What is Light Pollution?

~5 min

Can you imagine going out of your apartment and seeing the magnificent Milky Way glowing above your head? Unfortunately, it’s impossible if you live in a big city — and the reason is light pollution.


Twenty years ago, on June 1, 2002, the Czech Republic passed the world’s first law to reduce light pollution. We’d like to use this anniversary to talk about light pollution and its effects on our life in general and astronomy in particular.

Light pollution meaning

Light pollution is the presence of excessive artificial light. It is most common in large cities where it’s produced by streetlights, billboards, shopping malls, and exterior lights on buildings.

Astronomers are specifically concerned about light pollution because it prevents them from observing the night sky. However, environmentalists, health workers, and economists also insist on reducing light pollution — we’ll tell you why soon.

Different types of light pollution

There are multiple types of light pollution; let’s briefly consider those that can directly affect observational astronomy:

  • Skyglow: yellow or orange halo in the night sky;
  • Glare: bright light from passing cars or street lamps;
  • Light trespass: street light shining into your room;
  • Satellite glow: brightening of the sky by artificial satellites.

The first type of light pollution reduces your ability to view celestial objects. The second and third types ruin your night vision.

The fourth type has emerged relatively recently. Many experts think that satellite constellations, such as Starlink, pose a real threat to ground-based astronomy. Due to increasing numbers of satellites, our skies might be crawling with bright dots of artificial light in the nearest future.

Light pollution scale: Bortle scale

Astronomers often use the Bortle scale to measure the night sky’s brightness. This scale ranges from Class 1 (perfectly dark sky) to Class 9 (most light-polluted city sky). You can read detailed descriptions of each of the classes here.

There are also other scales for measuring light pollution. For instance, this interactive light pollution map uses a scale based on the light sources’ radiance. If you select “World Atlas 2015” from the drop-down list in the upper-right corner, you’ll see a map that utilizes a scale based on the sky's brightness. Try to find your city on these maps and see how light-polluted it is!

How to measure light pollution?

You can measure light pollution in your location by determining the faintest stars you can see. Astronomers call this the “naked-eye limiting magnitude” or NELM. Though this criterion is quite subjective as it depends on your eyesight, it can still help you understand how much skyglow there is in your skies.

If you live in the Northern Hemisphere, you can use the Ursa Minor constellation for your measurements, as it never sets below the horizon. Southern Hemisphere observers can use the Southern Cross. We’ll tell you how to determine the NELM with the help of Ursa Minor’s Little Dipper and the Sky Tonight app.

First, you need to find Ursa Minor. Open Sky Tonight, tap the magnifier icon at the lower part of the screen, and type “Ursa Minor” in the search field. Then tap the blue target icon on the corresponding search item — you’ll see Ursa Minor’s current location. Point your device at the sky and follow the arrow to find Ursa Minor.

Now it’s time to determine the limiting magnitude. Zoom in on the constellation to see individual stars in the Little Dipper asterism. Tap the panel at the very bottom of your screen. In the upper-left part of the panel, there’s a magnitude slider. Adjust the slider until the amount of Little Dipper stars you see in the sky matches the one in the app. The number you’ll see below the slider is your naked-eye limiting magnitude.

Light pollution effects

People who are not interested in observing the night sky usually don’t care much about light pollution. But if you think it impacts only astronomy lovers, you’re wrong — light pollution is harmful to most living creatures and to the environment as a whole.

Health effects

Humans are used to a particular day-night cycle, which acts as an internal clock for our bodies. This cycle is controlled by the amount of light around us. Excessive artificial light during the night can disrupt this cycle and cause sleep disorders, depression, and weakening of the immune system.

Effects on animals

Light also controls animals’ behavior, such as reproduction, sleep, and protection from predators. Excessive artificial light negatively affects animals in many ways: migratory birds get disoriented, baby turtles are drawn away from the ocean into the cities, populations of insects decline, etc.

Energy waste

Unnecessary lighting costs billions of dollars every year and harms the ecology. According to the International Dark-Sky Association, 35% of all outdoor lighting is wasted because of poorly-designed light fixtures. The total cost of this waste amounts to around three billion dollars every year in just the United States. Plus, millions of tons of carbon dioxide are emitted to power this lighting, which results in immense damage to the environment.


According to the article published in the Science Advances journal, more than 80% of the world’s population live under light-polluted skies. Because of the skyglow, many city dwellers have never seen the Milky Way in their life! People now need to travel pretty far from big cities to see anything more than a handful of stars. Light pollution also makes it more challenging for astrophotographers to take good pictures of the night sky.

How to reduce light pollution?

Сity authorities can fight light pollution by adjusting street lighting and improving the design of light fixtures. There are three main steps that should be taken:

  • Using warm yellow light instead of blue light;
  • Dimming all unnecessarily bright light sources;
  • Shielding light fixtures so that no light escapes into the sky.

Here’s an example of a good light fixture design.

There are also things that you personally can do to minimize light pollution — especially if you live in a house:

  • Turn off lights when you don’t need them;
  • Use warm-colored light bulbs;
  • Switch to LEDs — they waste less electricity;
  • Replace outdoor lights with shielded light fixtures;
  • Install motion sensors on outdoor lamps.

Keep in mind that even small steps in reducing light pollution are worthwhile!


Which country has the highest light pollution?

Singapore is considered the most light-polluted country in the world. Other countries with a high level of light pollution include Kuwait, Qatar, UAE, and Saudi Arabia. See the list of 20 most light-polluted countries.

Which country has the least amount of light pollution?

Chad, the Central African Republic, and Madagascar are the countries least affected by light pollution. Check out a light pollution map.

How much has light pollution increased?

According to a study led by the University of Exeter, global light pollution has increased by at least 49% over 25 years. However, researchers say that the actual increase could be up to 270% because they included only the light visible via satellites in their study.

How many stars can you see with light pollution?

In a dark location, the human eye can see about 2,500 individual stars. If you live in the suburbs, the number of visible stars decreases by ten times — to about 250. From a typical light-polluted city, you might only see about a dozen stars. Read an infographic about the brightest stars in the night sky.

The Brightest Stars and Their Constellations
Learn the brightest stars, their constellations, distance from the Earth, and best time to see! Check out this infographic.
See Infographic

Bottom line: Light pollution is excessive artificial lighting. It creates skyglow, making it hard to see stars in the night sky. It also negatively impacts human health, wildlife, ecology, and economics. Light pollution can be reversed, and anyone can contribute.