The fourth angel poured out his bowl on the sun, and the sun was allowed to scorch people with fire. Revelation 16:8
The sun undergoes several changes in the last days. The Great Tribulation begins with the darkening of the sun and the moon. There are those who believe we are presently in the days of tribulation. Among Covenant theologians the tribulation was in the reign of Nero. Some have said the tribulation began when Cain killed his brother Abel.
There sure are a wide range of beliefs concerning the time of this terrible period. Most are speculations by those who try to use Revelation to bolster their church agenda. For instance those who believe we are now in the millennium have to say that the tribulation is over.
But God has given a sign so unmistakeable that no one will miss it. It cannot be misinterpreted. Here it is:
The sun turned black like sackcloth made of goathair, the whole moon turned blood red, and the stars in the sky fell to earth... (Rev. 6:12,13)
Thus begins the great tribulation period. It does not concern the church. It touches Jews and Gentiles. We will be saved from this terrible time. Yes I will gladly say it again. Rev. 3:10 promises our deliverance from the period of testing. Other scriptures do also.
In come the "spiritualizers". To them, nothing in Revelation is literal. But this Day of the Lord is in both Old and New Testaments and cannot be understood in any way but literally.
I think of being in a movie theater. Lights are on so people can find their seats. Then the theater is darkened and the screen lights up. The sun and moon will momentarily darken, then the sun will be hotter than ever before.