Math Puts the Milky Way over LA

Phil Plait describes the math used to make the Milky Way over LA.

"The problem is the Earth spins, so the Milky Way and the stars in the sky move. Normally you could just shift all the pictures to line them up, but in this case, though, Kiely used a wide-angle 11mm lens, so the pictures are distorted. That makes a simple shift much harder to do. So instead, he used some math to make a model of how the stars moved across the frame of the picture over time. This created a series of curved lines, all different depending on where they were in the frame. That is essentially a map, a grid, showing where a star would be given its position and the time the picture was taken. He then used that model to warp each image, placing them all on a common frame, and added them together."