Of course, the ancient Greeks had no telescopes, and so could only see stars as dim as 6th magnitude. Today we can see stars with ground-based telescopes fainter than 22nd magnitude. There are also objects in the sky brighter than 1st magnitude. For example the star Vega has a magnitude of zero. In October 1999 the planet Jupiter has a magnitude of -3.
Negative magnitudes mean that the object is very bright.
When astronomers observe a meteor they try to estimate its brightness by comparing it to other objects in the sky. If a meteor is as bright as Jupiter, it has magnitude -3. If it's as bright as the full moon and casts shadows on the ground it has a magnitude of -13. The magnitude scale is logarithmic. So is the human eye.