Bortle Scale Chart: 2023 Easy Guide for Amateur Astronomers

By Chris Klein

Bortle Scale Chart

Are you tired of not being able to see the stars at night? Do you live in an area with bright city lights that make it impossible to stargaze? If so, you’re not alone. Many people struggle with light pollution, making seeing celestial objects in the night sky challenging. That’s where the Bortle Scale sky chart comes in.

The Bortle Scale is a handy numerical tool that allows for the measurement of the night sky’s brightness in a given location. With its nine distinct levels, this scale can accurately determine the extent to which you can observe celestial objects and the level of interference caused by light pollution. By providing this valuable information, the Bortle Scale enables us astronomers and stargazers alike to make informed decisions about where and when to observe the night sky, ensuring that we can fully appreciate the majesty of the universe in all its glory. 

In this article, you get

To learn how the Bortle Dark Sky Scale measures night sky quality

How to use the Bortle Scale to find the sky darkness in your area

Details about each class on the Bortle Scale Chart, from inner city sky to excellent dark sky sites

Examples of what you can observe in each Bortle Class

By the end of this article, you’ll know about the sky quality meter of the Bortle Scale and how to use it to find the best locations for exploring the night sky.

Let’s dive right in.

What Is the Bortle Scale Chart? Overview

Light Pollution Map of USA

The Bortle Scale Chart is a tool used to measure the night sky’s brightness at a particular location. It was created by John Bortle, an amateur astronomer, to help other amateur astronomers evaluate the darkness of the sky and the visibility of celestial objects.

The Bortle Scale Chart has nine levels, with Level 1 being the darkest and Level 9 being the brightest. The amount of light pollution in the sky and the visibility of celestial objects determines the levels. The lower the level, the darker the sky and the better the visibility of celestial objects.

Here is a brief overview of the nine levels of the Bortle Scale Chart:

  • Level 1: Excellent dark-sky site
  • Level 2: Typical truly dark site
  • Level 3: Rural sky
  • Level 4: Suburban sky
  • Level 5: Inner-city sky
  • Level 6: Bright suburban sky
  • Level 7: Suburban/urban transition
  • Level 8: City sky
  • Level 9: Inner-city sky

If you’re an amateur astronomer like me, you can use the Bortle Scale Chart to help you observe the night sky. It is a valuable tool for planning observing sessions, choosing observing sites, and evaluating the quality of a particular observing site.

I describe the chart’s nine classes (levels) in each of the following sections.

Class 1: Excellent Dark-Sky Site

Class 1 is the darkest level of the Bortle Scale Chart. It is for the darkest and most pristine skies on Earth. Class 1 offers you the best possible views of the Milky Way. There will be no light pollution, and the sky will be incredibly dark.

In Class 1, with the unaided eye, you’ll see:

  • the Zodiacal Light
  • Gegenschein
  • Zodiacal Band
  • Galaxies
  • Nebulae

The International Dark Sky Association has designated many Dark Sky Places in Class 1 zones, thanks to these spots having the darkest sky possible.

Read my articles on some of the best Dark Sky Places for exploring the night sky:

Class 2: Typical Truly Dark Site

Class 2 areas are also very dark but may have minor light pollution, such as a distant town or highway. The Milky Way is also clearly visible in Class 2. Class 2 is still an excellent location for observing the night sky. You’ll be able to see many of the same objects as Class 1, but the sky won’t be quite as dark. There may be some light pollution from nearby towns or cities, but it won’t be enough to impact your naked eye observations significantly.

Class 3: Rural Sky

Class 3 areas are rural or suburban. You’ll still be able to see many of the same objects as Class 1 and Class 2, but they may not be as bright or clear. The Milky Way is still visible, but it may be faint. Class 3 is still an excellent location for observing the night sky, but there will be light pollution from nearby sources such as street lights or buildings.

Class 4: Rural/Suburban Transition

Class 4 areas are transitional zones between rural and suburban areas. They have moderate light pollution from nearby towns or cities. In this class, you can see the Milky Way with some difficulty, and some deep sky objects like the Andromeda Galaxy are visible. However, light pollution from nearby cities may still be visible on the horizon.

Here are some characteristics of class 4:

Class 5: Suburban Sky

Class 5 is the Suburban Sky, with significant light pollution from street lights, buildings, and other artificial light sources. In this class, the Milky Way is no longer visible. Only the brightest stars and planets are visible.

