Q&A: Light Pollution Defined

Question: I keep hearing about this thing called “light pollution” and how it’s bad for astronomers. But how can light pollute? It’s not a toxic chemical, or airborne particulate, or radioactive … it’s just light. — KS, New River, AZ

Answer: Well, it’s clearly not the same type of pollution as hazardous chemicals, exhaust emissions, litter, or radioactive waste. But if you accept the definition of pollution as “harmful contamination of natural resources,” it is very real indeed.

This amazing satellite image shows North America at night. Two things about this image are worth noting. First, there’s not a lot of dark sky left within the continental US, save for some remote areas in the Rocky Mountains.

Second, all those lights you can see are wasted energy. If they are visible from space, that means the lights are directed upward. And but for a few exceptions (such as airport runways) the purpose of night-time lighting is to illuminate things on the ground.

The main culprits are things like unshielded dusk-to-dawn yard lights, decorative drive and walkway lights, and billboards with spotlights mounted along their bottom edge (which cost le$$ to replace).

Dark sky is under-appreciated as a natural resource because most people, living in cities, have no idea what they are missing. When was the last time you saw the stars under truly dark skies? Have you ever?

Two Inferior Planets Visible
Q&A: Polar Satellites