The angle from the Sun, to a moon, to the observer is called "phase angle." This montage shows Saturn's moon Titan as seen by Cassini through a clear filter at a variety of phase angles. At high phases, sunlight is forward-scattered through the atmosphere, making it brighten in a ring shape around the moon. The images have been resized to a constant pixel scale and rotated so that the terminator is up-and-down; the images sample a variety of latitudes and longitudes.

NASA / JPL / SSI / montage by Emily Lakdawalla