1543 Nicolaus Copernicus proposes a theory of a sun-centered cosmos, but proof is needed because Earth does not seem to move. 1608 Dutch eyeglass-makers develop the first telescopes.


1656 Dutch scientist Christiaan Huygens builds ever-bigger telescopes that are capable of detecting more detail and fainter objects. 1668 Isaac Newton produces the first reflecting telescope, an instrument that is far less affected by the distortion of chromatic aberration. 1733 The first flint glass/crown glass achromatic lens is made. This greatly improves the potential image quality of refracting telescopes.

Galileo Galilei’s effective use of a telescope marked a watershed in the history of astronomy. There have been other turning points—such as the introduction of photography, the discovery of cosmic radio waves, and the invention of the electronic computer—but the invention of the telescope was fundamental to the advancement of the subject.

“The Milky Way is nothing else but a mass of innumerable stars planted together in clusters.” Galileo Galilei

Limits of the naked eye

Before Galileo, the naked eye was all that was available to observe the sky. The naked eye is limited in two main ways: it is unable to record detail, and it can only detect objects that are reasonably bright.

When looking at a full moon, the lunar diameter subtends (spans) an angle of 1⁄2º at Earth’s surface. This means that two lines extending from opposite sides of the moon meet at the eye to make an angle of 1⁄2º. However, the naked eye can only detect separate objects that are more than about 1⁄60º apart. This is the eye’s resolution, and determines the level of detail it can detect. Looking at the full moon with the naked eye, the lunar diameter is resolved into only 30 picture elements, analogous to individual pixels in a digital photograph. Dark lunar seas and lighter lunar highland are discernible, but individual mountains and their shadows are beyond detection.

Looking up at the night sky on a cloud-free, moonless night in Galileo’s Italian countryside, 2,500 stars would be visible above the horizon. The Milky Way—the disk of the solar system seen side-on—looks like a river of milk to the naked eye. Only a telescope shows that the Milky Way seems to be made up of individual stars; the bigger the telescope, the more stars are visible. By turning his new telescope to the night sky, Galileo would become one of the very first people to appreciate the true nature of this band of stars across the sky.

The resolution of the naked eye is about 1⁄60°. The moon subtends an angle of 1⁄2° seen from Earth, meaning that the lunar diameter can be resolved into 30 picture elements.
Galileo demonstrates his telescope to Leonardo Donato, the Doge of Venice. Like other astronomers of his time, Galileo relied on patronage to fund and legitimize his work.

Building a telescope

Galileo did not invent the telescope himself. The idea of combining two lenses—a large one at the front of a tube to collect the light, and a small one at the back to magnify the image—had come from the Dutchmen Hans Lipperhey, Jacob Metius, and Sacharias Janssen in around September 1608. (It had taken over 300 years to progress from the invention of reading glasses to the invention of a telescope.) After hearing about this new instrument, Galileo had resolved to make one for himself.

A telescope does two important things. Its resolution (the detail a telescope can detect) is proportional to the diameter of the objective lens—the large lens at the front that collects the light. The larger the objective lens, the better the resolution. An eye that has fully adapted to the dark has a pupil that is about ¼ in (0.5 cm) across, and a resolution of around 1⁄60º. Put the eye at the back of a telescope with an objective lens of 1, 2, or 4 cm diameter, and the resolution improves to 1⁄120º, 1⁄240º, and 1⁄480º respectively. Details then spring into view. Jupiter, for example, looks like a disk and not just a point.

A telescope also acts as a “light bucket.” Every time the diameter of the objective lens is doubled, the light gathered increases by a factor of four, and objects of similar light output can be detected if they are twice as far away. Objective lenses of 1, 2, and 4 cm enable the eye to discern 20,000, 160,000, and 1,280,000 stars respectively.

Galileo was not satisfied with his first instrument, which only magnified three times. He realized that a telescope’s magnification was directly related to the ratio of the focal length of the objective lens to the focal length of the eyepiece. A longer-focus convex lens for the objective, or a shorter-focus concave lens for the eyepiece was required. Since these were not available, Galileo taught himself to grind and polish lenses and made them for himself. Living in northern Italy, the glassmaking center of the world at the time, helped him considerably. He eventually developed a new telescope with 33 times magnification, and it was with this improved instrument that he discovered the Jovian (“of Jupiter”) moons.

