I just watched a documentary called "In Search of Christmas". It spoke of a conjunction between Jupiter, Venus, and the moon in the constellation of Ares on April 17th, 6 B.C. They also mentioned that conjunctions of Jupiter and Venus were common in 3 B.C. and 2 B.C. (about the time Jesus was two years old).
The show noted that the Roman census was a historical event, but generally only involved Roman citizens. So, if Mary and Joseph were Roman citizens (as Herodians), their involvement in a census of some kind is more plausible.
The show made another interesting point. Apparently shepherds only watched over their flocks at night during the spring time when the females were giving birth to their young. The rest of the year, the flocks were protected at night by pens.
© Charles N. Pope, US Library of Congress. All rights reserved.