Light Pollution Has Completely Erased the Milky Way From India's Major Metro Cities

Stand on a rooftop in Chennai or Hyderabad at midnight and the sky glows a dull amber-orange. That glow is not moonlight. It is the sum of every streetlamp, billboard, shop sign, and construction floodlight in the city bouncing off dust and aerosol particles in the lower atmosphere, a phenomenon called skyglow. Astronomers measure sky darkness on the Bortle scale, which runs from 1 (a pristine, pitch-black site) to 9 (an inner-city sky so bright you can read a newspaper outdoors). Mumbai, Delhi, and Kolkata score between 8 and 9. At those levels, the Milky Way, the faint, diffuse band of 100 to 400 billion stars that forms our galaxy's disc, is completely invisible. The contrast between the galaxy's glow and the background sky simply collapses.

The Light Pollution Atlas, a global dataset compiled using satellite radiance data, places the entire Indo-Gangetic Plain among the most light-polluted regions on Earth. From Amritsar to Patna, there is almost no break in artificial brightness at night. A child growing up in Gurugram has roughly the same chance of seeing the Milky Way from their terrace as a child in central London.

The Milky Way's central band has a surface brightness of roughly 22 magnitudes per square arcsecond, a unit astronomers use to measure how bright an extended object appears across a patch of sky. A city sky in India's metros typically sits at 17 to 18 magnitudes per square arcsecond. Because the scale is logarithmic, that difference means the sky background is 100 times brighter than the galaxy's own light. The Milky Way does not disappear because the stars are gone. It disappears because the sky around it has been flooded with enough scattered artificial light to drown out any contrast.

Artificial light at night also scatters differently depending on wavelength. Older sodium-vapour streetlamps, common across Indian cities through the 2000s, emit a narrow orange band. The rapid shift to white LED street lighting, accelerated under India's UJALA scheme, which replaced over 360 million incandescent and CFL bulbs, has made skyglow worse in one specific way: white LEDs are rich in blue-wavelength light, and blue light scatters far more efficiently in the atmosphere than orange or red. Rayleigh scattering, the same physics that makes the daytime sky blue, amplifies blue-rich artificial light into a broader, more pervasive glow. Cities that switched to LEDs to save energy often brightened their night skies in the process.

A 2016 study published in Science Advances by Fabio Falchi and colleagues, using data from the Suomi NPP satellite, found that 99 percent of the population of Europe and the United States lives under light-polluted skies. India's data in the same study showed comparable urban saturation, with the added factor of dense population clustering. The researchers calculated that a person needs to travel to a site with a Bortle 4 sky or darker to see the Milky Way with any clarity. In practical terms for an Indian city dweller, that means leaving the metro and driving several hours into genuinely rural darkness.

From Delhi, the nearest Bortle 3 or 4 sites are in the Thar Desert near Jaisalmer or in parts of Spiti Valley in Himachal Pradesh, both requiring overnight travel. From Mumbai, the Sahyadri ranges offer partial relief, but even there, the glow from the city and from Pune bleeds into the western horizon. Bengaluru's rapid expansion has consumed what were once dark rural buffers; the observatories that once operated on the city's outskirts have had to contend with encroaching brightness for decades.

The Indian Astronomical Observatory at Hanle in Ladakh, operated by the Indian Institute of Astrophysics, sits at 4,500 metres above sea level precisely because it needs to be that remote and that high to find a sky dark enough for serious optical work. Professional astronomers in India have been voting with their equipment for years: the only usable darkness is at extreme altitude and extreme distance from any settlement.

Light pollution's damage is not limited to stargazing. Artificial night brightness disrupts the navigation of migratory birds that use stellar cues, confuses sea turtle hatchlings on coastal beaches, and suppresses melatonin production in humans exposed to blue-rich light after dark. The ecology of darkness is something cities are only beginning to account for. India's coastal cities, Mumbai, Chennai, Visakhapatnam, sit on migratory flyways and nesting beaches where artificial light has measurable effects on wildlife behaviour documented in field studies by the Bombay Natural History Society and others.

The human cost is subtler but real. A 2019 study in JAMA Internal Medicine found that exposure to artificial light at night was associated with increased risk of weight gain and metabolic disruption in women. Sleep medicine researchers at AIIMS have separately documented the role of nighttime light exposure in disrupting circadian rhythms among urban Indian populations, with blue-wavelength light from screens and street lighting identified as a primary driver.

The physics is not the obstacle. Skyglow is not a permanent chemical change to the atmosphere. Turn off or redirect the lights, and the sky recovers within minutes. Several cities globally have demonstrated this with dark-sky ordinances: Flagstaff in Arizona became the world's first International Dark Sky City in 2001, and its Milky Way is visible from the city limits. In India, Pench Tiger Reserve in Madhya Pradesh has made early moves toward reducing artificial lighting in buffer zones, and Ladakh's Hanle village has been designated as India's first dark sky reserve by the Ministry of Science and Technology, with restrictions on artificial lighting in the surrounding area formalised in recent years.

The barrier is not technical. Shielded fixtures that direct light downward rather than into the sky cost roughly the same as unshielded ones. Warm-spectrum LEDs with lower blue content exist and are commercially available. The barrier is that no Indian metro has yet treated night-sky darkness as infrastructure worth protecting, the way air quality indices or noise ordinances are treated, imperfectly, but at least as a named public concern.

The Milky Way that Aryabhata calculated the rotation of, that navigators used to cross the Indian Ocean before GPS, that every rural grandmother in India can still name in her own language, that sky is still there, physically intact, 26,000 light-years from the galactic centre to this planet. The city has not destroyed it. The city has simply decided, one streetlamp at a time, that it did not need to be seen.