The full Bortle scale, class by class
| Class | Name | Limiting magnitude | Milky Way appearance | Ontario examples |
|---|---|---|---|---|
| 1 | Excellent dark sky | 7.6–8.0 | Casts visible shadows. Zodiacal light is obvious. | Wabakimi, Polar Bear Provincial Park (extreme north) |
| 2 | Typical truly dark site | 7.1–7.5 | Highly structured, complex detail visible. | Killarney, Quetico, Pukaskwa, Lake Superior north coast |
| 3 | Rural sky | 6.6–7.0 | Bright with structure. Galactic core obvious. | Torrance Barrens, Algonquin interior, North Frontenac |
| 4 | Rural/suburban transition | 6.1–6.5 | Clear but loses fine detail. Light domes visible on horizon. | Algonquin Hwy 60 corridor, Bruce Peninsula tip |
| 5 | Suburban | 5.6–6.0 | Pale, washed out near horizon, visible overhead. | Small towns: Bancroft, Parry Sound, Wawa |
| 6 | Bright suburban | 5.1–5.5 | Visible only directly overhead, no structure. | Outer GTA suburbs, Barrie, London outskirts |
| 7 | Suburban/urban transition | 4.6–5.0 | Vague glow only. | Mississauga, Brampton, Hamilton |
| 8 | City sky | 4.1–4.5 | Not visible. Only brightest stars seen. | Toronto, Ottawa, Hamilton core |
| 9 | Inner city | <4.0 | Only Moon, planets, brightest few dozen stars. | Downtown Toronto, downtown Montreal |
What each class actually means for observing
The jump from Bortle 5 to Bortle 3 is more dramatic than the jump from Bortle 9 to Bortle 6. The reason is logarithmic. Each Bortle class roughly halves the sky brightness, but human vision responds non-linearly. A truly dark sky reveals 5–10x more stars than a suburban one. Most people who've never seen Bortle 2 don't really believe it until they experience it.
- Bortle 1–2: Galaxies and nebulae are naked-eye objects. The Milky Way casts shadows. Andromeda is huge.
- Bortle 3: The galactic core is the dominant feature of the sky. Maybe 2,500 stars visible.
- Bortle 4: Milky Way still clear but loses fine structure. About 2,000 stars visible.
- Bortle 5: Milky Way visible overhead. About 1,500 stars. Most deep-sky targets need binoculars.
- Bortle 6–7: Milky Way barely detectable. About 500 stars. Telescope work is heavily limited.
- Bortle 8–9: Around 50–200 stars. Moon, planets, brightest stars only.
Limiting magnitude: how to test your sky
Naked-eye limiting magnitude is the dimmest star you can see directly overhead. It's the most practical way to measure your Bortle class without an SQM meter.
Easy reference points: Polaris is mag 2.0 (visible anywhere). The dimmest star in the Little Dipper is mag 5.0 (you need Bortle 5 or darker to see all seven). The dimmest stars in the Pleiades naked-eye go down to mag 6.5 (Bortle 3).
Bortle is not the only factor
A Bortle 3 site under hazy, humid air or wildfire smoke can actually perform worse than a Bortle 5 site under crisp dry air. Transparency (the clarity of the atmosphere) and seeing (atmospheric turbulence) are separate, equally important variables.
The Ontario Night Sky tonight score combines all three (Bortle darkness, transparency from cloud and humidity, smoke from US-AQI) into a single 0–100 number per site per hour.
How to find your Bortle class
- Light Pollution Map (lightpollutionmap.info). Visualizes World Atlas 2015 / VIIRS satellite data overlaid on roads.
- Clear Outside app. Gives you Bortle class plus cloud, transparency and seeing in one view.
- DarkSiteFinder. Community-maintained Bortle map for North America.
Frequently asked questions
What is a good Bortle class for astrophotography?
Bortle 4 or darker is the practical minimum for nightscape and Milky Way photography. Deep-sky imaging of faint galaxies and nebulae benefits significantly down to Bortle 2. Below Bortle 5, light pollution becomes the limiting factor in your final image quality more than camera or lens choice.
How do I find the Bortle class of my location?
Use lightpollutionmap.info. It overlays satellite light pollution data on a world map. Zoom to your exact spot and the colour code maps directly to Bortle class (white = 9, red = 7–8, orange = 6, yellow = 5, green = 4, blue = 3, grey = 2, black = 1).
Is Toronto Bortle 8 or 9?
Downtown Toronto is Bortle 9. The outer suburbs (Mississauga, Markham, Vaughan) range Bortle 7–8. Reaching Bortle 5 takes roughly 60–90 km from the lake. Bortle 3 needs 200+ km north (Muskoka, Haliburton) or northeast (Frontenac).
Can I improve a Bortle 6 backyard for stargazing?
You can block local glare with fences and dark fabric panels, which improves your dark adaptation, but you can't reduce the regional skyglow that defines your Bortle class. The atmosphere itself is glowing from city lights miles away. The only real fix is driving to a darker site.
What's the darkest sky in Ontario?
The far northern parts of Ontario above the road network (Wabakimi Provincial Park, Polar Bear Provincial Park, and the Hudson Bay coastline) are Bortle 1, the darkest possible. Of road-accessible sites, Quetico, Pukaskwa and the north shore of Lake Superior are solid Bortle 2.