Hanle in Ladakh Is India's First Dark Sky Reserve, What It Protects and Why Astronomy Needed It

On a moonless night at Hanle, the Milky Way casts a shadow. That is not a figure of speech. At sufficient darkness, a Bortle scale rating of 1 or 2, which Hanle achieves, the galaxy's central band produces enough diffuse light to throw faint shadows on pale ground. Most Indians alive today have never seen this. Light pollution now blankets roughly one-third of the world's population so completely that the Milky Way is permanently invisible from where they live, according to a 2016 study published in Science Advances by Fabio Falchi and colleagues. Hanle, sitting at approximately 4,500 metres in the Changthang plateau of eastern Ladakh, is among the last places in India where that shadow is still possible.

The Indian Astronomical Observatory (IAO) has operated at Hanle since 2001, run by the Indian Institute of Astrophysics (IIA), Bengaluru. Its 2.01-metre optical telescope was, at the time of installation, among the largest in Asia. The site was chosen for reasons that compound: extreme altitude thins the atmosphere above the instruments, the Changthang plateau's arid cold keeps humidity and cloud cover low for roughly 270 nights a year, and the nearest city of any scale, Leh, was far enough away that artificial light had not yet contaminated the horizon. That last condition is the one that cannot be engineered. You can build a better mirror. You cannot filter out a town.

In September 2022, the Hanle Dark Sky Reserve was formally declared, the first of its kind in India and one of a small number across Asia. The reserve is administered through a memorandum of understanding between the IIA, the Ladakh Autonomous Hill Development Council (LAHDC), and the local community of Hanle village. The mechanism is specific: a defined radius around the observatory carries restrictions on outdoor lighting. Streetlights within the zone must be shielded so they direct light downward only, not outward or upward. New commercial construction is subject to lighting ordinances. Tourist camps, which have multiplied as Hanle became accessible on improved roads, must switch off or substantially dim external lighting after a fixed hour.

The reserve also formalises astrotourism as a sanctioned and regulated activity. This matters because unregulated tourist infrastructure, generator-powered camps with floodlights, vehicle headlights sweeping across hillsides, was already beginning to degrade the site before the declaration. By channelling tourism through the reserve framework, the administration creates an economic reason for the local community to protect darkness rather than treat it as an obstacle to development. A dark sky that generates income from visitors who come specifically to see it is a dark sky with a constituency.

The International Dark-Sky Association (IDA), which certifies dark sky places globally, uses a tiered system. Hanle has not yet received IDA certification, but the domestic declaration follows the same logic: set a perimeter, regulate light sources within it, and monitor sky quality over time with a Sky Quality Meter.

The most significant telescope at Hanle for current research is not the 2.01-metre optical instrument. It is MACE, the Major Atmospheric Cherenkov Experiment telescope. Commissioned in 2021, MACE is the world's highest-altitude imaging atmospheric Cherenkov telescope, sitting at 4,300 metres. It detects very-high-energy gamma rays not by collecting their photons directly, but by imaging the brief blue flash, Cherenkov radiation, that high-energy particles produce when they enter the atmosphere at speeds exceeding the local speed of light in air. The flashes last nanoseconds. MACE's 21-metre mirror, made of 356 panels, captures them.

Cherenkov telescopes are acutely sensitive to background light. A single bright source on the horizon can blind the detector during observations. The gamma-ray sources MACE targets, supernova remnants, pulsar wind nebulae, active galactic nuclei, emit at energies that require long observation windows and clean baselines. Darkness at Hanle is not ambience. It is a measurement condition.

India has several major observatories: the Vainu Bappu Observatory in Kavalur, Tamil Nadu; the Giant Metrewave Radio Telescope near Pune; the Mount Abu Infrared Observatory in Rajasthan. None of them sit inside a formally protected dark sky zone. Kavalur has faced pressure from the expanding light dome of nearby towns for decades. The GMRT operates in the radio spectrum and deals with radio frequency interference rather than optical light pollution, but the principle is identical: the signal you are trying to detect is being buried by human-made noise.

Hanle's declaration sets a precedent that India's astronomical infrastructure has needed. The IAO site represents a significant national investment in high-altitude, high-clarity observation. Losing its dark sky conditions to unmanaged development would not just inconvenience researchers, it would strand the instruments. A telescope at 4,500 metres with a compromised sky is not equivalent to a telescope at sea level with a clear one. The altitude advantage disappears if the light floor rises.

Leh's growth is real. The road to Hanle, once a punishing track, has improved. Tourist numbers in the Changthang region have increased substantially since the region became more accessible. Each of those developments is legitimate. The reserve does not stop them. It asks that lights point down.

The Hanle village community, primarily Changpa nomadic herders, are formal stakeholders in the reserve's governance. Their buy-in determines whether the ordinances hold in practice. The IIA has run astronomy outreach programmes in the village, and local guides now lead night-sky sessions for tourists. The darkness that was simply a condition of living at altitude has become, through the reserve, something the community has a name for and a reason to defend.

A protected sky and a functional observatory are the same asset looked at from two directions. The reserve makes that explicit for the first time in India, and the MACE telescope's gamma-ray detections, of sources that no other instrument in the country could observe at this energy range, are the argument for keeping it that way.