Galileo Discovers Moons of Jupiter

Jupiter, then, had moons like the earth — four of them in fact! — and they revolved round him in periods which were soon determined.

Continuing Galileo’s Discoveries,
our selection from Pioneers of Science by Sir Oliver Lodge published in 1893. For works benefiting from the latest research see the “More information” section at the bottom of these pages. The selection is presented in six easy 5 minute installments.

Previously in Galileo’s Discoveries.

Time: 1586-1638
Place: Padua, Italy

Now we come to his most brilliant, at least his most sensational, discovery. Examining Jupiter minutely on January 7, 1610, he noticed three little stars near it, which he noted down as fixing its then position. On the following night Jupiter had moved to the other side of the three stars. This was natural enough, but was it moving the right way? On examination it appeared not. Was it possible the tables were wrong? The next evening was cloudy, and he had to curb his feverish impatience. On the 10th there were only two, and those on the other side. On the 11th two again, but one bigger than the other. On the 12th the three reappeared, and on the 13th there were four. No more appeared. Jupiter, then, had moons like the earth — four of them in fact! — and they revolved round him in periods which were soon determined.

The news of the discovery soon spread and excited the greatest interest and astonishment. Many of course refused to believe it. Some there were who, having been shown them, refused to believe their eyes, and asserted that although the telescope acted well enough for terrestrial objects, it was altogether false and illusory when applied to the heavens. Others took the safer ground of refusing to look through the glass. One of these who would not look at the satellites happened to die soon afterward. “I hope,” says Galileo, “that he saw them on his way to heaven.”

The way in which Kepler received the news is characteristic, though by adding four to the supposed number of planets it might have seemed to upset his notions about the five regular solids.

He says:

I was sitting idle at home thinking of you, most excellent Galileo, and your letters, when the news was brought me of the discovery of four planets by the help of the double eye-glass. Wachenfels stopped his carriage at my door to tell me, when such a fit of wonder seized me at a report which seemed so very absurd, and I was thrown into such agitation at seeing an old dispute between us decided in this way, that between his joy, my coloring, and the laughter of us both, confounded as we were by such a novelty, we were hardly capable, he of speaking, or I of listening.

On our separating, I immediately fell to thinking how there could be any addition to the number of planets without overturning my Mysterium Cosmographicon, published thirteen years ago, according to which Euclid’s five regular solids do not allow more than six planets round the sun. But I am so far from disbelieving the existence of the four circumjovial planets that I long for a telescope to anticipate you if possible in discovering two round Mars — as the proportion seems to me to require — six or eight round Saturn, and one each round Mercury and Venus.”

As an illustration of the opposite school I will take the following extract from Francesco Sizzi, a Florentine astronomer, who argues against the discovery thus:

There are seven windows in the head — two nostrils, two eyes, two ears, and a mouth; so in the heavens there are two favorable stars, two unpropitious, two luminaries, and Mercury alone undecided and indifferent. From which and many other similar phenomena of nature, such as the seven metals, etc., which it were tedious to enumerate, we gather that the number of planets is necessarily seven.

Moreover, the satellites are invisible to the naked eye, and therefore can have no influence on the earth, and therefore would be useless, and therefore do not exist.

Besides, the Jews and other ancient nations as well as modern Europeans have adopted the division of the week into seven days, and have named them from the seven planets: now if we increase the number of the planets this whole system falls to the ground.”

To these arguments Galileo replied that whatever their force might be as a reason for believing beforehand that no more than seven planets would be discovered, they hardly seemed of sufficient weight to destroy the new ones when actually seen. Writing to Kepler at this time, Galileo ejaculates:

Oh, my dear Kepler, how I wish that we could have one hearty laugh together! Here, at Padua, is the principal professor of philosophy whom I have repeatedly and urgently requested to look at the moon and planets through my glass, which he pertinaciously refuses to do. Why are you not here? What shouts of laughter we should have at this glorious folly! And to hear the professor of philosophy at Pisa laboring before the Grand Duke with logical arguments, as if with magical incantations, to charm the new planets out of the sky.”

A young German protégé of Kepler, Martin Horkey, was travelling in Italy, and meeting Galileo at Bologna was favored with a view through his telescope. But supposing that Kepler must necessarily be jealous of such great discoveries, and thinking to please him, he writes: “I cannot tell what to think about these observations. They are stupendous, they are wonderful, but whether they are true or false I cannot tell.” He concludes, “I will never concede his four new planets to that Italian from Padua, though I die for it.” So he published a pamphlet asserting that reflected rays and optical illusions were the sole cause of the appearance, and that the only use of the imaginary planets was to gratify Galileo’s thirst for gold and notoriety.

When after this performance he paid a visit to his old instructor Kepler he got a reception which astonished him. However, he pleaded so hard to be forgiven that Kepler restored him to partial favor, on this condition, that he was to look again at the satellites, and this time to see them and own that they were there.

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