For some time scholars have seen in the ending of Luke and the opening of Acts a sticky apparent contradiction about the timing of the ascension. Luke 24:51-52 ends by saying, "And it happened that while he was blessing them, he was removed from them and was taken up into heaven. And they, worshipping him,returned into Jerusalem with great joy." (as it reads in most Bibles today) And this, according to a simple reading of the narrative, was the day of his resurrection. Yet Acts opens (1:1-11) with Jesus ascending to heaven after 40 days.
Now I am aware that this is a sticky mess being that Acts seems to have undergone at least 2 substantial revisions and Luke too has been worked over here and there. But assuming that the original forms of these two books were by the same hand, as is generally agreed, what did this early/mid 2nd century author (we'll call him "Luke" to honor tradition) mean to convey?
Bart Erhman has posed a solution. (others have as well of course but his seems to have merit). It is recognized that two key phrases from Luke 24:51-2 are absent from certain Western manuscripts. The phrases in question, "and he was taken up to heaven" (absent from D a b d e ff2 l syr-s geo1 and Sinaiticus) and "worshipping him" (all the before mentioned except the Sinaiticus) greatly alter the meaning of the passage. Other intrinsic arguments can be made for the shorter form as being the original, (or at least the older). If this be so then we have Luke ending his first volume with Jesus simply walking away from his disciples who then go off to Jerusalem with joy. This would easily mesh with the opening of Acts that have Jesus still roving about for 40 days. Acts 1:2, "the things Jesus began to do and teach until the day when, having through Holy Spirit commanded the disciples whom he had chosen, he was taken up", is often understood to be saying that GLuke have recounted experiences of the risen Jesus prior to his ascension. This of course is weird since GLuke mentions no such thing. However if we remove the idea that Luke closed with an ascension (as posed above)then the verse in Acts would naturally be understood as speaking about the stories of Jesus prior to his death spoken from the perspective of the story teller. This also better harmonizes with the ending of Mark (and Matt) his primary source that ends without an ascension. The words od Acts 1:2 "having through Holy Spirit commanded the disciples whom he had chosen" appears to be a later harmonization with Matt 28:19 which is itself corrupted with interpolation.
If this shorter form of Luke 24 is in fact the older, why was it interpolated? A number of possibilities exist, perhaps the scribe when reading Acts 1:2 interpreted it as referring to an ascension story in Luke 24 and therefore added a few words "and ascended to heaven". Or perhaps as Erhman suggests, the books were not generally read side by side (as shown by the order in the Canon) and the intended purpose of the interpolation served an entirely different cause. The words as they read in most Bibles serves to counter Docetic Christian teaching that no actual physical body was taken to heaven. And adds for good measure an affirmation that Jesus was to be identified with God and worshipped. These were raging controversies of the 2nd and 3rd centuries. The proto-orthodox scribes have left abundant proof they they made numerous such alterartions to gain headway in this controversy. Carelessly zealous to add support for their interpretation they inadvertently created a contradiction that has puzzled readers for centuries.
As a side thought, many reputable scholars, even clergy, have suggested that the doctrine of "ascension" is itself a late addition to the Jesus story. The Pauline concept of ascension being a more figurative expression of divine approval and blessing in the mystery cult tradition. The Gospel tradition seems to represent a halfway point in this evolving theology. ie. resurrection without ascension. Acts (mid 2nd century) seems to be first explicit claim for ascension to heaven. I am not entirely convinced this was, even then, the work of the original author (but at least it doesn't create a glaring contradiction in light of this proposal).