Jesus said that the darkening of the sun
and the moon takes place after the Great
Tribulation.

"Immediately after the tribulation of those
days shall the sun be darkened, and the
moon shall not give her light, and the stars
shall fall from heaven, and the powers of
the heavens shall be shaken."
Matthew 24:29

Since the opening of the Sixth Seal occurs
after the Great Tribulation, the first five
seals must represent the time of the actual
Tribulation.