[NOTE (May 11, 2000): This next Bad/Good section deals with Ellie's trip taking no time to the outside world. I originally argued that this was not possible. I was wrong; at least, I was wrong in the context of the movie. Sagan established both in the book and in the movie that the aliens have faster-than-light travel, and even may have the limited ability to travel in time. Basically, they took Ellie out of her coordinates in space and time, chatted with her for a while, then put her back to her original coordinates in space and time. Therefore, no time elapsed for people back on Earth. I have received copious emails about this pointing out my error! I have left it in so you can read it anyway.]
Bad:
When Ellie returns to Earth, 18 hours have passed for her, but no time at all has passed on Earth.
Good:
This point isn't really astronomy, but physics. What is implied is that the travel through the wormhole took no time at all, as if she were moving at some super-relativistic speed. Einstein showed that an object moving near the speed of light relative to some outside observer has time slow down relative to that observer (for more detail about relativity, take a look at the Relativity FAQ). It's possible, if you fudge it a bit, to assume that no time elapses at all if you are moving faster than light, which Ellie did. However, she had a conversation with her ``father''. That conversation took up time. No matter what, some small amount of time must have elapsed during her trip.
The only way around that is to assume that the aliens could somehow manipulate time, and I have to draw the line there. Her ``father'' admitted they did not build the transit system, so I have severe doubts they could shape time the way they want. It's a neat plot twist, but the zero-time trip won't work.