Did Bhishma knew about the events happening in the universe all the time?
If yes, how did he get that power?
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"Learning by his Yoga prowess of the devotion of Bhishma, Madhava, otherwise called Hari, (entering his body) bestowed upon him heavenly knowledge compassing the Past, the Present, and the Future, and went away. "
Krishna did it for a reason.
"Subject to universal time, when the divine Surya turns to his northerly course, thou, O foremost of men, shalt go to those regions whence no man of knowledge ever returns to this earth! When thou, O Bhishma, wilt leave this world for that, all Knowledge, O hero, will expire with thee. It is for this, that all these persons, assembled together, have approached thee for listening to discourses on duty and morality. Do thou then speak words of truth, fraught with morality and Yoga, unto Yudhishthira who as firm in truth but whose learning has been clouded by grief on account of the slaughter of his kinsmen, and do thou, by this, quickly dispel that grief of his!'
Krishna also could have taught Yudhisthira the duties of king, dharma, and statecraft himself so why did he tell Bhishma to teach Yudhisthira and even Bhishma asked the same question to Krishna?
"Vasudeva said, 'Know, O thou of Kuru's race, that I am the root of fame and of everything that leads to good. All things, good or bad, proceed from me. Who on earth will wonder if the moon be said to be of cool rays? Similarly, who will wonder if I were described as one possessed of the full measure of fame? I have, however, resolved to enhance thy fame, O thou of great splendour! It is for this, O Bhishma, that I have just inspired thee with great intelligence. As long, O lord of earth, as the earth will last, so long will thy fame travel with undiminished lustre through all the world. Whatever, O Bhishma, thou wilt say unto the inquiring son of Pandu, will be regarded on earth to be as authoritative as the declarations of that Vedas. That person who will conduct himself here according to the authority of thy declarations, will obtain hereafter the reward of every meritorious act. For this reason, O Bhishma, I have imparted to thee celestial understanding so that thy fame maybe enhanced on earth.
So Krishna gave Bhishma the boon of the past, present, and the future so that he could instruct Yudhisthira the duties of king, moral duties, morality, state affairs, statecraft, vedas and puranas because Krishna wanted Bhishma to have immortal fame on earth.