AstroMap

Interactive space visualization

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Use the Layers panel on the map to toggle these features:

Day/Night
Light Pollution(2016-2024)
Aurora(KP filter)
ISS Tracker
Moon Phase

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Bortle Scale - How light pollution affects the dark night skies

© ESO/P. Horálek, M. Wallner - Representation of the Bortle scale

Bortle Scale

The Bortle Scale is a nine-level numeric scale that measures the night sky brightness at a specific location. It was created by amateur astronomer John E. Bortle and published in 2001 in Sky & Telescope magazine.

1
Class 1 - ExcellentPerfect dark sky, Milky Way casts shadows
2
Class 2 - Typical dark skyZodiacal light visible
3
Class 3 - RuralSome light pollution on horizon
4
Class 4 - Rural/Suburban transitionMilky Way still visible
5
Class 5 - SuburbanFaint Milky Way
6
Class 6 - Bright suburbanMilky Way only visible at zenith
7
Class 7 - Suburban/Urban transitionMilky Way invisible
8
Class 8 - Urban skyOnly bright stars visible
9
Class 9 - Inner cityFew stars visible

What is SQM?

SQM (Sky Quality Meter) is an instrument that measures the night sky brightness in magnitudes per square arcsecond (mag/arcsec²).

Difference between Bortle and SQM:

  • Bortle: Subjective scale based on visual observations (1-9). Easier to use without equipment.
  • SQM: Objective measurement with electronic instrument. Typical values: 16 (city center) to 22+ (pristine sky).

Approximate conversion:

Bortle 1: SQM 21.99+
Bortle 2: SQM 21.89-21.99
Bortle 3: SQM 21.69-21.89
Bortle 4: SQM 20.49-21.69
Bortle 5: SQM 19.50-20.49
Bortle 6: SQM 18.94-19.50
Bortle 7: SQM 18.38-18.94
Bortle 8-9: SQM <18.38

Data updates:

Light pollution data is based on images from the VIIRS (Visible Infrared Imaging Radiometer Suite) satellite. The map is updated annually by D. Lorenz from NOAA data. Use the year selector in the Layers panel to view historical data (2016, 2020, 2022-2024) or play a timelapse.

Light Pollution

VIIRS satellite imagery (2016-2024). Shows artificial sky brightness using the Bortle scale (1-9).

Select year in Layers panel
Data: D. Lorenz Atlas

Aurora Forecast

Real-time aurora probability from NOAA OVATION model. Filter by KP level (0-9) in Layers panel.

Data: NOAA SWPC

Live Satellites

Track the ISS and other space stations in real-time. Positions update every 5 seconds.

More satellites

Day/Night

Live terminator showing sunrise/sunset line. Includes civil twilight transition zones.

Updated in real-time

Tips

  • • Click on the map to see the location coordinates and Bortle class
  • • Use the Layers panel (right side) to toggle overlays and adjust opacity
  • • Enable Aurora layer to see the KP level filter
  • • Enable Light Pollution layer to select historical data year (2016-2024) or play timelapse
  • • Click the moon icon to see detailed lunar phase based on your location
  • • The ISS trajectory shows 45 minutes past and future
  • • Aurora is best visible in low light pollution areas (Bortle 1-3)