Light Pollution – A Comprehensive Guide for the Backyard Astronomer

Have you put off buying a telescope because light pollution means you can’t see the stars from your backyard?

You don’t have to avoid astronomy as a hobby just because you live in an urban setting or deal with a lot of light pollution. In fact, we’ve written a 3,000 word guide to urban astronomy (the link opens a new tab).

In this article, we’ve given you loads of information about what light pollution is, what do astronomers mean by light pollution, how to measure it, and the best ways of mitigating its effects so you get more from your telescope.

Feel free to read the whole article from start to finish, or use the ‘Table of Contents’ dropdown, above, to go directly to the section which most interests you.

What Is Light Pollution?

Light pollution is the brightening of the night sky that comes from man-made sources of light.

One hundred years ago, you could look into the darkness and easily see the Milky Way. Today, those of us who live in and around cities see much fewer stars. As urban astronomers, it’s harder to find planets and deep sky objects than under a dark sky. 

The International Dark-Sky Association says that the sky over urban areas is 100 times brighter than it would be if it were lit only by stars.

How Is Light Pollution Measured? 

You can see light pollution with the naked eye but there are different types.

According to Astronomy.com, sky glow is the orange illumination that appears near the horizon. It is a combined effect of all of the lights in an area radiating some of their luminosity upward.

Line-of-sight light pollution happens when a bright light in your home or yard prevents the sky from looking as dark as it can.

The Lighting Research Center explains that people only started measuring light pollution fairly recently. Sky glow measurements vary depending on the way the light is distributed from a particular bulb, how it reflects off the ground, the effects of humidity and the interaction of aerosols (tiny particles in the atmosphere) with light. Measurements can change from moment to moment.

Measurements can’t be made from a satellite because they have to be taken from the ground. Although researchers still haven’t come up with a perfect standard for measuring, you have several resources available to you. 

The Sky Quality Meter is an affordable tool (check Amazon price – opens a new tab) that anyone can use to measure the sky’s brightness. Many online skyglow monitoring stations use this or similar light meters.

Some initiatives, such as Globe at Night, crowdsource sky glow observations. The Dark Sky Meter app lets people use their smartphones to collect data about the sky’s brightness.

This isn’t the most effective way of measuring light pollution, but it does give you general information that can help if you’re scoping out a site from which to view the stars at night.

Types Of Light Pollution

Sky glow is one of the primary types of light pollution and the one we normally think about when it affects our astronomy or taking night sky photos.

It is caused by light that reflects off of both the ground and molecules in the sky. It makes the entire sky look lighter and can cause fainter objects, like stars, to disappear from view. 

Most of the glow in the sky is caused by artificial light. However, you can witness natural sky glow when the full moon is present. Dust in the solar system can also cause this phenomenon.

How far away do you need to get from an urban area to find a pocket of darkness?

That depends…

If you’re at a lower altitude, the topography of the landscape will shield some of the light pollution. If you’re on top of a mountain, you’ll be affected by sky glow from cities that are farther away.

Lights from a lower city causing light pollution

A general guideline is to drive about an hour away from a city with a population of 1.5 million people (e.g. San Antonio or Phoenix) and three hours away from a city of 4 million people (e.g. Chicago or Los Angeles).

However, you can still be affected by other types of light pollution no matter where you are. 

Glare is a bright light that affects objects within its view. This is why turning your headlights on can prevent you from seeing the stars even if you’re in the middle of nowhere. 

Many lights are inefficient and cast illumination where they’re not supposed to. They can cause a type of light pollution called light trespass. This is what happens when your bedroom is lit up at night because your neighbor kept their porch light on. 

What do Astronomers Mean by Light Pollution?

Light pollution prevents you from seeing dimmer objects in the night sky. The more light pollution you have, the fewer objects you’ll be able to see, either with or without binoculars or a telescope.

