What gave us close up views of the gas giants and showed us their moons?
The telescope, of course!
Two of the earliest telescopic discoveries, by Galileo:
Jupiter's large moons, still collectively called the "Galilean moons".
Our own Moon is rough, showing a jagged boundary between lighted and dark sides when viewed from Earth.
Later on, use of the telescope revealed that Uranus is a planet and empirically verified the presence of Neptune.