Can you see stars in light polluted skies?

Light pollution is a big problem, and it’s getting bigger. Recently published research led by Dr Alejandro Sánchez de Miguel at the University of Exeter, UK suggests that a global rise in artificial lighting has substantially boosted light pollution levels over the past few decades – some regions have suffered as much as a fivefold increase! The problem is intrinsically linked to energy waste, so it would be reasonable to expect that efforts are underway to tackle it, but not all approaches are beneficial to our view of the sky. LED lighting for example, while energy efficient and cost-effective, has become the bane of stargazers everywhere as its broad emissions flood the sky with even more light than traditional low-pressure sodium lamps. This is compounded by the dazzling nature of often poorly shielded LEDs, which impacts the way our eyes adjust to the darkness.

Light pollution entails more than just the loss of natural beauty in the night sky. It affects ecology and human health too, so perhaps one day we’ll see meaningful legislation to regulate it on a wider scale. But in the meantime, stargazers are having to adapt and make do with soupier skies. Fortunately, light pollution need not be an insurmountable barrier to stargazing. As a Londoner, I’ve spent more than a decade living under some of the most heavily polluted skies in Europe, and that hasn’t stopped me from enjoying the stars and witnessing many special events in the sky. By understanding the impact of light pollution, we can set our expectations and make the most of even the brightest skies.

Even in the bright stars of London, starlight reaches the ground

(Image credit: Tom Kerss)

 What does light pollution do to our view? 

It’s not uncommon to hear about light pollution “blocking” our view of the stars, but this is a mischaracterization of its effects. Light pollution doesn’t prevent starlight from reaching the ground. Rather, it competes with it. Imagine using a black pen to scribble on a sheet of white paper. As long as your pen is in good order, your creativity will be easy to admire. If you use darker paper – say, a dark grey – the black doodles become harder to discern, particularly if they include fine details. On a sheet of nearly black paper, it would be virtually impossible to see anything made with a black pen. The ink isn’t lost, but it’s contrast with the paper is. In the night sky, it’s the same phenomenon in reverse. Light pollution makes the canvas of the sky brighter, so that some natural light sources fall below the contrast threshold that makes them discernible by eye.

The appearance of the Great Square of Pegasus in rural, suburban and urban skies

(Image credit: Tom Kerss/Stellarium)

Aside from the Sun and Moon, everything else in the sky can be regarded as a point source (a point of light) or a diffuse object (a patch of light). Stars are point sources, whereas deep sky objects such as nebulae and galaxies are diffuse. In our solar system, planets and satellites are point sources, and the comas and tails of comets are usually diffuse. Diffuse objects are easily suppressed by even moderate light pollution – the Milky Way and elusive Zodiacal Light are among the first to go – while point sources, owing to their relatively high contrast, put up more of a fight. The brightest few hundred stars are resistant to light pollution, making the outlines of most of the constellations accessible in suburban skies, although fainter constellations are more challenging. The naked-eye planets are typically brighter still, and can be seen without trouble from the brightest city center. 

How to measure and make the most of your light pollution level