They would have recognized the imagery of earthquakes and wars being triggered by the failure of the sun and moon (again quoting from Morna Hooker who posits that Mark is quoting Isaiah 13:10 and 34:4. She also cites Ezekiel 32:7;; Joel 2:10; 5:15; Amos 8:9).
She raises the question of whether these signs are to be taken metaphorically or literally.
In any case, we are promised that at the time of disaster, our rescuer will appear. And that appearance will be as inevitable--forseeable--as figs ripening on a blossoming fig tree.