Page "Io (moon)" Paragraph 9

The first reported observation of Io was made by Galileo Galilei on January 7, 1610 using a 20x-power, refracting telescope at the University of Padua.

However, in that observation, Galileo could not separate Io and Europa due to the low power of his telescope, so the two were recorded as a single point of light.

Io and Europa were seen for the first time as separate bodies during Galileo's observations of the Jupiter system the following day, January 8, 1610 ( used as the discovery date for Io by the IAU ).

The discovery of Io and the other Galilean satellites of Jupiter was published in Galileo's Sidereus Nuncius in March 1610.

In his Mundus Jovialis, published in 1614, Simon Marius claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery.

Galileo doubted this claim and dismissed the work of Marius as plagiarism.

Regardless, Marius ' first recorded observation came from December 29, 1609 in the Julian calendar, which equates to January 8, 1610 in the Gregorian calendar, which Galileo used.

Given that Galileo published his work before Marius, Galileo is credited with the discovery.