“My dear Kepler, what would you say of the learned who … have steadfastly refused to cast a glance through the telescope?” Galileo Galilei

“Three little stars”

Galileo discovered the moons of the planet Jupiter on the night of January 7, 1610. At first, he thought he was looking at distant stars, but he quickly realized that the new bodies were moving around Jupiter, At the time, Galileo was a 45-year-old professor of mathematics at the University of Padua near Venice. When he published his pioneering telescopic observations, he wrote: “Through a spyglass, Jupiter presented himself. And since I had prepared for myself a superlative instrument, I saw (which earlier had not happened because of the weakness of other instruments) that three little stars were positioned near him—small but yet very bright. Although I believed them to be among the number of fixed stars, they nevertheless intrigued me because they appeared to be arranged exactly along a straight line and parallel to the ecliptic ….”

Refracting telescopes suffer from a problem known as chromatic aberration. The different wavelengths of light are brought to slightly different foci, so the final image is surrounded by a halo of color.

“Galileo had the experience of beholding the heavens as they actually are for perhaps the first time.” I Bernard Cohen

Repeated observations

Galileo’s unexpected discovery fascinated him. As he observed Jupiter night after night, it soon became clear that the new stars were not beyond Jupiter, in the distant heavens. They not only accompanied the planet as it moved along its path across the sky, but also moved around the planet.

Just as the moon orbits Earth every month, Galileo realized that there were four moons in orbit around Jupiter, staying with it as it orbited the sun. The more distant moons took longer to complete their orbits than the closer ones. The time to complete one orbit from the inner to the outer moon is 1.77, 3.55, 7.15, and 16.69 days, respectively. The Jovian moon system looked like a small model of the sun’s planetary system. It was proof that not everything in the cosmos orbited Earth, as had been thought in pre-Copernican days. The observation of these four moons was a boost to the theory of the sun-centered cosmos.

Galileo quickly published his discovery in his book Siderius Nuncius (The Starry Messenger), published on March 10, 1610. In the hope of advancement, Galileo dedicated the book to a former pupil of his who later became the Grand Duke of Tuscany, Cosimo II de’ Medici. He named the moons the Medicean Stars in honor of the four royal Medici brothers. This political thoughtfulness won him the position of Chief Mathematician and Philosopher to the Medici at the University of Pisa. However, the name did not catch on.

At first, many were sceptical, suggesting that the moons were no more than defects in the telescope lens. However, other pioneering telescopic astronomers such as Thomas Harriot, Joseph Gaultier de la Vatelle, and Nicolas-Claude Fabri de Peiresc confirmed their existence when Jupiter returned to the night sky later in 1610, after passing behind the sun.

Galileo’s telescope had a concave lens as an eyepiece. When viewing a celestial object a great distance away, the distance between the two lenses would equal the focal length of the objective lens minus the focal length of the eyepiece.
Kepler’s telescope, developed soon after, had a convex lens as an eyepiece. The length of the telescope was equal to the objective focal length plus the focal length of the eyepiece.

Disputed priority

In 1614, German astronomer Simon Marius published Mundus Iovialis, in which he described Jupiter’s moons and claimed to have discovered them before Galileo. Galileo would later accuse Marius of plagiarism, but it is now generally accepted that he made his discovery independently at around the same time. Marius named the moons Io, Europa, Ganymede, and Callisto after the Roman god Jupiter’s love conquests, and these names are still used. They are now known collectively as the Galilean moons.


There were two kinds of early refracting telescope: the Galilean, and the Keplerian, developed in 1611 by Johannes Kepler. They both had a long-focus, large diameter lens at the front, called the objective. This collected the light and brought it to a focus. The image at the focus was magnified using the smaller, short-focus eyepiece lens.

The magnification of the instrument is equal to the focal length of the objective lens divided by the focal length of the eyepiece. A flatter convex objective lens reduced chromatic aberration, gave a longer focal length, and, for a fixed eyepiece, greater magnification. For this reason, telescopes became longer in the 17th century. The minimum focal length of eyepieces at the time of Galileo and Kepler was about 1–1½ in (2–4 cm). This meant that, for a magnification of x30, an objective lens with a focal length of 24– 48 in (60–120 cm) was needed. Built in 1888, the huge James Lick Telescope on Mount Hamilton, California (above), has a 36-in (90-cm) lens and a focal length of 57 ft (17.37 m).

