Question: what is there in the center of earth

title: History of the center of the Universe
text: On 7 January 1610 Galileo used his telescope , with optics superior to what had been available before . He described `` three fixed stars , totally invisible by their smallness '' , all close to Jupiter , and lying on a straight line through it . Observations on subsequent nights showed that the positions of these `` stars '' relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars . On 10 January Galileo noted that one of them had disappeared , an observation which he attributed to its being hidden behind Jupiter . Within a few days he concluded that they were orbiting Jupiter : Galileo stated that he had reached this conclusion on 11 January . He had discovered three of Jupiter 's four largest satellites ( moons ) . He discovered the fourth on 13 January .