Before we come to our title question , let me ask a counter question. There is a quotation,, that history repeats itself and those, who do not learn from the mistakes of history, are condemned to repeat them.
My question is very simple : why does history repeat itself at all ?
The answer to this question, will lead us in our quest for scientific basis of astrological predictions.
The answer to both these questions is rather simple and convincing !
We all know that all heavenly bodies like Sun, Earth , Moon etc are of spherical shape and they revolve and rotate in a circular ( elliptical) manner . That is the secret behind repetition of events ! Just as every clock partially repeats the same environment of Sun Rise at 6 AM every day , though date is not the same , in the same way all events are likely to repeat with some modifications and new players. It is like staging of Shakespeare's drama in different theatre and different actors but core drama remaining same.
That is why, when a major planet configuration is repeated, similar events happen with new set of players. We all experience our day-night and seasons due to above reasons , but still we do not want to accept the repeatability of events in our individual life.
Astrology takes the help of this parameter to help predict partially the nature of coming time and it succeeds where no other science succeeds !