A Jovian clock

Galileo carefully studied the changing positions of the Jovian moons from day to day. He concluded that, like the planets, their positions could be calculated in advance. Galileo saw that, if this could be done accurately, the system would act as a universal clock and could solve the problem of measuring longitude at sea. To establish longitude requires the ability to tell the time, but in Galileo’s day, there were no timepieces that would work on a boat. Because Jupiter is at least four times farther away from Earth than the sun, the Jovian system looks the same from anywhere on Earth, so a “Jovian clock” would work from anywhere. The longitude problem was finally solved with the introduction of accurate chronometers by the English clockmaker John Harrison around 1740. This was well before the orbits of Jupiter’s moons had been worked out in detail.

Galileo’s discovery of four satellites around Jupiter had another interesting consequence. When Jonathan Swift published Gulliver’s Travels in 1726, he predicted, in the chapter on Laputa, that Mars would have two moons simply because Earth had one and Jupiter four. In 1877, this prediction was fortuitously proved to be correct when Asaph Hall discovered Mars’s two small moons, Phobos and Deimos, using a new 26-in (66-cm) refracting telescope at the US Naval Observatory in Washington.

Starting with the closest to Jupiter, the Galilean moons are, from left to right, Io, Europa, Ganymede, and Callisto. Ganymede is larger than the planet Mercury.

Support for Copernicus

In Galileo’s time, there was still a heated debate between believers of the old biblical theory that Earth was stationary at the center of the cosmos and Copernicus’s new idea that the Earth was in orbit around the sun. The geocentric (Earth-centered) idea stressed the uniqueness of the planet, while the heliocentric (sun-centered) proposal made Earth just one of a family of planets. The assumption that Earth does not occupy a privileged place in the cosmos is now known as the Copernican principle.

The challenge now was to find observations to prove that one theory was correct and the other false. The discovery of moons around Jupiter was great support for a sun-centered system. It was now clear that everything did not orbit around a central Earth, but there were still unanswered questions. If the sun-centered system was correct, Earth must be moving. If Earth had to travel around the sun every year, it had to have an orbital speed of 20 miles/sec (30 km/sec). In Galileo’s time, the exact distance from Earth to the sun was not known, but it was clearly far enough that Earth would need to be moving quickly, and humans cannot apprehend this movement. Also, this orbital motion should make the stars appear to swing from side to side every year in a phenomenon called stellar parallax. This again was not observed at the time. Galileo and his contemporaries did not suspect that the typical distance between stars in the Milky Way was about 500,000 times larger than the distance between Earth and the sun, which makes stellar parallax so small that it is difficult to measure. It was not until the mid-19th century that vastly improved instruments made it possible to detect this swing.

Despite these questions, Galileo considered that his findings had proved Copernicus correct beyond reasonable doubt. His discoveries also included the phases of Venus, which are best explained if the planet is in orbit around the sun, and the fact that the sun is spinning, shown by the movement of sunspots. By 1619, Galileo’s pugnacious defense of Copernicus had drawn him into conflict with the Church, which had declared in 1616 that heliocentricism was heretical. In 1633, he appeared before the Inquisition. His books were banned, and he spent the last 10 years of his life under house arrest.

“The Bible shows the way to go to heaven, not the way the heavens go.” Galileo Galilei

New moons

Jupiter only had four known moons for 283 years. A fifth satellite, Amalthea, was discovered by the American astronomer E. E. Barnard in 1892, using the 36-in (91-cm) refractor at the Lick Observatory in California. It was the last solar system satellite to be discovered by direct observation. Subsequently, satellites have been found by the meticulous examination of photographs. The number of known Jupiter satellites had crept up to 12 by the mid-1950s, and has now reached 67. Many smaller moons may be found in the future.


Galileo Galilei was born in Pisa, Italy on 15 February 1564. He was appointed to the Chair of Mathematics at the University of Pisa in 1589, moving to the University of Padua in 1590. Galileo was an astronomer, physicist, mathematician, philosopher, and engineer, who played a pivotal role in the process of intellectual advances in Europe now known as the Scientific Revolution.

He was the first person to effectively turn the refractor telescope on the heavens. During 1609–10, he discovered that the planet Jupiter had four moons, Venus underwent phase changes, the moon was mountainous, and the sun was spinning round once in about a month. He was a prolific writer and made his findings accessible to a wide audience.

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