The sun shall be darkened (ho helios skotistheĢsetai). Future passive indicative. These figures come from the prophets (Isa 13:9.; Eze 32:7.; Joe 2:1., Joe 2:10.; Amo 8:9; Zep 1:14-16; Zec 12:12). One should not forget that prophetic imagery was not always meant to be taken literally, especially apocalyptic symbols. Peter in Act 2:15-21 applies the prophecy of Joel about the sun and moon to the events on the day of Pentecost. See Mat 24:29-31 for details of Mar 13:24-27.