On October 3, 2003 three astronomers, Mike Brown, Chad Trujillo and Chad Trujillo took three photographs of a portion of the night sky using the Samuel Oschin Telescope at Palomar Observatory, California Institute of Technology. The photographs were taken at one and a half hour intervals. The three images were then compared by superimposing them on top of each other. What they noticed was that one of the objects on the photograph had moved relative to the others. There conclusion was that this object is not a star but a planet, the tenth planet in our solar system, beyond Pluto. It has the uninteresting name UB313. You can see the three photographs with the planet identified at Mike Brown's web site.
The idea of superimposing we more often use to create an identical image. That's what you do when you pin a dress pattern on the fabric and then cut along the lines on the pattern.
I hope this helps,