Using a telescope Galileo was able

Using a telescope Galileo was able to see several things invisible to the naked eye. Firstly he could see many stars not visible without a telescope. Secondly the ancient Greeks believed that the Moon was smooth. Looking through a telescope Galileo could see the Moon's surface is actually rough, with mountains and craters. He also discovered 4 small 'moons' orbiting the planet Jupiter. At the time these were astonishing discoveries. Until then nobody knew that any of the other planets, apart from Earth, had 'moons'.