Here are some characteristics of class 5:

  • Milky Way is not visible
  • The limiting magnitude is around 5.0
  • Only the brightest stars and planets are visible
  • Light pollution from street lights and buildings

Class 6: Bright Suburban Sky

Class 6 areas are bright with high light pollution from suburban street lights, buildings, and other sources. The sky is bright, and only the brightest stars are visible. In this class, the only celestial objects visible are the Moon, planets, and a few bright stars.

Here are some characteristics of class 6:

  • Only Moon, planets, and most luminous stars are visible
  • The limiting magnitude is around 4.5
  • Heavy light pollution from street lights, buildings, and cars

If you live in a class 4-6 location, there are still ways to enjoy stargazing. You can use a telescope or binoculars to observe bright objects like the Moon and planets. You can also find a darker location nearby, like a park or nature reserve, to escape the worst of the light pollution.

Class 7: Suburban/Urban Transition

Class 7 areas are transitional zones between suburban and urban areas. This class represents areas on the outskirts of urban or suburban areas. They have high levels of light pollution from many sources of artificial light. The sky is bright, and only a few stars are visible. While some dark areas may exist, the overall sky brightness is still relatively high. You may be able to see some of the brightest stars and planets, but fainter objects will be difficult to observe.

Here are some characteristics of Class 7:

  • Sky brightness: 18.0-19.0 mag/arcsec²
  • Visible stars: 4th-5th magnitude
  • Limiting magnitude: 5.5-6.0
  • Milky Way: invisible
  • Nebulae and galaxies: very difficult to see

Class 8: City Sky

Class 8 areas are urban areas with very high levels of light pollution. The sky is very bright, and only the brightest planets and stars are visible. Class 8 represents areas that are within a city or town. The sky brightness is exceptionally high.

Here are some characteristics of Class 8:

  • Sky brightness: 19.0-20.0 mag/arcsec²
  • Visible stars: 3rd-4th magnitude
  • Limiting magnitude: 5.0-5.5
  • Milky Way: invisible
  • Nebulae and galaxies: impossible to see

Class 9: Inner-City Sky

Class 9 areas are the brightest and most light-polluted areas on the Bortle Scale Chart. They are located in the heart of major cities and have incredibly high levels of light pollution. These areas are in the heart of a city or town, and the sky brightness is so high that only the Moon and a handful of the brightest stars are visible.

Here are some characteristics of Class 9:

  • Sky brightness: >20.0 mag/arcsec²
  • Visible stars: 2nd-3rd magnitude
  • Limiting magnitude: 4.0-4.5
  • Milky Way: invisible
  • Nebulae and galaxies: impossible to see

It may seem impossible to observe the night sky in these high Bortle Scale classes, like downtown Los Angeles, but you can still do some things to enjoy the night sky. Try stargazing early in the morning hours when fewer lights are on. You can also use a telescope or binoculars to observe the brightest objects in the sky, such as the Moon, planets, and bright stars.

In summary, the Bortle Scale Chart is a helpful tool for evaluating the darkness of the night sky. It helps amateur astronomers and stargazers like you and me choose the best observing locations and plan our sessions accordingly.

How to Use the Bortle Scale Chart

The Bortle Scale is a valuable tool that can help you determine the quality of the night sky and plan your observing sessions accordingly.

Benefits of Using the Bortle Scale

One of the most significant benefits of using the Bortle Scale is that it can help you find dark-sky locations. The lower the Bortle number, the darker the sky. For example, if you’re looking for a place to observe the Milky Way, you’ll want to find a location with a Bortle number of 3 or lower.

The Bortle Scale can also help you plan your observing sessions by showing what objects you can see. For example, if you’re in a location with a high Bortle number, you may not be able to see faint objects like galaxies or nebulae.

Limitations of the Bortle Scale

While the Bortle Scale is a helpful tool, it does have its limitations. For one, the Bortle Scale only measures the brightness of the night sky and doesn’t consider other factors like weather or atmospheric conditions to determine if it’s clear outside. Additionally, the Bortle Scale is subjective and can vary depending on who is making the observation.

You’ll need to find your location on a light pollution map. Once you have your location, look for the corresponding Bortle number by clicking on the light pollution level of your site. Use the Bortle number to determine the brightness of the night sky and plan your observing sessions accordingly.

