Brightness and magnitudes

The 'magnitude' of a star, a planet, or the International Space Station is a measure of how bright it is. The magnitude scale was first developed by the ancient Greeks, but is still in use by astronomers today.

'First-class stars' (stars of magnitude 1) are the brightest; 'second-class stars' (stars of magnitude 2) are a bit fainter, and so forth. Sixth-magnitude stars are the faintest stars that can be seen by the naked eye on a truly dark night.

The qualitative magnitude scale of the Greeks has now been precisely quantified. It turns out there are even objects of magnitude 0 (the very brightest stars) and of negative magnitude (bright planets like Jupiter and Venus). Also, the scale has been refined, so a particular star can have a brightness of, say, magnitude 3.67.

A difference of one magnitude corresponds to a brightness ratio of about 2.5. In other words: if ISS reaches a brightness of -1, it is about 2.5 times as bright as a star of magnitude 0.

Twisst classifies ISS passes depending on the maximum altitude:

  • Bright: brightness between magnitude 2 and magnitude 0
  • Very bright: brightness between magnitude 0 and magnitude -2
  • Extremely bright: brighter than magnitude -2