Starry Messenger Study Guide

Starry Messenger By Galileo Galilei

Published in New Latin by astronomer Galileio Galilei in March of the year 1610, Sidereus Nuncius (Starry Messenger in English) was the first scientific treatise published that was based on observations made through a telescope. It contains Galileo's observations on the Moon, the stars, and the moons of Jupiter.

Of the Moon, Galileo noted that the line separating the dark side of the moon from the light was smooth where it crossed the darker areas, but jagged when it crossed the brighter areas. From this, he deduced that the mares (the darker areas) were flat and low-lying, whereas the brigher areas are mountainous, and at least 4 miles in height (much larger than Mt. Everest.) This directly contradicted the then-popular Aristotelian notion that, since heavenly bodies are more perfect than earthly ones, the Moon must be a smooth sphere. This was the first element in a siege on Aristotelian cosmology that would eventually see Galileo killed by the Catholic Church, which had wholeheartedly adopted Aristotle's ideas.

Of the stars, Galileo reported at least ten times more stars visible through a telescope than with the naked eye. Also, he reported that some of the 'nebulous' stars in Ptolemy's star catalogue were actually clusters of many tiny stars. He deduced that the nebulae were galaxies (though he didn't use that word), and that the Milky Way itself was not a cloud, but a continuous collection of millions of similar tiny stars that even his telescope was not powerful enough to identify.

In the last portion of the treatise, Galileo reported seeing four objects that he established by recording their positions nightly were orbiting around Jupiter. As he was hoping to return to Florence and gain the patronage of Cosimo II of Medici, he named the four moons 'Medicean stars'. Since then, however, they have been properly named, though they are still often referred to as 'Galilean moons'.