Here are some actionable tips for using the Bortle Scale:

Improving Light Pollution Awareness

what is light pollution

Light pollution is a growing problem affecting our view of the stars, our health, and the environment. It’s caused by excessive artificial light, which can harm nocturnal animals and disrupt their natural behavior. As an amateur astronomer, you can help raise awareness about light pollution and its effects.

It’s vital to preserve dark skies for future generations. Reducing light pollution can improve our view of the stars, protect nocturnal animals, and reduce energy waste.

Here are some actionable tips to help you reduce light pollution and preserve dark skies:

  • Use outdoor lighting fixtures with shielding to prevent light from shining upwards.
  • Use motion sensors or timers to turn off lights when unnecessary.
  • Encourage your community to switch to energy-efficient lighting, such as LED bulbs.
  • Support local efforts to reduce light pollution and preserve dark skies.

Bortle Scale in Amateur Astronomy

As an amateur astronomer, understanding the Bortle Scale is crucial in determining the best observing conditions for stargazing.

Here are some interesting points you should know about the Bortle Scale:

  • The Bortle Scale was created by John E. Bortle in 2001.
  • The scale measures how visible celestial objects are and how much they are affected by light pollution.
  • The scale considers the naked-eye limiting magnitude, the sky’s background brightness, and the visibility of the Milky Way.
  • Astronomers widely use the Bortle Scale to determine the quality of the observing site.

As an amateur astronomer, you should observe from a location with a lower Bortle Scale rating to get the best observing conditions.

Here are some tips to help you make the most of the Bortle Scale:

  • Use the Bortle Scale to find a dark observing site. Look for areas with a Bortle Scale rating of 3 or lower.
  • Avoid observing from areas with a Bortle Scale rating of 7 or higher. These areas are heavily light-polluted and will make it challenging to see celestial objects.
  • Consider traveling to a dark sky site to observe the night sky. Many national parks and remote areas have low Bortle Scale ratings and offer excellent observing conditions.

Understanding the Bortle Scale is essential for amateur astronomers who want to observe the night sky. By using the scale to find a dark observing site, you can enjoy the beauty of the night sky and see more celestial objects.

Astrophotography

The quality of your astrophotography depends on many factors, including your equipment, your skills, and the conditions in your location. But by using the Bortle Scale to find the best site for your astrophotography, you can increase your chances of capturing stunning night sky images.

Here are some tips for using the Bortle Scale in astrophotography:

  • Choose a location with a low Bortle Scale rating (Class 1-3) for the best astrophotography results.
  • If you can’t travel to a low Bortle Scale location, try to find a spot with a clear view of the sky facing toward a lower Bortle Scale Class.
  • Use a camera with a high ISO sensitivity and a fast lens to capture images of the night sky.
  • Try varying your exposure times to achieve the ideal balance between capturing sufficient light and preventing overexposure.
  • Use a tripod to keep your camera steady and reduce the risk of blurry images.
  • Consider using a tracking mount to keep your camera pointed at the same spot in the sky as the Earth rotates.

Summary: Bortle Scale Chart

Thank you for reading my article “Bortle Scale Chart.” If you’re looking to stargaze, understanding the Bortle Scale Chart is crucial. The chart measures the sky’s brightness and helps you determine the quality of the night sky for stargazing. The lower the number, the darker the sky, and the better the stargazing experience.

To use the chart, start by finding your location’s Bortle Class ranking, which ranges from 1 to 9. Class 1 represents the darkest skies, while Class 9 indicates the most light-polluted skies. Once you know your location’s ranking, you can use the chart to identify what celestial objects you can observe at that location.

For example, if you’re at a Class 1 location, you can expect to see faint objects such as the Milky Way’s dust clouds and galaxies. At a Class 5 location, you can observe brighter objects such as the Andromeda Galaxy and the Orion Nebula. However, at a Class 9 location, you may only see the brightest stars and planets.

If you’re looking for a genuinely awe-inspiring stargazing experience, try to find a location with a low Bortle Class ranking. Using the Bortle Scale Chart can help you plan your stargazing trips and ensure the best possible experience. Remember, the darker the sky, the more celestial objects you’ll be able to see.

About the Author

Chris Klein

Chris Klein is an enterprising astrophotography adviser. Go here to read his incredible story "From $50,000 in Debt to Award-Winning Photographer Living in Switzerland". If you want to send Chris a quick message, then visit his contact page here.

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