Stars in New York City
Full disclosure: I've never lived in NYC, but the light pollution where I live precludes seeing any but the brightest stars. I think it's highly improbable that Sara and Lars would be able to see Casseopia whilst sitting by the skating rink near the end.
Understandably, there are many plot points requiring the willing suspension of disbelief to enjoy this movie.
Further full disclosure: I do in fact love this fantasticly enjoyable little tale for the hopeless romantic that exists in many of us.