Why are the sun, moon and stars darkened at the fourth Trumpet?

At the sounding of the fourth trumpet, Revelation 8:12 tells us that a third part of the sun was smitten, and the third part of the moon, and the third part of the stars, so that the day was darkened for a third of it and the night likewise. This darkening is not random. It is one of the specific divine judgments that God unleashes as part of the Day of the Lord, matching what He had already declared through the Old Testament prophets long before John saw it in his vision.

Isaiah makes the reason plain when he ties this very kind of darkening directly to the coming wrath of God: "Behold, the day of the Lord cometh, cruel both with wrath and fierce anger, to lay the land desolate: and he shall destroy the sinners thereof out of it. For the stars of heaven and the constellations thereof shall not give their light: the sun shall be darkened in his going forth, and the moon shall not cause her light to shine." So the darkening of the heavenly bodies is bound up with God's determination to lay the earth desolate and to destroy the sinners in it.

Joel likewise connects this darkening to the nearness of judgment, saying, "The sun shall be turned into darkness, and the moon into blood, before the great and the terrible day of the Lord come," and again, "The sun and the moon shall be darkened, and the stars shall withdraw their shining." Amos adds that the Lord Himself will cause the sun to go down at noon and will darken the earth in the clear day, while Ezekiel speaks of the Lord covering the heaven, making the stars dark, covering the sun with a cloud, and setting darkness upon the land.

Taken together, these witnesses show that the darkening at the fourth trumpet happens because it is God's own act of judgment, a visible sign in the heavens that His wrath, His day of reckoning against sin, has arrived on the earth.