The Starry Messenger title page, 1645

Galileo got his hands on one of the first telescopes and decided to point it at the sky. Amazed at what he was seeing, he wrote this short book to share his observations in elegant and clear writing.

The Starry Messenger is the first work that describes the cratered surface of the moon (at the time it was thought to be smooth) and the moons of Jupiter. He shares his findings with exuberance:

This part of the moon’s surface, where it is spotted as the tail of a peacock is sprinkled with azure eyes, resembles those glass vases which have been plunged while still hot into cold water and have thus acquired a crackled and wavy surface, from which they receive their common name of “icecups.”


To assert that the moon’s secondary light is imparted by Venus is so childish as to deserve no reply. Who is so ignorant as not to understand that from new moon to a separation of sixty degrees between moon and sun, no part of the moon which is averted from the sun can possibly be seen from Venus? And it is likewise unthinkable that this light should depend upon the sun’s rays penetrating the thick solid mass of the moon, for then this light would never dwindle, inasmuch as one hemisphere of the moon is always illuminated except during lunar eclipses. And the light does diminish as the moon approaches first quarter, becoming completely obscured after that is passed.


I have observed the nature and the material of the Milky Way. With the aid of the telescope this has been scrutinized so directly and with such ocular certainty that all the disputes which have vexed philosophers through so many ages have been resolved, and we are at last freed from wordy debates about it. The galaxy is, in fact, nothing but a congeries of innumerable stars grouped together in clusters. Upon whatever part of it the telescope is directed, a vast crowd of stars is immediately presented to view. Many of them are rather large and quite bright, while the number of smaller ones is quite beyond calculation.


…now we have not just one planet rotating about another while both run through a great orbit around the sun; our own eyes show us four [moons] which wander around Jupiter as does the moon around the earth, while all together trace out a grand revolution about the sun in the space of twelve

Even though Galileo’s equipment was primitive, the rudimentary astronomy he uses still takes a good amount of background information and mathematics to understand. His observations may be confusing to the layman.

You won’t help but smile when Galileo relays how stunned he is to notice that there are more stars that exist than what is visible to the naked eye. “Inconceivable,” he says. The knowledge and science that we take for granted today was brand new in his time, and I can only imagine what a rush it must have been for him to be the first to discover so many truths of the universe.

He ends the book with a cliffhanger:

Time prevents my proceeding further, but the gentle reader may expect more soon.

This future work, centered on proving the Copernican model of the solar system, led to house arrest until his death.

Read More: “The Starry Messenger” via PDF