For example, the dimmest objects we can see with the naked eye in a dark location are about magnitude 6. However, look at the same patch of sky in the middle of a city and you may only see the small handful of objects which shine at magnitude 2 or 1 and brighter.

As we explain in our article on stellar magnitude, magnitude is a measurement of the brightness of a celestial object. Although the difference between magnitudes 6 and 2 doesn’t sound like much, it is dramatic.

There are over 8,700 objects that shine with a magnitude down to 6.

Reduce this to magnitude 4, which is normal in the light pollution of a large town, and you can now only see around 900 objects. Try doing astronomy in the center of a big city and you may have just 100 visible objects, which is not enough to navigate around and certainly not much to see.

It doesn’t really matter how bright an object actually is, though. What matters to you is how bright it seems.

Even if a star has an impressive absolute magnitude, it might not appear that remarkable to you if the sky is glowing with light pollution. For example, a really bright magnitude 1 star like Aldebaran in Taurus is very bright under a rural sky but may appear as an unimpressive dim star in the middle of London or New York.

You can see from this that being able to adjust the magnitude visible to you, whether by going to a darker location, shielding visible lights or improving your equipment, will make a big difference to the quality of your astronomy.

Stargazing.net explains that increasing the faintness of stars you can see by one level of magnitude allows you to see three times as many of them. 

If you have never observed the sky without light pollution, you might not know what you’re missing. The video below provides a good portrayal of what you can view in the sky with and without light pollution.

How Society Could Reduce Light Pollution

Many communities have launched efforts to keep light pollution at bay.

In 1991, Toronto initiated the Fatal Light Awareness Program (which is to protect birds rather than stars, but bird watchers and astronomers alike benefit from the aims of the program), and Chicago followed about ten years later. Today, several cities across the United States take measures to educate their citizens about protecting the dark skies. 

Unfortunately, many of these efforts have faded, but there is nothing to stop you (or your local astronomy club) from reaching out to your local government to encourage action. 

There are many ways to make a positive difference. Many beach towns, for example, are changing the way that they mount streetlights. Some are experimenting with adding filters to the lights. Residents in some coastal areas are advised to keep outdoor lights off during turtle hatching season.

One of the easiest ways to reduce your personal light pollution is to shield lights properly. Don’t let light from your yard leak into the sky – put a ‘hat’ on it to make sure all the light goes downwards, avoid using especially bright lights when only soft lighting is required and turn them off when they are not being used.

The color of your lighting matters too. The Illinois Coalition for Responsible Outdoor Lighting demonstrates that red, orange, yellow and amber lighting contributes to sky glow less than blue, green and violet lights.

When you purchase a white LED, you might actually be getting a light on the blue spectrum. This could be contributing to light pollution. Low-pressure sodium lamps and narrow-band amber LEDs are the best options for outdoor lighting if you want to be mindful of sky glow.

As cities begin using LED bulbs in their street lamps, scientists are wondering about how they affect light pollution. Astronomers already know that sodium vapor streetlights are better than mercury vapor when it comes to interfering with the light from cosmic objects.

The Myskyatnight project is counting on locals to analyze whether this is making things better. The data from these observations are helping researchers understand how sky glow is changing around the world, which is a vital step in working out how to mitigate it.

How To Measure Your Light Pollution

Whilst this is all interesting, the reality for us as stargazers is we want to find the best location in our neighborhood from which to view the sky. If you have to use blackout curtains because of the streetlight that shines on your bed at night, you already know that your own property is affected by light pollution. 

But how far do you need to travel to get somewhere you can see the stars, planets and deep space objects?

To measure your neighborhood’s light pollution, start by simply using your eyes. As you walk around, observe areas that are brightly lit and those that are dark. Often parks in urban centers give the darkest sky. If you live more on the fringes of your town, then a trip to the fields will give better seeing than your own backyard.

If all else fails, find shadowy locations in your neighborhood shielded from glare and light trespass. Just be aware that you still might not have a great view of the sky if you’re in an area with a lot of light pollution. 

