Galileo’s Telescope – The What, When and How

If there’s one man who could be called the father of modern astronomy, it’s Galileo Galilei. Now internationally famous and memorialized by rock songs and space probes to Jupiter, Galileo started his career humbly and died in controversy. Who was Galileo Galilei?

Did Galileo Invent the Telescope?

Born in Pisa, Italy, on February 15th 1564, Galileo was the son of a musician, but it was the spiritual life that first caught his

Painting of Galileo Galilei
This painting of Galileo dates back to about 1640.
Image Credit: National Maritime Museum, Greenwich, London, Caird Collection

attention. A devout Roman Catholic, Galileo had wanted to join the priesthood but, at the age of 16, his father persuaded him to study for a medical degree instead.

It was while he was studying at the University of Pisa that he noticed a swinging chandelier and his interest in physics was awakened. After attending a lecture on geometry, he switched his studies to mathematics, natural philosophy and fine art.

At only 24 years old, he briefly became an instructor at the Academy of Arts of Drawing in Florence before returning to Pisa as the chair of mathematics the following year. Three years later, in 1592, he moved to Padua, where, as professor of mathematics, he taught geometry, mechanics, and astronomy.

No one seems to know what drew Galileo to astronomy in the first place, and while he made a number of inventions (including an early thermometer and a water pump) it’s not true to say he invented the telescope.

Again, no one quite knows for sure, but it’s thought the telescope may have been invented by a German-Dutch spectacle maker called Hans Lippershey. It was Lippershey who submitted the earliest known patent for a refracting telescope in October 1608.

A replica of one of Galileo's earliest telescopes
A replica of one of Galileo’s earliest telescopes. Credit: Jim and Rhoda Morris

Once Galileo heard about the telescope; he was soon building his own and throughout 1609, he worked to improve his creations. By August that year, Galileo had built an 8 power telescope while just two or three months later, he had built another with a magnification two and a half times greater. Less than six months later, he had made discoveries that would alter our view of the universe forever.

What Did Galileo Observe With His Telescope?

Galileo’s telescope was primitive compared to even the most basic we might own today. With an objective lens of just 37mm and a magnification of 20x, it provided a field of view of only 15’ – or roughly half the size of the full Moon. But it was with this Galileo Galilei’s telescope that he began to uncover the true nature of the universe.

Up until that time, it was thought that the Earth was at the center of everything and that the Sun, Moon, and planets all orbited it. The stars themselves were believed to be tiny points of “aether” affixed to crystal spheres that surrounded the Earth.

The Sun, Moon, and planets were thought to be perfect creations. They were; after all, created by God and therefore flawless. So when Galileo turned his telescope toward the Moon at the end of November 1609, he was in for something of a surprise.

Sketches of the Moon by Galileo, as it appeared in his book Sidereus. redit: Wellcome Trust

To the naked eye, the Moon appeared quite smooth, with the lunar “seas” appearing as dark patches upon its surface. Galileo, however, noticed something else. Through his low powered telescope, he saw craters, mountains, and shadows cast by the Sun rising over the lunar surface.

Clearly, the Moon was not as smooth and perfect as it seemed.

He also turned his telescope toward the Milky Way. This misty river flowed across the heavens but no one clearly understood its true nature. One theory was that it was where the northern and the southern celestial hemispheres were joined. Another stated that it was a belt of compressed fire.

Democritus of ancient Greece was, perhaps, closest to the truth. He speculated that the Milky Way was made up of tiny stars, too small to be seen with the naked eye. Galileo was of the same opinion, and after studying the Milky Way with his telescope, came to the conclusion that Democritus was right.

Galileo didn’t stop there. The universe was obviously full of hidden treasures previously unseen by the naked eye observers of centuries past. He went on to make observations of the stars in Orion and the Pleiades. He noted that while six or seven stars could be seen in the cluster with the naked eye, some thirty-five could be seen through a telescope.

Galileo also looked toward some of the other “nebulous” stars that Ptolemy had listed, including the Praesepe, or Beehive Cluster in the constellation of Cancer. Galileo noted that it was made of many tiny stars, and with these observations, he effectively discovered the true nature of star clusters.

When Did Galileo Discover the Moons of Jupiter?

By the beginning of January 1610, the planet Jupiter, just one month past opposition, was now moving through the eastern stars of Taurus, the Bull. On January 7th, Galileo noticed a line of “three fixed stars, totally invisible by their smallness” – two on one side of the planet and the third on the other.

The following night he looked again and noted that the three stars were all on one side. On the 10th, only two could be seen,

A page from Sidereus Nuncius, showing Galileo’s observations of Jupiter and its moons. Credit: History of Science Collections, the University of Oklahoma Libraries

causing Galileo to correctly surmise that the third was hidden by Jupiter itself.

And so it continued until the 13th when a fourth appeared. By now, Galileo was convinced the tiny stars all orbited Jupiter. It was the first discovery of celestial bodies orbiting something other than the Earth and it was to turn the astronomical world upon its head.

Until that time, many believed in the Earth-centered universe, where everything orbited the Earth. Favored by Aristotle and published in his book On the Heavens in 350 BC, this model had existed for nearly 2,000 years and had largely remained unchallenged.

