1. What is light pollution and what are its causes?

Just over a hundred years ago, people in towns and cities could look up at the night skies and clearly observe the wonder of stars on a clear night. Now, millions of children across the world have never had the chance to see the wonders of the Milky Way.

This is because of light pollution.

Light pollution is defined as the unwanted presence of anthropogenic (caused by human activity) light in the night sky. It is an unwanted consequence of outdoor lighting and the increase in human population living in cities over the same length of time.

Different elements include:

  • Glare – this is unrestricted brightness that hurts your eyes if you look at it for too long
  • Skyglow – this term is used to describe the brightening of the night sky over urban areas and can be a mixture of natural and unnatural light sources
  • Light trespass – this refers to light encroaching where it is not wanted or needed
  • Clutter – a grouping of lights that collectively generate too much light

This is an inevitable side effect of industrial civilization. Common sources of light pollution include street lights, exterior and interior building lights, commercial offices and properties, and floodlit stadiums.


2. Why is it a problem?

The growing use of man-made light has serious environmental consequences for the natural world – humans, wildlife, and the climate.

It has profoundly negative consequences for nocturnal animals, such as badgers and foxes. Birds that hunt or migrate at night can become disoriented by artificial light. Every year, millions of birds needlessly lose their lives after crashing into tall unnecessarily illuminated office buildings. Countless other species are affected by light pollution, many in ways which we are only just discovering.

Humans are also affected by light pollution, which can have a negative impact on circadian rhythms and generally contributing to sleeping disorders, which can lead to many health-related issues.

Light pollution also affects the ability of astronomers and astrophotographers to observe and study the sky.

Unshielded lights send rays in all directions, including upwards. This makes astral bodies much harder to observe and photograph. More insidiously, light pollution also plays havoc with spectroscopy, the study of spectra of an object and one of the most important tools for land-based astronomy.


3. When did light pollution become a problem?

Light pollution is a fairly recent problem and was first observed in the early years of the twentieth century, around the time when towns and cities first adopted electric lighting.

However, the effects of early electric and gas lights were negligible compared to the visual assault caused by the sheer number and intensity of modern lights, particularly with the advent of inexpensive LED lighting, which resulted in more lights for less energy.


4. How is it measured?

Light pollution can be measured in several ways, for example, by:

  • Taking measurements from the Earth’s surface to calculate night sky brightness;
  • Taking aerial measurements to measure direct or reflected luminance;
  • Taking measurements from outer space via the International Space Station or satellites.

The Bortle Scale measures Night Sky Brightness by using the limited magnitude of stars for comparison. It uses a scale of 1 to 9, with 9 being the worst.

The Sky Quality Index is another way of measuring Night Sky Brightness and uses a calibrated photometer to produce extremely accurate results.

As well as photometers, astronomical CCD and DSLR cameras are used to capture images of the entire night sky and create a luminance calibrated color map.


5. How can we reduce light pollution?

The good news is that pollution caused by light is one of the easiest types of pollution to ameliorate. This doesn’t mean we all have to sit around in the dark, it’s about taking sensible steps to make light more efficient and less intrusive. We can do this by:

  1. Lobbying governmental bodies to put light pollution laws into place – these could fine companies who leave office lights on in tower blocks all night, for example;
  2. Adopting intelligent street lighting solutions that use wireless technology to be controlled from a central management system;
  3. Joining campaign groups, such as the Astronomical Association’s Campaign for Dark Skies and lobby for light pollution legislation.

There are also simple steps that we can all take:

  1. Installing reflectors in a garage driveway or garden path rather than a row of lights – and lobbying for the banning of ‘Rottweiler’ lights – 100-watt halogen lights that emit an ultra strong and completely unnecessary glare;
  2. Only using outdoor lighting when necessary. Lighting operated by motion sensors is a good solution;
  3. Swapping high wattage bulbs for dimmer ones.


6. How to find dark skies

If you would like to find some lovely dark skies, there are some brilliant online resources:

  • This light pollution map by Dark Site Finder is a free resource that allows you to quickly see the best places in the world to find a dark night sky.
  • Dark Sky Discovery offers a dark sky map service with details of stargazing events for those based in the UK. It’s a pretty interactive site; you can nominate local places that are good for astronomers.
  • The International Dark Sky Association also has a world map showing the positions of locations designated as part of the International Dark Sky Places Program and is a useful resource although it isn’t a comprehensive map of general dark sites.

You may also find a light pollution smartphone app useful when out and about. Here are three of the best:

  • SkyLive for those with iPhones has everything you need to plan your star observations. It takes weather, light pollution, and interesting astronomical events into account and much more.
  • The Light Pollution Map by Photographer’s Arsenal is available across a range of platforms including Android and provides up-to-date information about light pollution, as well as many other features, such as lunar eclipse alerts and meteor showers. It’s free with an option to upgrade for a fee.
  • The Loss of the Night app for the Android OS invites users to take part in a worldwide project that aims to help scientists quantify the problem of light pollution. The app basically works by guiding you to a certain star and asking you if you can see it. Educational and useful, this app gives a high level of user satisfaction and is well worth exploring.


7. Light pollution filters for astronomy & astrophotography

Light pollution filters are an essential piece of kit if you want to take clearly defined photos of astral objects in the sky or anything at night without the streetlights showing as a horrible glow.

There are many kinds of filter and, basically, their job is to reflect or absorb the wavelengths of light generated by artificial city lighting, while allowing the rest of the visible spectrum to get through to your lens and sensor. This results in a crisper image with much less blurred light.

You can buy different sizes and strength to fit on telescopes or for use with a DSLR camera – and prices to suit all budgets.