https://homepages.hass.rpi.edu/ruiz/AdvancedDigitalImagingSpring2021/ReadingsADI/Galileo%20The%20Starry%20Messenger.pdf

It is easily readable for any observational practitioner just like the Commentariolus of Copernicus-

http://copernicus.torun.pl/en/archives/astronomical/1/?view=transkrypcja&

Galileo quickly created the short treatise after he put two and two together in observing the change in position of Jupiter's satellites, no mean feat given that he had to visually observe changes against the background stars that nobody previously
could notice.

Galileo did equate the motions of Jupiter's satellites with the motion of Venus and Mercury around the Sun, however, it was not possible to frame the motions of the inner planets using the prevailing Ptolemaic framework in operation among the first
heliostatic astronomers at the time.

The compulsion to get an insight out there is present among those seeing observations for the first time and certainly the back and forth motions of Venus and Mercury using a human satellite free from daily rotational influences had that effect on me as
it should others-

https://www.youtube.com/watch?v=w2uCtot1aDg

The observational limitations of Galileo's era were mostly arising from being attached to a rotating Earth, so they were forced to use a stationary field of background stars to gauge planetary motions and then try to convert them into a heliocentric
framework. It may work for the slower moving planets further from the Sun than the Earth but it doesn't for the faster moving Venus and Mercury-

"Now what is said here of Jupiter is to be understood of Saturn and Mars also. In Saturn these retrogressions are somewhat more frequent than in Jupiter, because its motion is slower than Jupiter's, so that the Earth overtakes it in a shorter time. In
Mars they are rarer, its motion being faster than that of Jupiter, so that the Earth spends more time in catching up with it. Next, as to Venus and Mercury, whose circles are included within that of the Earth, stoppings and retrograde motions appear in
them also, due not to any motion that really exists in them, but to the annual motion of the Earth. This is acutely demonstrated by Copernicus" Galileo

The current modification should be immediately apparent to 21st century observers just as modifications supplied by Galileo and Copernicus resolve outstanding observational issues which may have puzzled previous observers or left them uncomfortable with
a less than sufficient solution.

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