Q:

Who could have watched the veil of the temple tear when Jesus died?

A professor today made the claim that the following claim is spurious:

And the curtain of the temple was torn in two, from top to bottom. Mark 15:38 (NRSV)

His rationale is that no-one but the high priest would be allowed to see the veil in the holy of holies in the temple, so the author of Mark could not have knowledge of such an event occurring. Is this claim accurate? Who would have seen or had knowledge of the tearing of the veil that could have communicated that event, directly or indirectly, to Mark? How could this event have made it to Mark?

A:

The Holy of Holies, or the Most Holy Place, was separated from the outter room (the Holy Place) by the veil. Only the High Priest could go beyond the veil into the Most Holy Place, but other priests could enter the Holy Place. Thus any priest at the Temple would have been able to see it.
The non-priest Levites may have also been able to enter the Holy Place (they were originally charged with tearing down and setting up the Tabernacle in the wilderness)
So any number of people could have seen it. Even if they weren't immediately present, it would have been known within a day (if crucified Friday as many believe) or within a few days (if crucified on wed. as others argue). It would have been found within the week at the latest. 
Obviously the two events couldn't be watched simultaneously, so there is no doubt some interpretation/correlation being done by Mark and others between these two events. Mind, the veil ripping in half would be no small event.
Holy of Holies (Wikipedia)

