Adam Clarke on Revelation 6:12

The sixth seal - This seal also is opened and introduced by Jesus Christ alone.

A great earthquake - A most stupendous change in the civil and religious constitution of the world. If it refer to Constantine the Great, the change that was made by his conversion to Christianity might be very properly represented under the emblem of an earthquake, and the other symbols mentioned in this and the following verses.

The sun - the ancient pagan government of the Roman empire, was totally darkened; and, like a black hair sackcloth, was degraded and humbled to the dust.

The moon - the ecclesiastical state of the same empire, became as blood - was totally ruined, their sacred rites abrogated, their priests and religious institutions desecrated, their altars cast down, their temples destroyed, or turned into places for Christian worship.