Seeing Light Pollution from Space: How Satellite Data Powers SkyQI's Map

Every night, a NASA satellite photographs Earth's artificial light. Here's what that data actually tells us — and how SkyQI puts it on your screen.

When you open SkyQI's light pollution map and toggle on the satellite overlay, you're looking at data collected from 824 kilometers above your head. A sensor called VIIRS, mounted on a satellite that circles the Earth 14 times a day, measures how much artificial light escapes upward from every patch of land on the planet.

This is not a photograph. It's a precision measurement — and understanding what it measures changes how you think about the night sky.

What Is VIIRS?

VIIRS stands for Visible Infrared Imaging Radiometer Suite. It's a sensor aboard NASA and NOAA's Suomi NPP and JPSS satellites, and it has a special trick: a Day/Night Band (DNB) that can detect extremely faint light — down to the level of a quarter moon illuminating clouds.

Every night, as the satellite passes over each part of Earth, the DNB records how much light is radiating upward from the surface. Scientists then compile these nightly passes into monthly and annual composites, filtering out moonlight, clouds, fires, and other transient sources.

The result: a clean map of persistent artificial light across the entire planet. Updated monthly.

Key facts about VIIRS:

• Spatial resolution: ~750 meters per pixel (each measurement covers roughly 15 arc-seconds of latitude and longitude)
• Sensitivity: Can detect radiance as low as 0.1 nW/cm²/sr — dim enough to map rural villages
• Coverage: Global, pole to pole, every night
• Archive: Data available from 2012 to present, enabling trend analysis

What Does the Number Mean?

The raw measurement is radiance, expressed in nanowatts per square centimeter per steradian (nW/cm²/sr). That's a mouthful, so let's break it down:

• Nanowatts — a billionth of a watt. The light escaping upward from a city block is tiny in absolute terms, but VIIRS is sensitive enough to measure it.
• Per square centimeter — normalized to area, so measurements are comparable regardless of pixel size.
• Per steradian — normalized to the angle of observation, accounting for the satellite's viewing geometry.

In plain terms: higher radiance = more light escaping upward = worse light pollution.

Here's what the numbers look like in practice:

From Satellite to Your Screen: The Conversion Pipeline

Raw radiance is useful for scientists, but most people think about night skies in terms they can relate to: "Can I see the Milky Way?" or "How many stars are visible?" SkyQI converts satellite data through a three-step pipeline:

Step 1: Radiance → SQM

SQM (Sky Quality Meter) is the standard unit for sky brightness, measured in magnitudes per square arcsecond. Higher SQM = darker sky.

The conversion uses a logarithmic relationship based on the foundational work of Falchi et al. (2016), who created the New World Atlas of Artificial Night Sky Brightness. The key insight: the relationship between upward light emission and sky brightness is not linear — doubling the radiance does not halve the sky quality, because light scatters through the atmosphere in complex ways.

SkyQI's conversion maps:

• Very low radiance (< 0.5) → SQM 22.0 (the natural sky background)
• Low radiance (0.5–2) → SQM 21.0–21.5 (dark rural sky)
• Moderate radiance (2–10) → SQM 19.5–21.0 (rural to suburban)
• High radiance (10–50) → SQM 18.0–19.5 (suburban to urban)
• Very high radiance (50+) → SQM 15.0–18.0 (city sky)

Step 2: SQM → Bortle Scale

The Bortle Dark-Sky Scale (created by amateur astronomer John Bortle in 2001) provides an intuitive 1-to-9 classification:

Step 3: Visible Stars (NELM)

Finally, SkyQI estimates Naked Eye Limiting Magnitude (NELM) — the faintest star you could see with your eyes alone. From a pristine site, that's about magnitude 7.5 (roughly 14,000 stars across the whole sky). From an Indian metro, it drops to magnitude 3–4 (maybe 50 visible stars).

The formula is straightforward: NELM ≈ (SQM − 11) / 2, capped between 3.0 and 7.5.