How Dark Is Your Sky?

The World Atlas of the Artificial Night Sky Brightness was one of the first resources that showed the level of light pollution across the globe. It is a contour map that shows the predicted levels of upward-directed light over most of our planet.

It’s based on satellite data measured in the 1990s and computations of ground measurements. As a ‘first pass’, it is great, but it doesn’t take into account the effects that topography has in blocking some light pollution. Other resources have since been developed as more accurate markers of sky glow.

We recommend using the Dark Site Finder to gauge how your neighborhood fares when it comes to light pollution. On the screen grab below you’ll notice how much brighter the night skies of the east US are compared to the west.

The skies above the eastern US are markedly brighter than in the west

Montana, the Dakotas, Oregon and Nevada have some incredibly dark skies, whereas Maryland, New Jersey, New York and Massachusetts have some of the most light-polluted skies in the world. We’ve compile a list of the darkest skies in each of the 50 US States (link opens a new tab).

Yet, even in these places, there are patches of darkness for good astronomy. Use the zoom function to find your town and zoom into your neighborhood to find the darkest sites closest to you. Anywhere grey or white is to be avoided, but even in a ‘red’ area, there are enough objects visible for quality astronomy to take place.

If you are looking to pin down an actual street where you can get your telescope out, then the Light Pollution Map is a more detailed tool for pinpointing light pollution issues. It works by selecting different overlays to go on a base map of your neighborhood.

Our recommendation is to choose the newest VIIRS overlay to view the data picked up by satellites. This provides a measure of the light escaping into the atmosphere around you. 

There is an SQM data overlay you can choose too. This is created by people all over the world entering data from their Sky Quality Meters. It is nowhere near as comprehensive as the VIIRS overlay, but a useful indicator nevertheless.

Yellow, green and blue areas are best for astronomy. Town centers tend to be orange and red and here the light pollution is severe enough to limit what you can see.

Use The Bortle Scale

You might be happy with where you do your astronomy already but would like a way to ‘grade’ the quality of your sky’s brightness. That is exactly what the Bortle Scale is: a standard for measuring the night sky’s brightness. It helps you identify the light level based on the celestial objects you can see. 

Understanding the Bortle Scale can help you predict what you’re going to be able to see using your naked eye, a telescope or another device. When you’re recording unusual events or objects, such as an unusually long comet tail, you need precise benchmarks for gauging sky conditions.

The Bortle Scale can also help you understand what other backyard astronomers are talking about when they tell you about a secret pocket of dark sky in your town.

If someone informs you that they’ve found a truly dark location but describe what they can see, you can evaluate the information based on the Bortle Scale and compare it to your current location. 

Examples of Bortle Scale Measurements

The full Bortle Scale (click here to open it in a new tab) grades the sky in colors from gray/white for the worst light pollution to black for the best. The grade is given based on the faintest object that can be seen from your location.

To make it easier for the average backyard astronomer to grade the sky, guidance is also given on the visibility of the Milky Way, Astronomical Objects and Night Time Scene. Selected examples of the Bortle Scale are:

• Blue = Rural Sky. Naked eye limiting magnitude (see below) is 6.6-7.0. The Milky Way appears complex with dark voids and light patches visible. M33 is only visible with averted vision but Andromeda Galaxy (M31) is clearly visible [to the naked eye]
• Yellow = Suburban Sky. Naked eye limiting magnitude is 5.6 to 6.0. The Milky Way is ‘washed out’ overhead and invisible at the horizon. M31’s oval shape is visible and so is the glow of Orion Nebula.
• Red = Suburban / Urban Transition. Naked eye limiting magnitude is 4.6 – 5.0. The Milky Way is completely invisible, even directly overhead. Andromeda is rarely glimpsed but the brighter constellations are still easy to recognize.

Traditionally, people have used the naked-eye limiting magnitude scale, or NELM, to communicate their measurements of dark sky brightness.

