The fourth angel sounded his trumpet, and a third of the sun was struck, a third of the moon, and a third of the stars, so that a third of them turned dark. A third of the day was without light, and also a third of the night. (Revelation 8:12 NIV)
When the forth trumpet sounds God will once again strike the skies like He did back in the sixth seal (Revelation 6:12-17). The sun and moon will lose some of their brightness and a third of the stars will apparently disappear out of the sky.
I have another theory about this passage, which I admit is a bit farfetched. Perhaps when it says that “a third of the days was without light and also a third of the night,” it means that a third of the day will literally dissappear. This could mean (again this is farfetched) that the Earth will increase it’s spin so that one day is only 16 hours long instead of 24. This would corroborate what Jesus said in Matthew 24:22
“If those days had not been cut short, no one would survive, but for the sake of the elect those days will be shortened.
Anyway, it’s just a theory.