How the Map Overlay Works

When you view SkyQI's light pollution map, the satellite layer adapts to your zoom level:

Zoomed out (all of India): The server aggregates thousands of data points into larger cells (~25 km squares), averaging the radiance within each cell. This gives you the big picture — the bright corridor of the Indo-Gangetic Plain, the dark pockets of Ladakh and the Western Ghats, the glow of coastal cities.

Zoomed in (a single city): The resolution increases to ~1 km cells, showing neighborhood-level variation. You can see how a highway corridor is brighter than the residential blocks beside it, or how a park creates a small dark pocket within an urban area.

The color code:

• 🔵 Dark blue → Bortle 1–2 (pristine)
• 🟢 Green → Bortle 3–4 (rural)
• 🟡 Yellow → Bortle 5 (suburban)
• 🟠 Orange → Bortle 6–7 (bright suburban to urban)
• 🔴 Red → Bortle 8–9 (city to inner city)

You can adjust the overlay opacity to see the base map beneath, and click any point to get the exact radiance, SQM, Bortle class, and NELM estimate.

Satellite vs. Smartphone: Two Complementary Views

VIIRS and SkyQI's photo analysis measure different things:

This is why SkyQI uses both. The satellite overlay gives you the broad picture: "This region is probably Bortle 5." Your smartphone measurement says: "Actually, from this specific hilltop, facing away from the city, it's Bortle 4 tonight."

When the two agree, you can be confident in the assessment. When they disagree, it usually means local conditions (terrain shielding, recent weather, seasonal haze) are creating a micro-environment different from the regional average.

What VIIRS Cannot See

Satellite data has real limitations worth understanding:

Shielded light doesn't register. A well-designed streetlight that points all its light downward emits almost nothing upward — VIIRS can't see it. This means a city that upgrades to shielded fixtures might show lower radiance on satellite maps while still having significant ground-level light trespass.

Atmospheric scattering is invisible from above. Light that scatters off humidity, dust, or pollution creates "sky glow" — the orange dome you see over cities at night. This scattering happens below the satellite, so VIIRS underestimates its effect. A dry night in Rajasthan and a humid night in Kerala with the same radiance will produce very different sky quality at ground level.

Resolution misses details. At 750 meters per pixel, a single bright stadium can dominate the reading for an entire neighborhood. The dark park next to it won't show up as a separate measurement.

Time averaging smooths extremes. Monthly composites blend every clear night together. The exceptionally dark night after a power cut and the festival night with a thousand extra lights both disappear into the average.

These limitations are exactly why ground-level measurements matter — and why your SkyQI uploads are valuable even when satellite data already exists.

India Through the Satellite's Eyes

SkyQI currently processes over 2,900 VIIRS grid points covering India. Some patterns in the data stand out:

The Indo-Gangetic Corridor — From Amritsar to Patna, an almost unbroken band of Bortle 6+ light pollution stretches across northern India. Individual cities are merging. The dark gaps between them shrink every year.

Coastal Concentration — India's west coast from Mumbai to Goa and east coast from Chennai to Visakhapatnam show continuous light along the shoreline. Fishing harbors, which use high-intensity lights to attract fish, contribute disproportionately.

The Himalayan Gradient — Move north from the plains into the hills and the radiance drops steeply. Within 100 km of the foothills, you can go from Bortle 7 to Bortle 3. This sharp gradient makes the Himalayan foothills some of the most accessible dark sky sites for India's urban population.

Island Darkness — Andaman and Lakshadweep islands register among the lowest radiance values in India, rivaling Ladakh. But they're ocean environments, so atmospheric conditions (humidity, salt haze) can reduce actual sky quality below what the satellite predicts.

Try It Yourself

Open SkyQI's light pollution map, toggle on the satellite overlay, and explore. Check your home, your favorite travel destination, or that camping spot you've been considering.

Then take your phone outside tonight, upload a sky photo, and see how the ground-level measurement compares to what the satellite predicts.

Every photo you upload makes the map more accurate.

The VIIRS Day/Night Band data used by SkyQI is produced by NASA's Earth Observation Group and is freely available for research and public use.