Light Pollution

Overview of the Problem:

The image above shows North America at night as seen from orbit. If you’d like to see the entire global image from which this North American image was cropped, just click on it. It vividly shows how much artificial light is lost globally straight up into space.

An estimated $1 billion of electricity is wasted each year by bad lighting in the United States alone. The purpose of night-time lighting is, with few exceptions, to illuminate objects on the ground. Properly shielded lighting fixtures (now required in many municipalities) minimize the upward escape of light. Light that escapes into the sky creates light domes over cities, reducing contrast and limiting the visibility of stars and planets. Here’s what the light dome over Phoenix looks like from ground level:

This is what astronomers call light pollution. And this is why my byline proclaims: The only good night lights are billions of miles away.

What Can Be Done:

Truth be told, the vast majority of light pollution comes from municipal street lighting and commercial lighting (billboards, parking lots, marquees). This is where state and local building codes need to act. Arizona, because of its huge astronomy industry, has enacted some of the most comprehensive lighting ordinances in the country. When you consider there are almost 5 million people living under the Phoenix light dome, the city hasn’t done too bad. But Tucson and Flagstaff, where the large telescopes reside, have done much better.

At the individual level, simply eliminate unnecessary outside lighting at night. If you’re concerned about security, use motion detectors to activate outside lights. For those lights you just can’t do without, ensure they are properly shielded fixtures using bulbs of no higher wattage than needed. To learn more about light pollution and proper light shielding, visit the website of the International Dark-Sky Association (IDA) at:

OK, so you’ve done what you can with your own lighting, but what if your neighbor does not? The documents below will give you some ideas about how to engage a neighbor with an offending yard light. The two contain essentially the same information, but the second is a bit more technical. Before you file a complaint with Planning and Zoning, try to start a friendly dialog first.

From UW Madison: Neighbor-1.pdf (425k)
From IDA: Neighbor-2.pdf (1372k)

Arizona Solutions:

Dark skies are a precious and endangered natural resource. Astronomy brings millions of dollars into our state each year via professional research and tourism. To protect our dark skies, lighting ordinances have been enacted in many jurisdictions. If you feel you are a victim of light trespass, there may be a remedy. You need to familiarize yourself with the law. It is up to you to know your rights. It is the responsibility of your local Planning Department to enforce these lighting laws. Links to some Arizona ordinances in my home county of Maricopa are provided below:

Arizona Revised Statutes, Title 49 – The Environment, Chapter 7 – Light Pollution: ARS-49-7.pdf (64k)
Maricopa County Zoning Ordinance (lighting): MCZO-1112.pdf (53k)
Maricopa County Zoning Ordinance (signage): MCZO-1401.pdf (516k

Here are the phone numbers of contacts for reporting bad streetlights or insensitive neighbors. If your problem is with a nearby streetlight that intrudes on your property, you can request a light shield to be installed and better direct the light (a generous option provided at no cost to you). Again, this is for Maricopa County residents:

City of Phoenix street lighting request line: 602.495.5125
APS street lighting: 602.371.6134
Maricopa County Planning & Development: 602.506.3011

Or, if you’re really in a hurry, and in Maricopa County, you can submit a violation complaint form online at:

Wherever you live, you can find links to your local lighting ordinances here:

Complaint Confidentiality:

Complaints about code violations are a matter of public record. Although normally classified as “confidential,” the name of the complainant will be released if so ordered by a court of law. The online complaint form requires both your name and email address. However, if you are truly paranoid, be aware that the Maricopa County site does state they act on anonymous complaints as well. Of course, this would require either a letter or a phone call on your part.

The Worst Lights:

The single worst offending light is what we call D2DHg (dusk-to-dawn mercury vapor) lights. Their blue spectrum causes the light to scatter strongly in air, contributing to overall sky glow even when not line-of-sight. These lights are prohibited by law as of Jan 1 2011, but there are still many in use. Replacing these offenders are unshielded incandescent and halogen lamps. In Arizona, ANY outside light brighter than 150 watts is required to be shielded. But many are not. In 1997 I wrote an article entitled “The Top 10 Reasons for Not Installing a Dusk-to-Dawn Yard Light.” It was first published in The Desert Advocate newspaper that same year, and reprinted many times since. Even after all the D2DHg lights are gone, the environmental impact of bad lighting will persist. You can get that article, copyright-free, distribution unlimited, here:

BadLight.pdf (35k)

Measuring Light Pollution:

It can be said that you know light pollution when you see it. Nonetheless, there is a relatively inexpensive device manufactured by Unihedron that can measure light pollution quantitatively. It’s called a Sky Quality Meter (SQM) and can be had for $135 (as of 2021). It looks like this:

To take a measurement, just aim the meter straight up at the sky, push the button, and you get a reading in seconds. You almost have to be an astronomer to know what that number 19.66 actually means, and it’s explained in the manual, but for now, suffice it to say, higher numbers are darker skies. Here’s a quick primer with examples of a range of possible readings, as well as those from my home:  SQM-2.pdf

I’m pleased to report that, even with the encroachment of Phoenix, my skies remain well above 20 on the SQM. I can still see the Milky Way and enjoy meteor showers. How long that will last I can’t say, but that’s why I monitor readings once or twice each year.

Additional Resources:

The IDA has a page that lets you search for approved outside lighting fixtures. If you’re looking to add or change an outside light, go here:

I’ve created a PowerPoint lesson on light pollution. Notes are not yet included, but if you know enough about light pollution to want to use this in your own presentation the slides will be self-explanatory. It’s obviously Arizona-centric, and was created in 2016, but it’s easily adapted and all still relevant. You can download it here:  LP.ppt (14.6 MB)

For a more scientific discussion of what really qualifies as “dark” skies, see this excellent explanation by Brian Skiff of Lowell Observatory in Flagstaff: