Who was Senior? Bhishma or Vyasa?

Who was Senior? Bhishma or Vyasa? Bhishma and Vyasa are two prominent personalities in Mahabharata. Both are reputed as highly learned. Who was senior in age? First impression may be that Vyasa was senior. A little thinking will get us the answer.

Bhishma was a young man, maybe around 20 years, but had no interest in marrying and settling down. He was spending his time in hunting. His father was not much hopeful that he would be instrumental in continuing his Vansha. He saw Satyavati and wanted to marry her to get sons to continue his Vansha. Satyavati had earlier fallen in love with Parashara and had given birth to Vyasa. Parashara had however left her to follow his life of a seeker of divine knowledge. How many years had elapsed after that? Mahabharat does not state this clearly. One has to make a conjecture. After Parashara left and Vyasa also went with him, Satyavati must have spent a few years in greif. She was however a very young girl and her whole life was there for her to live!

We can presume that 7-8 years after Vyasa's birth she overcame her grief. When Shantanu saw her and asked her father for her hand Vyasa may have been at most 10 years old. Bhishma was 20 years old, give and take a year or two! That makes Bhishma senior to Vyasa by around 10 years!