Our moon is one of the largest satellites in the solar system, which is impressive considering how much smaller the Earth is compared to Jupiter or Saturn. It has a diameter of 2,160 miles, as opposed to 3,280 miles, the diameter of Jupiter's Ganymede, the largest satellite. Most experts agree that the moon formed when a Mars-sized planet collided with Earth several billion years ago. The ensuing debris cloud reformed into the moon. Here the moon is seen in a NASA composite image transiting the sun from the STEREO-B spacecraft.