NELM describes the magnitude of the faintest star that you can see without a telescope. However, everyone has a different quality vision, i.e. some people’s eyesight is better than others, so NELM estimates are subjective.

The Bortle Scale is based partly on NELM. However, it introduces other criteria, such as whether clouds look dark or light, to help individuals standardize their measurements of dark skies. You can assign the sky a certain level of brightness based on what you measure using the scale.

Despite what you might think, there are some fabulous deep sky objects to see from a Bortle 7 sky.

How To Combat Light Pollution

Even if you live in a populated area with major sky glow, you can become a backyard astronomer. The right location and equipment will help you explore the cosmos as effectively as possible. 

Telescope Filters

Many telescope filters can enhance your night vision. Broad-band light pollution reduction, or LPR, filters are designed to improve the visibility of deep-sky objects by blocking out some of the wavelengths that contribute to light pollution. This screens out some of the sky glow and enhances the contrast of the objects that you’re looking at.

Whilst not all of the sky glow is eliminated, getting rid of some of it will bring better views to your eyepiece. Broad-band filters deliver the best results with emission nebulae. They can also improve the contrast of reflection nebulae and diffuse galaxies. 

Some of the objects whose visibility can be improved with an LPR filter are:

• The Orion Nebula
• The Lagoon Nebula
• The Merope Nebula
• The Trifid Nebula

An LPR filter delivers modest results for enhancing the view of the following galaxies:

• M33
• M81
• M101
• NGC 253
• NGC 2403

You may not see star clusters any better when using an LPR filter because their emissions fall within the range of wavelengths that are blocked by these filters. In fact, if you are in a dark site, you might notice that your view is dulled when you use a filter. That’s because some of the useful wavelengths, which allow you to view certain objects, are filtered out with the sky glow.

Narrow-band nebula filters are designed for viewing emission nebulae. They dim even more wavelengths than the broad-band filters. However, if these are the objects you’re interested in viewing, you’ll be able to see them more clearly in mild to moderate light pollution if you use a narrow-band filter.

With a nebula filter, you should be able to see objects that would otherwise be indiscernible. Even when viewing nebulae that are visible without filters, you’ll be able to distinguish more details and contrast with a nebula filter.

This is one type of filter that you might want to use at a dark-sky site. You’ll get better results when using a nebula filter with minimal sky glow. However, it will still help you make out certain objects in a light-polluted neighborhood.

Line filters block out the most light pollution. They also reduce light from the cosmos. You can use these in places with extreme sky glow, but you’ll also limit what you can see in the deep sky.

Myths About Using Filters

1. LPR filters completely eliminate light pollution.

While LPR filters can dim sky glow significantly, they can’t get rid of all of the light pollution. They don’t get rid of wavelengths emitted by LED lights, which are being used in many communities. LPR filters also don’t dim the light from incandescent bulbs. 

2. Filters work best when there is more sky glow.

All filters work better in darker areas. However, because some stars emit the same wavelengths as street lamps and other artificial lights, you’ll dim their glow by using certain filters. Therefore, you might not want to use an LPR filter in a dark area if you’re trying to see certain objects. 

3. Filters enhance the brightness of nebulae.

Filters can’t make a deep-sky object brighter. They actually make things darker. It is improved contrast against the background which makes the object seem brighter. LPR filters can make nebulae pop out against a backlit sky and can improve detail around the periphery of the nebulae.

4. Filters will help you see stars. 

An LPR filter won’t help you see galaxies, stars or star clusters better. It also won’t help when you’re viewing the moon or planets.

A narrow-band filter actually reduces the visibility of blue-green stars. A broad-band filter tends to let you see stars’ natural colors.

Use The Right Equipment

You don’t need a huge, expensive telescope to view the sky even if you live in an urban area. This Astronomy.com article compares the pros and cons of a smaller, easy-to-use telescope with a more expensive, complex one. The more you use your telescope, the better you’ll get at identifying objects. 

