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A ray of light pouring through an opening in the roof of a building falls on a particular spot. Normally, we shall not be able to tell where the same ray of light will fall next year. But a prediction can be made with the help of Jyotisa. This is how it was done in the olden days. A pearl attached to a thread was hung from the roof. If a man was able to indicate correctly in advance where its shadow would fall on a particular day, he received a reward from the king. One's competence in other sastras is established through argument, but in Jyotisa it has to be proved by actual demonstration. You cannot deceive anyone by employing the methods taught by this science. The sun and the moon are witness to what you do. "Pratyaksam Jyotisam sastram. "
For a general background, please see here