SlashFilm recently posted a great article about how Neil deGrasse Tyson noticed that the sky in the scene were Rose (Kate Winslet) is floating on a plank and gazing at the sky. According to Neil deGrasse Tyson, the sky is completely inaccurate for the time of night on that particular date (April 15, 1912 at 4:20 AM).
Below is an interesting video in which Neil deGrasse Tyson describes the mini saga which finally resulted in a phone call from a guy who works post production for James Cameron. The call was requesting his assistance on a sky upgrade for the 10 year anniversary release of the film. This sky grade is also used in the 3D version of Titanic. Skip ahead to 26:00 or 26:10 for the bit about Titanic. Neil deGrasse Tyson is quite a funny guy.