What is the Bortle scale?

The Bortle scale is a nine-level numeric scale created by John E. Bortle in 2001 to measure the darkness of the night sky. Class 1 is the darkest sky measurable on Earth; Class 9 is an inner-city sky with very few stars visible.

What does limiting magnitude mean?

Limiting magnitude is the faintest star magnitude visible to the unaided eye. Magnitude 6.5 corresponds to about 9,000 stars visible; magnitude 4.0 to around 500. Darker skies have higher limiting magnitude numbers.

Is this estimate perfectly accurate?

No. The Bortle class is an approximation that depends on seeing conditions, humidity, and individual eye sensitivity. This tool produces a reasonable estimate; for scientific accuracy use a Sky Quality Meter (SQM).

How do I improve my Bortle class?

Travel away from city cores, gain elevation, observe during new moon, and choose nights with low humidity. Each step away from a major metro area typically drops 1-2 Bortle classes.

What is a good Bortle class for astrophotography?

Bortle 4 or below is excellent for wide-field Milky Way shots. Narrowband imaging can be done from Class 6-7 cities; broadband deep-sky imaging benefits from Class 3-4 or darker.

Light Pollution Helper

Name: Light Pollution Helper
Availability: InStock
Author: Nham Vu

Enter your sky observations to estimate the Bortle class and get tips for finding darker skies.

Sky Observations

Faintest star visible (limiting magnitude)

Look at a familiar star field for at least 5 minutes. What is the faintest star you can consistently see?

Milky Way visibility

Zodiacal light (spring/autumn evenings)

Light dome / horizon glow

Fill in your sky observations and click Estimate Bortle Class.

Summary

Enter your sky observations to estimate the Bortle class and get tips for finding darker skies.

How it works

Select the faintest stars you can see with the naked eye (limiting magnitude).
Rate the visibility of the Milky Way core from your site.
Indicate whether zodiacal light or airglow is visible.
Note any horizon glow from nearby towns or cities.
The tool weighs all inputs and returns an estimated Bortle class with a description.
Use the tips panel to identify steps for reaching a darker site.

Use cases

Rate a new observing site before a long drive.
Document sky quality changes over months or years.
Plan astrophotography targets that match your sky class.
Compare rural camping spots for star-party suitability.
Educate students about the impact of artificial light pollution.
Determine whether a goto telescope mount needs bright alignment stars.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu