Galileo Galilei’s Sidereus Nuncius: A Latin-English Summary
Haven’t read this milestone of science? This short and accessible version leaves no excuses.
Having built a simple telescope, Galileo Galilei was the first to observe the universe up close. In the year 1610 he published his first observations in a little booklet titled Sidereus Nuncius (“Starry Messenger”). His observations not only challenged the Earth-centric worldview espoused by the Catholic church, they also contributed to the birth of modern science, as a discipline based on observation and experiment.
This summary contains the most important parts of the Sidereus Nuncius (classical spelling: Nuntius) both in Latin and English for your reading pleasure. Note that I shortened some of the quotes to make them easier to digest.
Mountains And Valleys on The Moon
Starting with the Moon, Galileo gets down to business:
Ex saepius iteratis inspectionibus in eam deducti sumus sententiam, ut certo intelligamus, Lunae superficiem, non perpolitam, aequabilem, exactissimaeque sphaericitatis existere, ut magna philosophorum cohors opinata est, sed, contra, inaequalem, asperam, cavitatibus tumoribusque confertam, non secus ac ipsiusmet Telluris facies, quae montium iugis valliumque profunditatibus hinc inde distinguitur.
Repeated observations have led me to believe with certainty that the Moon’s surface is not polished, equal, exactly spherical, as many philosophers have assumed, but, on the contrary, unequal, rough, filled with cavities and hills, not unlike the Earth’s surface, which is distinguished by mountain ridges and deep vallies.
Galileo concluded this from several observations. For one, he found that the borderline between the bright and the dark part of the Moon was not even, but rugged:
Iam terminus partem obscuram a luminosa dividens non aequabiliter secundum ovalem lineam extenditur, veluti in solido perfecte sphaerico accideret; sed inaequabili, aspera et admodum sinuosa linea designatur: complures enim veluti excrescentiae lucidae ultra lucis tenebrarumque confinia in partem obscuram extenduntur, et, contra, tenebricosae particulae intra lumen ingrediuntur.
The border dividing the dark from the bright part does not extend evenly along an oval line, as it would happen in a perfect sphere, but rather along an uneven, rough and sinuous line: several bright prongs cross the dividing line and extend into the dark part, and, vice versa, shady fragments protrude into the bright part.
You can see this in Galileo’s drawings:
Galileo also observed bright spots on the dark part of the Moon, indicating mountain tops hit by sunlight:
Quod maiorem infert admirationem, permultae apparent lucidae cuspides intra tenebrosam Lunae partem, omnino ab illuminata plaga divisae et avulsae. Huius exemplum eadem figura nobis exhibet. At nonne in terris ante Solis exortum, umbra adhuc planities occupante, altissimorum cacumina montium solaribus radiis illustrantur?
What is even more amazing, many bright tips appear within the Moon’s dark area, distinctly separated and removed from the illuminated region. See the same figure for an example. Is it not the same on Earth? Before dawn, while shadow still occupies the plains, the highest mountain tops are already illuminated by the rays of the sun.
Overall, Galileo’s observations showed that the Moon and the Earth have more similarities than differences — a finding that many people found hard to accept.
The Milky Way is Not a Cloud
When you look at the night sky from the countryside, far away from urban light pollution, you will notice a whitish cloudy band spanning the sky: the Milky Way. Since antiquity, astronomers have speculated about its nature: is it a cloud? A nebula? Stardust? Galileo solved the conundrum:
Est enim Galaxia nihil aliud, quam innumerarum Stellarum coacervatim consitarum congeries: in quamcumque enim regionem illius Perspicillum dirigas, statim Stellarum ingens frequentia sese in conspectum profert, quarum complures satis magnae ac valde conspicuae videntur; sed exiguarum multitudo prorsus inexplorabilis est.
The Milky Way is nothing else than an aggregation of countless stars that are tightly packed together: at whatever part of it you direct the telescope, a huge number of stars readily comes into view, many of which are large and conspicuous; the small ones, however, are inexplorably numerous.
Galileo found the same when he aimed his telescope at some well-known star clusters such as the Pleiades. Seen with the naked eye, the Pleiades consist of six (seven if your eyesight is above average) tightly packed stars. When viewed with the telescope, around forty more stars appear:
If you want to see Galileo’s other sketches of star clusters, have a look at a scanned version of the Sidereus Nuncius.
Jupiter Has Moons
Galileo’s biggest claim to fame was his discovery of four moons revolving around Jupiter. This proved beyond doubt that the solar system doesn’t have a single center of rotation: while Jupiter revolves around the Sun, his moons revolve around him. Let’s retrace how Galileo made the discovery.
On the night of January 7th, 1610, Galileo scanned the sky with his telescope:
Cum caelestia sidera per Perspicillum spectarem, Iuppiter sese obviam fecit. Tres illi adstare Stellulas, exiguas quidem, veruntamen clarissimas, cognovi; quae, licet e numero inerrantium a me crederentur, nonnullam tamen intulerunt admirationem, eo quod secundum exactam lineam rectam atque Eclipticae parallelam dispositæ videbantur, ac caeteris magnitudine paribus splendidiores.
When I watched the stars through the telescope, I came across Jupiter. I detected three little, but bright stars standing next to him. While I believed them to be fixed stars, it astounded me that they were located along an exact straight line parallel to the ecliptic (i.e. the Sun’s orbit), and shinier than other stars of the same magnitude.
Galileo included this sketch of his observation:
In the sketch, Ori. means “east” (oriens) and Occ. means “west” (occidens). The big circle in the middle is Jupiter, the asterisks are his moons.
On the following night, the unsuspecting Galileo stumbled on Jupiter again:
Cum autem die octava, nescio quo fato ductus, ad inspectionem eandem reversus essem, longe aliam constitutionem reperi: erant enim tres Stellulae occidentales omnes, a Iove atque inter se, quam superiori nocte, viciniores, paribusque interstitiis mutuo disseparatae.
When on the 8th of January, I don’t know by which fate, I returned to observing Jupiter, I found an entirely different configuration: all three little stars were in the west, closer to each other and to Jupiter than on the night before, spaced at equal distance from each other.
Again, Galileo made a sketch:
He started to become suspicious:
Hic haesitare coepi, quonam pacto Iuppiter ab omnibus praedictis fixis posset orientalior reperiri, cum a binis ex illis pridie occidentalis fuisset.
I began to wonder how Jupiter could be located east of all the aforementioned fixed stars, since he had been west of two of them on the night before.
In the following weeks, Galileo continued to observe Jupiter as often as possible, finding a different configuration of “stars” every time, like in this modern time-lapse animation of Jupiter and his moons:
It thus became clear to Galileo that the bright moving globules were not stars, but moons:
Statutum ideo omnique procul dubio a me decretum fuit, tres in caelis adesse Stellas vagantes circa Iovem, instar Veneris atque Mercurii circa Solem; quod tandem luce meridiana clarius in aliis postmodum compluribus inspectionibus observatum est; ac non tantum tres, verum quatuor esse vaga Sidera circa Iovem suas circumvolutiones obeuntia.
I therefore determined beyond any doubt that three planets revolve around Jupiter, the same way that Venus and Mercury revolve around the Sun. This became crystal clear through many more observations. Moreover, I found that there are not only three, but four planets rotating around Jupiter.
Galileo called the moons “planets”, or “wandering stars”, as a more accurate word was not invented yet. (In Latin, luna means only the Earth’s moon; other planets’ moons later came to be called satellites.)
Fuel to The Copernican Revolution
Long before Galileo, Copernicus had posited that the solar system did not have a single center of rotation, but at least two, with the Earth revolving around the Sun and the Moon around the Earth. Many scholars found that implausible.
Now, Galileo added a third center of rotation:
Nunc enim, nedum Planetam unum circa alium convertibilem habemus, dum ambo magnum circa Solem perlustrant orbem, verum quatuor circa Iovem, instar Lunae circa Tellurem, sensus nobis vagantes offert Stellas, dum omnes simul cum Iove, 12 annorum spatio, magnum circa Solem permeant orbem.
Now we have not only one planet revolving around another, while both are on a large orbit around the Sun, but we see four planets wandering around Jupiter (like the Moon around the Earth), while all of them together with Jupiter are on a large orbit of 12 years around the Sun.
In Galileo’s judgment, this lent further credibility to the Copernican model. Still, it would take a long time (and a trial against Galileo!) until the Copernican system became accepted.
Did you enjoy this summary? Are there other classics that I should summarize? Let me know in the comments!
