Mahabharata - Sahadev as greatest Astrologer

Sahadev knew about the up-coming Mahabharata war well in advance.

He was a great astrologer and was supposed to have known the events of the Mahabharata War beforehand. But, it's said that he was also cursed that if he discloses the knowledge, his head would split in pieces.

Probably due to this, we see him playing a relatively silent role in the epic as compared to other brother pandavas. 

The Bhagavata Purana has a passage in which Sahadeva predicts events of future when asked by his elder brother Yudhisthira, the king.

Of the five Pandavas, Sahadeva was the youngest. Still he has been referred to as the wisest of all of them. 

Yudhishtra even speaks of him as wiser than Brihaspati, the divine teacher of the Devas.

Sahadeva's fury

Source: Amar Chitra Katha

Also note that he was one of the few people (like Bhishma and Vidura) to be able to realize the that Lord Krishna was the almighty Para Brahman himself and lived at that time.

And he was the only one who performed Agrapuja to Lord Krishna, declaring openly among Kings, in the face of opposition, that Lord Krishna, being Parabrahma Swarupi, deserves the first respect.

As per the Bhagavata Purana, he was one of the greatest devotees of Lord Krishna. 

Lord Krishna once asked Sahadeva, what should be done to stop the war. 

Sahadeva told him that Krishna himself must be tied down and imprisoned. And all the Pandavas along with Duryodhana must be sent to forest. And Karna must be made the king.

When Krishna challenged him to tie him down, Sahadeva started meditating and envisioned Krishna as a small baby and tied him down. 

Since Krishna could not move out of the bondage created by Sahadeva in his meditative trance, he blessed him with divine vision and then only Sahadeva released Krishna from the bondage.

At the time of Mahabharata War, both the sides were trying to give sacrifice to Goddess Durga seeking success. 

Sahadeva being a great astrologer, Duryodhana approached him to determine the most auspicious time for performing the sacrifice. 

It is believed, in Hindu school of thought, that the time of sacrifice, denotes the commencement of war and thus decides the very outcome of the great war. 

Sahadeva advised him to perform the sacrifice on the new moon day to ensure success in war.

When questioned by his brothers and Lord Krishna, Sahadeva replied politely that he is ready to die doing his swadharama of being an astrologer.

To this, Lord Krishna came to rescue and played a trick on the Sun and Moon gods to create the Bodhayana Amavasya which occurs one day earlier than the regular Amavasya. 

The Pandavas performed the sacrifice on Bodhayana Amavasya day and obtained the boon of success from Goddess Durga, which sealed their fate to win the war.

Note that Sahadev was probably following the teaching of Lord Krishna even before the Bhagavad Gita was given out by Lord Krishna.

Also note that, similar characteristics were also demonstrated by Ravana in Ramayana, when he agreed to determine the auspicious muhruth for Lord Rama's before the war.