Galilean moons are very easy to observe using some kind of instrument to get some magnification. With just a binoculars would be enough to spot the four moons of Jupiter. They are constantly rotating around Jupiter. Every day their relative distances and positions changes. Their names are Io, Callisto, Europa & Ganymede. They all have a similar size and brightness. When passing in front of the big planet, they cast a shadow over the gaseous surface of Jupiter. It is required a telescope to observe such events.
Here it is a picture I took tonight with a digital pocket camera through a 14mm eyepiece and a 150 mm newtonian reflector telescope with no-tracking system.
Computer Physics Lab 