.

Is Dione (698 miles or 1123 kilometers across) suddenly larger than Rhea (949 miles or 1527 kilometers across)? No, of course not. Cassini simply captured an image when Dione was much closer to the camera, making the moon appear much bigger than her larger sister moon.
Besides their beauty, images like these can help Cassini's navigators determine exactly where the spacecraft is and confirm that it's on course.
This view looks toward the trailing hemisphere of Dione. North on Dione is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 11, 2015.
The view was obtained at a distance of approximately 68,000 miles (110,000 kilometers) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 29 degrees. Image scale at Dione is 2,165 feet (660 meters) per pixel. Rhea was 300,000 miles (500,000 kilometers) away at a phase (Sun-Rhea-spacecraft) angle of 30 degrees. The image scale at Rhea is 2 miles (3 kilometers) per pixel.
The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

Quelle: NASA