Even if your complicated equipment lets you see more deep-sky objects, you’re not going to benefit from it if you only pull it out once a year. Therefore, you should choose your telescope’s aperture based on your living situation and storage accessibility as well as the light pollution in your area. 

Computerized telescope mounts can help you locate targets when reference stars are washed out by sky glow. These electronics also increase the price of a telescope, though, so you’ll end up with a smaller aperture than for the same price without motorization.

Add a tube extension to block stray side light. This will especially increase contrast if your telescope is a Newtonian reflector. 

You can even make your own barrier if nearby line-of-sight lighting is interfering with your hobby. Many people create viewing shields from opaque tarps and PVC pipes. You may even consider planting a tree or bush to block out your neighbor’s porch light.

The quickest way to improve that situation though is to politely ask them to turn it off (offer to show them the rings of saturn as a sweetener).

Follow Proper Lighting Practices

The best place to start fighting light pollution is on your own property. Evaluate your outdoor lighting to make sure that it is properly shielded. 

One way to do this is to draw an imaginary line that extends horizontally from the top of your lamps. If any rays shine above that line, the light is not well shielded. 

Those who have spotlights or floodlights should check for light trespass. Are you illuminating your own property, or is your light spilling into your neighbor’s yard? Adding shields to your own lights can help reduce irritating glare and reduce sky glow. 

Globe lights are notorious for causing glare. Many communities still use these because they look quaint and charming. If they don’t have coverings above the light, there is no safeguard against light pollution.

You can also make sure that lights are installed under overhangs when possible. Fixtures should be placed low enough to provide efficient illumination. A light that is installed too high isn’t always energy efficient and can produce light trespass issues. 

Turn off your lights after 11 p.m. or when you’re not using them. Motion sensors can reduce sky glow and save you money on your energy bills while keeping your property safe. 

Finally, avoid using landscape lighting that points upwards. Many people shine lights on flagpoles, trees and the exterior of their homes. If a light is so bright that you can see the shadow of the illuminated object in the sky, you are contributing to light pollution.

Find A Dark Sky Park Or Community

If your own backyard isn’t perfectly suited to stargazing, look for a local spot that is. According to CNN, there are 22 official dark sky areas in the world. 

Dark Sky Parks and Dark Sky Reserves are titles awarded to communities that take measures to preserve the quality of the night sky. The intent is to highlight accessible dark sky areas. 

USA today also highlights some of the best places to stargaze. If you live in the UK or Europe then check out Dark Sky Discovery which says that’s where you’ll find some of the largest expanses of dark sky in the northern hemisphere.

If you can’t make it to one of these areas, find the darkest place in your neighborhood. The top of a high-rise building is ideal because it sits above most sources of street-level lighting. However, heat that rises from a roof can distort the view.

A grassy area in a local park may be far enough away from lighting sources to help you view the night sky. Always keep safety in mind, though. A dark park may not be the most secure place to hang out at night. 

Joining a local astronomers club can help you locate the best places to set up at night. You may even create safety in numbers by heading to local dark spots together.

If you are fortunate enough to find a dark sky location, check out our guide to the astronomy phenomena that need a dark sky for you to see them.

Start A Dark Skies Group

Dark Skies groups have popped up in some communities to educate local citizens and officials about light pollution and help reduce unnecessary light emissions. The International Dark-Sky Association gives tips for starting a group like this where you live. 

The first step to taking action is to educate yourself. Many people are worried about making changes to outdoor lighting because they’re concerned about safety. When you know the best way to shield lights while providing adequate illumination, you can share the information with your community. 

You might be more persuasive with numbers on your side. Therefore, getting together with like-minded individuals can help lawmakers take notice. Using social media and contacting reporters can help increase awareness of the problem. 

Arm yourself with details to make reducing light pollution easier for others. Put together a vendor list of dark-sky-friendly fixtures. Find out if your power company offers discounts to customers who convert to energy-efficient products.

