How Are Dark Skies Measured?

We know Nevada has dark skies, but just how dark are they? Friends of Nevada Wilderness uses standard professional equipment and practices to monitor the amount of light in night skies on public lands. The data is use to maintain the certification of places like Massacre Rim as some of the darkest skies in the continental United States. But there are many ways stargazers, including you, can measure darkness!

Naked-Eye Measurements

To begin, we’ll look at ways to measure the sky’s darkness using only your eyes. When you look up at the night sky, some objects appear brighter than others. Each celestial object has a magnitude based on its brightness. The star Vega has a magnitude of zero. Brighter objects have a lower magnitude; for example, the planet Venus has a magnitude of negative 4.6, and the sun has a magnitude of negative 26.74, which is very bright indeed! Objects with a positive magnitude are dimmer than Vega; for example, the star Polaris has a magnitude of positive 2.0. The darker the sky, the better you can see stars, but even under very dark skies most people can’t see a magnitude much greater than seven.

Some astronomers use this to describe how dark the sky is; for example, in a bright city, you may only be able to see stars of magnitude 2.0 or brighter. You’re looking at a ‘magnitude two sky.’ A researcher named John Bortle took this a step further and developed a scale to classify dark skies. The Bortle Scale rates the night sky from one, the darkest skies available on earth, to nine, the bright sky of downtown at night. To use the Bortle scale, you check to see if specific stars and other celestial objects are visible. This handy flowchart walks you through the steps.

Three people look for stars in the night sky

Photo by Kurt Kuznicki

Sky Quality Meters

What’s the downside of using our eyes to measure the skies? Everyone’s vision is different! A sharp-eyed seven year old can see stars that an adult might miss. Using a light meter helps eliminate this discrepancy. The Unihedron Sky Quality Meter-Lens is the most common device, and it’s easy to use. Just hold the device above your head and push the button! The meter reports the sky's darkness in astronomer units, or ‘magnitudes per square arc second’ (mpsas). Imagine taking one of the magnitude numbers for a celestial object and spreading it over a small area of the sky- that’s what the meter is reporting. Since dimmer stars have higher magnitudes, the bigger the number on the meter, the darker the sky. To compare: a Bortle Class 1 sky is the darkest possible. That corresponds to stars of magnitude 7.6 - 8.0 visible to the naked eye, and a reading of about 21.75 mpsas from the Sky Quality Meter.

To take proper readings, you need to follow a few rules. Wait until astronomical darkness, when the sun is at least 18 degrees below the horizon, and don’t take measurements when the moon is in view. This website can help you figure out the right time! Wait for a night with no clouds, and take several different measurements with the meter. These guidelines from the Dark Sky Association go over everything, and you can even report your findings here.

Even if you don’t have a sky quality meter, you can use your phone with the help of these apps: Loss of the Night and Dark Sky Meter.

Satellite Readings

There’s one big disadvantage to all of the methods above: you need people to go out and take measurements. It can be hard to gather that kind of data for large areas. Thus, many scientists use satellite imagery to measure the darkness of our night skies. This map shows different levels of light pollution across the globe; black areas maintain the natural brightness of the night sky, while lighter colors have more light pollution. The bright white areas on the map are forty-one times brighter than a natural night sky. Looking at the map, can you find Nevada? Hint: it’s pretty dark!

The downside to using satellites is that they don’t pick up the exact same wavelengths as the human eye. Specifically, blue light is left out of the satellite data, which is an increasing source of light pollution as LED lights become more widespread. You can read more about how the light pollution map was made here; the scientists even used sky quality meter data collected by people like you and me!

No matter how you measure, Nevada has some of the darkest skies in the country! Let everyone know how cool they are with the new Save Starry Skies license plate, and check out these resources if you’re looking for great places to enjoy the skies.

Light pollution map of North America with Nevada outlined