Mark's Gospel, written approximately 70 CE, tells us (13:24-26) that Jesus said to the disciples that they would personally experience social turmoil and persecution, there would be earthquakes and, "But in those days, after that tribulation, the sun shall be darkened, and the moon shall not give her light and the stars of heaven shall fall, and the powers that are in heaven shall be shaken and then shall they see the Son of man coming in the clouds with great power and glory." He concluded by saying, "Verily I say unto you, that this generation shall not pass, till all these things be done." What is important here is that Mark's Gospel was written only about 40 years after it says Jesus predicted the end of the world within the current generation.
Luke (21: 5-28) copied from Mark 13:1-30, but had to alter the material to suit the times. By the end of the first century, it was no longer plausible that the world would end before Jesus' generation shall pass - the generation had already passed. So Jesus no longer says, "Verily I say unto you, that this generation shall not pass, till all these things be done." The destruction of the Temple and the civil war and carnage of the Roman-Jewish War still presage an imminent end of the world, but the emphasis has shifted subtly so that even if the end will be very soon, no one can know when.
Luke 21:25-26 give an more emotionally powerful summary of the final signs in Mark 13:24-25, and each gospel then tells of "the Son of man coming in the clouds with great power and glory."
The chronology of the end time prophecies all precede Jesus' return to earth with the darkening of the sun and the moon, heightened distress among the nations and unusual astrological activity. Luke 21:25-26, too, is followed by: "...and then shall they see the Son of Man coming in a cloud..." (verse 27).
The Old Testament prophecies speak of the same thing: "...the sun shall be turned into darkness, and the moon into blood, BEFORE the great and the terrible Day of the Lord come." (Joel 2:31)
The sixth seal of Revelation chapter 6 reveals the same events just before the "seventh" and last seal is opened in chapter 8 - the "seven trumpets" of which, Jesus is said to return when the seventh and "last trump" sounds (I Cor.15:52).
But, the darkening of the sun and moon don't occur until AFTER the "fifth" seal of Revelation 6: the Great Tribulation... the persecution of God's children and the suppression of His Truth - on a global scale. (see also: Matt.24:9-29 for a description of the Great Tribulation)
http://wiki.answers.com/..._chapter_21_verses_25-26
"One man gives freely, yet gains even more; another withholds unduly, but comes to poverty. A generous man will prosper; he who refreshes others will himself be refreshed." Rev Canon Karanja.