Many people are persuaded by money. Explain that reducing unnecessary lighting can lower electricity bills. This may be all the encouragement that property owners and commercial businesses need to change their lighting practices.

Dark Skies Inc. of the Wet Mountain Valley says that most local ordinances have been put in place after community activists raised awareness of the problem. One person can make a difference when it comes to combating light pollution.

Helpful Tips To Keep Your Night Vision

Part of being an astronomer is making sure you are as prepared as possible for your telescope viewing and, in part, that means making sure your eyes are adjusted to see the faintest objects they can, i.e. getting your ‘night vision’ in place.

It takes up to half an hour for your eyes to fully adjust from seeing in light to seeing in darkness. And if you make the mistake of looking at a bright light after you are dark adapted it goes in a second… and you have to begin the 30-minute adjustment all over again.

So, it makes sense to protect your night vision.

When you’re getting ready to view the sky, turn off as many lights as you can. Even the glow from the windows of your house can interfere with your viewing. You can also take some other steps to optimize your night vision.

Time Your Viewing

Sky and Telescope explain that where you look is important when you’re trying to view celestial objects at night. Sky glow is minimal when you look straight up because there is less atmosphere between you and what you’re looking at. 

Air scatters light, which makes it harder to see what you’re trying to view. Therefore, time your observations to look for faint objects when they’re highest in the sky.

Pull Out The Telescope On Clear Nights

The color of the daytime sky can help predict the darkness of the night. Bright blue near the horizon forecasts a darker nighttime sky. 

If the sky is relatively white during the day, there are more tiny particles scattering sunlight. These particles will also scatter atmospheric light when the sun goes down. High humidity has the same effect. You’ll have a better view if you try to see the stars on a dry day.

Adapt Your Eyes To The Dark

One-Minute Astronomer explains that the human eye evolved with two modes. When your eyes are in the photopic mode, the cones in the retina respond to bright lights, and the rods are not used to pick up much light at all. In the scotopic mode, the rods in the retina help you see low-intensity light.

When they’re exposed to brightness, though, the rods get bleached. They can take up to an hour to regain their sensitivity. Therefore, it’s important to adapt your eyes to the darkness to get the most out of your night vision.

If you’re getting ready to stargaze, don’t use a white flashlight to search for gear or look at sky maps. Even the glow from your phone can bleach the rods and prevent you from seeing well in the dark. Some backyard astronomers use red LED flashlights (find yours on Amazon – link opens a new tab) in the dark. Red light with a wavelength greater than 620 nanometers can’t bleach your rods.

You can also keep one eye dark adapted by covering it with an eye patch. Use that eye to look through the telescope. Block out stray light by putting a towel over your head while you’re checking out the cosmos.

Where To Find Out More

There are hundreds of resources available to help you learn more about and manage light pollution. We have compiled a convenient list for you below. 

• International Dark-Sky Association – Comprehensive resource with a wide range of information about light pollution, combating it and avoiding it as a backyard astronomer. Use the website to find a local chapter or find out how to start one in your community.
• Globe at Night – International citizen-science campaign to improve public awareness of the impact of light pollution. Learn about observing constellations and report your observations to help the organization collect data.
• World Atlas of Light Pollution – A print resource that shows the spread of light pollution on Earth.
• YouTube – A quick search for “light pollution” on YouTube brings up about 920,000 results. The short film below imagines unpolluted skies above the world’s big cities.
• Mobile Apps – Some apps help you measure the night sky yourself. The Dark Sky Meter app lets people submit measurements to be used for scientific research. The Loss of the Night app compliments the Globe at Night project and lets you tag visible stars to provide a measurement of sky brightness.
• Astronomy Forums – Cloudy Nights has a light pollution forum where people can share ideas relating to the topic. The Society for Popular Astronomy also has a forum where you can ask questions and find resources about light pollution.