Dark Sky Bortle

ATRONOMY – TELECOPE & OPTIC (46) CALCULATOR Dark Sky Bortle A precise tool.
📖
What is the Dark Sky Bortle & How does it work?

The Bortle Dark‑Sky Scale classifies the darkness of the night sky on a numeric scale from 1 (excellent dark‑sky site) to 9 (inner‑city sky). Astronomers use the visual limiting magnitude – the faintest star visible to the naked eye – as a practical proxy for this classification.

Empirical observations have shown a roughly linear relationship between limiting magnitude (mlim) and Bortle class (B). A convenient approximation is mlim ≈ 7.5 − 0.5 B, which can be rearranged to estimate the Bortle class from a measured limiting magnitude.

B = \frac{7.5 – m_{\text{lim}}}{0.5}
B = Bortle class (1‑9)
m_{\text{lim}} = visual limiting magnitude (mag)

Because the scale is discrete, the computed Bortle value is rounded to the nearest integer and constrained between 1 and 9. This simple calculator helps observers quickly assess sky quality and compare sites worldwide.

⚙️
Parameters
Result —
❓
Frequently Asked Questions
What is the Bortle Dark-Sky Scale?
The Bortle Dark-Sky Scale is a numeric scale from 1 (excellent dark-sky site) to 9 (inner-city sky) that classifies the darkness of the night sky based on the faintest star visible to the naked eye.
How do I estimate my Bortle class?
To estimate your Bortle class, use the formula mlim ≈ 7.5 − 0.5B, where mlim is the visual limiting magnitude of stars visible in the night sky.
What does a higher Bortle number mean?
A higher Bortle number indicates less dark skies and more light pollution, typically found in urban or suburban areas.
Can I use this calculator for any location?
Yes, you can use this calculator for any location where you can estimate the visual limiting magnitude of stars visible to the naked eye.
What is the relationship between limiting magnitude and Bortle class?
There is a roughly linear relationship between limiting magnitude (mlim) and Bortle class (B), with mlim ≈ 7.5 − 0.5B, which can be rearranged to estimate the Bortle class.
How accurate is this approximation?
This approximation provides a convenient way to estimate the Bortle class but may not be perfectly accurate for all locations due to various factors like atmospheric conditions and observer experience.
What should I do if I can’t see any stars at night?
If you can’t see any stars at night, your location likely has a Bortle class of 9 or higher, indicating severe light pollution.

Results are for informational purposes only and do not constitute professional advice.