Does Luke Flat Out Contradict Himself?

Sometimes readers ask questions that have answers they probably would not suspect in a million years.  My guess is that this is true of the following interesting query about a contradiction between the Gospel of Luke and the book of Acts (written by the same author) about the ascension of Jesus.

 

QUESTION:

Talking of authors who contradict themselves any idea why Luke has Jesus ascending on the day of his resurrection but Acts places it 40 days later!? This seems like quite an obvious mistak...