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"On the 7th day of January in the present year 1610, in the 1st hour of the following night (...) Jupiter presented itself to my view, and (...) I noticed (...) that three little stars, small but very bright, were near the planet"

- G. Galilei, 'Sidereus Nuncius', March 1610 https://twitter.com/Nereide/status/1214671804897406977

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On 11 Jan 1610, #Galileo recognized that there were four "stars", not only three:

" (...) and also that there are not only three, but four wandering stars making their revolutions about Jupiter."

- From 'Sidereus Nuncius'

(The red underline, on the page in Latin, is mine)

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On 13 January 1610, #Galileo eventually observed all four Jupiter's moons at once, for the first time, as they appear in this page of the 'Sidereus Nuncius' in Latin.

He originally thought he saw four little stars.

#galileogalilei
#galileanmoons

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References

'Sidereus Nuncius':

in Italian https://bit.ly/35jOfwL 
in Latin https://bit.ly/3s5ss5G 
in English https://bit.ly/3pVjlTo 

Image: Page from Galileo’s notebook about his observations of Jupiter’s satellites. https://go.nasa.gov/3pXhj55