The Polish astronomer Nicolaus Copernicus had proposed a Sun-centered universe some 70 years earlier, but his model had failed to completely take hold. Galileo, however, was a believer, and Jupiter and its moons were proof that Aristotle’s model was wrong. Clearly, not everything orbited the Earth.

Galileo published his findings in a book called Sidereus Nuncius (“Starry Messenger”) in March 1610. Some welcomed his observations while others dismissed the discovery of Jupiter’s moons, attributing them to defects in Galileo’s telescope.

Soon enough, support began to grow. Johannes Kepler wrote a letter of support the following month while additional observations by other astronomers confirmed Galileo’s claims. (Simon Marius, a German astronomer, claimed he had discovered the moons first. While he later named them, the discovery is still credited to Galileo. Collectively, the four moons are still popularly known as the Galilean Satellites.)

The Catholic Church, however, took a far less enlightened view. They had tolerated Copernicus’ Sun-centered model as being merely a theory, but Galileo stated it was fact. The Church’s argument was that it directly contradicted scripture and was therefore heretical. All the same, they didn’t immediately persecute Galileo but allowed him to continue his work instead.

Did Galileo Discover the Rings of Saturn?

By July 1610, Galileo was turning his telescope to planets further afield. At that time, only five planets were known, with the outermost and faintest being Saturn. It was then traversing through eastern Capricornus and headed toward Aquarius.

Given its position in the sky, with the planet still months away from the opposition, Galileo must have stayed up until around midnight to observe it. What he saw baffled him. Saturn was not a single planet, but rather a triple planet!

He announced the discovery in a letter dated July 30th, 1610. “The fact is that the planet Saturn is not one alone,” he wrote, “but composed of three, which almost touch one another and never move nor change with respect to one another.”

Having discovered the four largest moons of Jupiter just 6 months earlier, Galileo surmised these worlds were, in fact, large moons that quickly orbited the planet.

In reality, Galileo was observing Saturn’s rings, but the optics of his telescope were too inferior to show their true nature. Instead, he saw the rings as ill-defined, unfocused circles beside the planet.

Confusingly, nearly two and a half years later he observed the planet again and was surprised to see the moons had completely vanished. In another letter, dated December 4th 1612, he wrote: “What is to be said concerning so strange a metamorphosis?”

Two sketches of Saturn by Galileo, showing the differing views from 1610 to 1616
Two sketches of Saturn by Galileo, showing the differing views from 1610 to 1616

He wondered if the globes had disappeared or if the planet, like its mythological name-sake, had devoured its children. More seriously, he speculated that maybe the optics of his telescope were at fault.

Convinced the “moons” would return, he continued his observations. Sure enough, he saw the planet begin to change again as the “moons” become larger and brighter. By 1616, he was able to see the rings for what they were, but he still wasn’t able to understand them. He drew and described them as “handles” on the side of the planet and so the mystery remained.

Galileo, of course, was observing the planet over the course of nearly seven years. During this time, the planet and its rings appeared to tilt from our vantage point as it orbited the Sun. In doing so, the rings appeared edge-on to us (in 1612) and then re-appeared and widened (as Galileo observed in 1616.)

It wasn’t until Christiaan Huygens observed the planet in 1655 – thirteen years after Galileo’s death – that he realized the nature of the rings.

Galileo’s Observations of Venus and His Final Days

Back in 1610, all of this lay in the future and Galileo was still discovering the solar system. By October that year, the planet Venus was returning to the evening sky and Galileo took the opportunity to make his first observations of our nearest planetary neighbor.

Again, he discovered something unexpected. Venus had phases, like the Moon. When he first observed the planet, it showed a tiny, full disc. Over the next three months, the planet appeared to grow larger, but more slender, as it turned from full, to half, and then to a crescent.

The only possible explanation was that the planet orbited the Sun – and not the Earth. It was clearly further proof that Copernicus was correct: the Sun was at the center of the solar system.

In February 1615, the Church ordered Galileo to abandon his belief in a Sun-centered solar system and one of his books was banned. Galileo was free to discuss the concept as an idea and as a belief but was not permitted to present it as fact.

Galileo, however, couldn’t stay away from the subject. In 1632, with permission from the Church, he published Dialogue Concerning the Two Chief World Systems. In it, he again outlined his belief in a Sun-centered solar system. However, his fatal mistake was in presenting the words of the Pope in a way that made the leader of the Church look foolish.

This was one step too far. Accused of heresy, he stood trial in early 1633 and, after being found guilty in June that same year, his book was banned and he was sentenced to house arrest. He never left his home again and died nearly nine years later, on January 8th, 1642.

Galileo’s legacy is hard to understate. Besides the discovery of Jupiter’s moons, the rings of Saturn and the phases of Venus,

The Galieo space probe at Jupiter. Credit – NASA

he also made observations of sunspots. In 1612 he narrowly missed out on discovering Neptune (before the discovery of Uranus) but he failed to notice the planets’ slow, gradual movements and mistook it for a star.

Galileo’s offending book remained banned for nearly another two hundred years. It wasn’t until October 31st, 1992 – nearly 460 years after his trial – that the Church finally pardoned Galileo. By that time, a space probe named in his honor was on its way to Jupiter. Its discoveries would prove to be as astonishing as those of Galileo himself, and this time, no one dared to doubt them.

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