What observation suggested to Galileo on January 8th that one of the initially seen objects had not vanished?

Answer

It had likely been obscured by Jupiter itself.

On January 8th, after only one object remained visible on one side, Galileo deduced that one of the initial objects had likely been obscured by Jupiter as the planet moved across the sky relative to the background stars.

What observation suggested to Galileo on January 8th that one of the initially seen objects had not vanished?
discoveryGalileotelescopeJupiterMoon