How to Find a Dark Sky Spot and Beat Light Pollution

The single biggest upgrade most stargazers can make costs nothing but a drive. NASA's guide to finding stargazing spots starts with a simple rule: "you need dark skies away from city lights to see more stars." The reason is that faint stars far outnumber bright ones, and only the brightest stars and planets can punch through a glowing city sky.

This guide covers what light pollution actually is, how far you need to go to escape it, the scale stargazers use to rate a site, and the other factors that make one dark spot better than another.

What light pollution is

The National Park Service defines light pollution as "the excess or inappropriate artificial light outdoors" and splits it into three kinds:

• Glare: bright light shining straight into your eyes.
• Light trespass: light spilling into places it was not meant to go.
• Skyglow: "the brightening of the night sky from human-caused light scattered in the atmosphere."

Skyglow is the one that hides the stars. The NPS explains that it lowers contrast so stars and planets become impossible to see, and that light pollution "prevents the human eye from fully dark-adapting." Glare from a single nearby light can ruin your night vision even under an otherwise decent sky.

How far from town is far enough?

NASA's guidance is specific: skyglow fades as you move away from a population center, "so heading outward from town, at least 20 to 30 miles from the city limits, is key." Even then, NASA notes, you will likely still see a glow on the horizon in the direction of the city.

The NPS describes how this works with a rule of thumb called Walker's Law: predicted skyglow from a city grows with its population and falls with distance raised to the power of 2.5. Working through that relationship for the same city:

Moving from	To	Predicted skyglow drops by a factor of about
10 miles	20 miles	5.7
10 miles	30 miles	15.6
20 miles	40 miles	5.7

Every time you double your distance from the same city, the predicted glow falls to roughly one sixth. That is why the last 10 miles of a drive often matter more than you expect.

The Bortle scale

NASA describes the Bortle scale as a way "to help would-be stargazers know how bright the sky is likely to be above a given location." It has nine levels:

• Class 1: the darkest sky you can find on Earth.
• Class 9: the most light-polluted city sky, where you will only see the brightest stars and planets.

NASA's practical advice: "For most of us, getting to a Bortle-class 3 or 4 location is dark enough for a great evening of stargazing." Darker sites usually mean long drives to remote places. NASA adds that various websites and apps map light pollution and Bortle ratings, which makes it easy to scout before you go.

Four things that make a site better

1. Put something between you and the city. NASA suggests that a ridge of mountains between you and town "can make a big difference," as can the far side of a large wilderness area or a body of water.

2. Go up. Higher altitude helps because you get above the dense, low air that holds haze, fog, and smoke. NASA says a spot "a couple thousand feet above the surrounding area" can make for clearer skies.

3. Face away from the glow. A city 30 to 50 miles away may light up one horizon, but NASA points out that you can "spin around the opposite direction" and find a much darker sky. If you want to see something in a particular direction, such as the Milky Way rising in the southeast, choose a site with no large city between you and that horizon.

4. You do not need to be far from roads. NASA notes you can find decent spots close to well-traveled highways, as long as there are not lots of bright lights nearby, because light pollution is a cumulative effect of development.

Check the conditions, not just the map

NASA recommends checking the weather, and also winds, temperatures, and even wildfire smoke. The NPS adds a reason clouds matter so much near towns: haze, dust, and air pollution scatter light further, and "clouds over a city light dome can enhance the sky brightness by as much as 1000 times compared to a clear night." A cloudy night near a city is far brighter than a cloudy night in the wilderness.

The problem is growing

The NPS cites satellite data showing that continuously lit areas brightened by 2.2 percent per year from 2012 to 2016, and citizen scientist measurements showing the average night sky got brighter by 9.6 percent per year from 2011 to 2022, "equivalent to doubling the sky brightness every 8 years." The math checks out: growing 9.6 percent a year for 8 years multiplies brightness by about 2.08.

What you can do at home

If your own yard is your observing site, the NPS notes that light pollution is "completely reversible." Its tips include lighting only if needed, only when needed (timers or motion sensors), and only where needed (shielded fixtures pointed down), plus choosing warm-white or amber light and using the minimum amount necessary.

Key takeaways

• NASA recommends getting at least 20 to 30 miles beyond the city limits to cut skyglow.
• Under Walker's Law, doubling your distance from the same city cuts predicted skyglow to about one sixth.
• A Bortle class 3 or 4 site is dark enough for a great night; class 9 shows only the brightest stars and planets.
• Mountains between you and town, higher altitude, and facing away from the city all improve a site.
• Clouds over a city can brighten its sky up to 1000 times, so check the weather as well as the map.