, 
Calculated eV
Cosmic rays
Observed eV
1.7 × 1015
the first knee
3 × 1015
the midpoint between 
and 
2 × 1016
the first ankle
2 × 1016
2.4 × 1017
the second knee
3 × 1017
the midpoint between 
and 
2.8 × 1018
the second ankle
3 × 1018
3.2 × 1019
the toe
4 × 1019
4.4 × 1021
beyond the GZK limit
not observed