What causes the sun to turn black and the moon to turn red during the sixth seal?
Cataclysmic events at the midpoint of the tribulation—earthquakes and probably volcanoes—fill the air with pollution. That pollution causes the moon to turn red as blood and the sun to be blackened out, matching the sixth seal’s great earthquake judgment.
Les explains that the sixth seal occurs at the midpoint of the tribulation, accompanied by a great earthquake. He points to the description that the sun became black as sackcloth of hair and the moon became as blood. He thinks this pictures what happens because of cataclysmic events—earthquakes and probably volcanoes—that put pollution in the air. The moon turns red as blood from that pollution, and the sun is actually blackened out because of it. This is tied to the specific earthquake at the midpoint, when the day of His wrath is come.
And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood;