The Galilean moons, or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit a planet other than the Earth.
This was a photo taken by myself using an unprofessional telescope, there should be four Galilean satellites, but in this picture there is only three. The one on the right lower corner was an optical illusion of Jupiter itself. It is not the telescope’s problem, in fact, it is my phone camera’s fault.