The Darkening at the End of the Tribulation is Not Caused by Eclipses

"And the fifth angel sounded, and I saw a star [Satan] fall from heaven unto the earth: and to him was given the key of the bottomless pit. And he opened the bottomless pit; and there arose a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit. And there came out of the smoke locusts upon the earth: and unto them was given power, as the scorpions of the earth have power" (Rev. 9.1-3).

This darkening carries on through to the end of the Tribulation and is most pronounced at the end of the Tribulation.