Generally Sudbury is considered the dividing line between Northern and Southern Ontario. Keep in mind that the far north of Ontario is almost uninhabited save for a few small reserves:

(map) https://en.wikivoyage.org/wiki/Ontario

If you think of where the centre of mass of the population of Ontario is, it is insanely far north. But yeah in its official definition Northern Ontario is more of a political designation than a geographical one.

https://www.reddit.com/r/Damnthatsinteresting/comments/rw4jd...

> Looking at a map, I'm surprised that's considered Northern Ontario

Check out a map of California.

Latitude of San Diego ("Southern California"): 32.7°

Latitude of San Jose ("Northern California"): 37.3°

Latitude of Crescent City ("doesn't count"): 41.8°

"Northern California" is pretty close to being dead in the center of the state, as far as north-south goes.