[maxxxxx]

Wasn't Galileo first to observe them with his telescope or were they mentioned somewhere before? I assume back then the skies were darker and people probably had better eyes so if they are observable I would assume somebody would have mentioned them.

[cgriswald]

There are a few confounding factors:

1. Jupiter is very bright.

2. The moons aren't always visible (they're behind or crossing Jupiter).

3. The magnitude of the moons changes with the relative positions of Earth and Jupiter.

It's possible that Galileo was not the first to see them, but if you don't know what they are or how you managed to see them, you're probably not going to be able to get anyone else to believe you. The telescope allowed Galileo to track their movement and also to convince others, who could also look through a telescope and see them easily.

[oska]

There is an argument that a Chinese astronomer observed and documented a Jovian satellite in the 4th century BC. [1]

[1] http://en.cnki.com.cn/Article_en/CJFDTOTAL-TTWL198102000.htm