history of nocturnal artificial illumination but also its future. That future includes which environmentalism best addresses coming challenges. Since 2020, the new movement around light justice, rooted in principles of environmental justice, offers the most compelling moral and political framework for addressing the benefits and costs of both artificial light and darkness, for both humans and nonhumans alike. The next step is a fight for more equitable, just access to light—and night.
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From Transforming Night: The History and Science of Light Pollution by Sara B. Pritchard. Copyright © 2026. Available from University of Washington Press.
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