OBSERVATIONS AND EXPERIMENTS OF GALILEO

  The astronomical observations of Galileo had fundamental importance for the success of the Copernican theory. His studies of Mechanics and Thermology have been important not only for the knowledge and technology that they brought, but also because they revealed the mathematical nature that is the foundation of all physical phenomena. Moreover, they highlighted the importance of a quantitative investigation. 

The Moon

When the telescope came into usage, it completely demolished the concept of the perfection of the celestial bodies. Using his telescope, Galileo could observe not only the Lunar "seas", but also many smaller regions, surrounded by dark lines. He noticed that the width of these lines changes according to the lunar phase, that is according to the angle formed with the incoming solar light beams. Galileo then concluded that these are shadows, and that the lunar surface has mountains and craters.
The Moon is then neither spherical nor perfect.

Jupiter's satellites

The four largest satellites of Jupiter (Io, Europa, Ganymede and Callisto) are rather bright, in particular when the planet is in opposition, but they cannot be seen by the naked eye, since they are hidden by the glare of Jupiter.
Galileo first discovered them, towards the end of 1609, while he was concluding his telescope observations of the Moon. He noticed first three and then four "starlets" close to the planet. He observed them for several weeks, and he noticed that they seemed to follow the motion of Jupiter across the sky. They also changed both their relative positions and their position with respect to the planet.
On January, 1610, Galileo came to the conclusion that they were not stars, instead they are four "moons" which rotate around Jupiter, just like the Moon around the Earth. He announced his discovery in the work that made him famous, the "Sidereus Nuncius", which was published in Venice on March, 1610.
This was a capital discovery that lead to the success of the Copernican theory of the planetary motions. In Aristotelian cosmology a single center of motion (the Earth) was considered, and all celestial bodies rotated around it. Copernicus instead supported the idea that the Earth moves around the Sun, and the Moon around the Earth. Two centers of the motion were then considered by him. Now it was shown that also Jupiter is a center of motion, so the Ptolemaic theory was no longer valid.
 
 

Saturn

According to Aristotelian cosmology, all celestial bodies are spherical and perfect, but the first observations of Saturn with a telescope were a real surprise. After publishing the "Sidereus Nuncius", Galileo kept observing the sky with his telescope hoping that new discoveries were to come.
On July, 1610, he observed Saturn in opposition. His instrument was not powerful enough to recognize the rings, so they appeared as lateral swellings of the planet. This is what he wrote: "....Saturn is not a single body, it is composed by three bodies, almost touching one another, they do not change neither their appearance nor their position, they are aligned along the Zodiac, and the central body is three times larger than the other two..."
The scientist then called the planet "Saturn three-bodied". Later, he also observed that the lateral bodies had disappeared. In fact, as Saturn moves along its orbit, the rings' plane changes its direction with respect to the Earth. When they were seen edge on, they could not be seen at the telescope.
Later, other astronomers confirmed the strange look of Saturn and its changes, and it was in 1659 when the astronomer Christiaan Huygens explained it with the presence of a ring around the planet.
 

Venus' phases

The planet Venus is illuminated, during its revolution around the Sun, just like the Moon around the Earth, so that it shows its phases. Galileo verified it with his observations at the telescope, and wrote: "Cynthiae figuras aemulatur mater amorum" (the loving mother, Venus) imitates the configurations of Cynthia (the Moon).
Venus' phases falsified the Ptolemaic system, and proved that Venus rotates around the Sun, as predicted by the Copernican system.

Sun spots

Sun spots are dark regions, irregular and variable, on the surface of the Sun. They can be seen also by the naked eye, even though the direct observation of the Sun is very dangerous. The first unaided eye observations of the Sun spots go back to the Chinese astronomers, and are dated at least 28 B.C.
Their systematic study began just after the introduction of the telescope in Astronomy, and the first investigator was Galileo, in 1609. The scientist was joined by Thomas Herriot, Johannes and David Fabricius, and Christoph Scheiner.
The irregularities on the Sun's surface, and its changing appearance, were more proofs against the Ptolemaic theory, since it stated that what belongs to the celestial kingdom is perfect and unchanging.
 
 

Isochronism of the pendulum

Galileo was very interested to a mathematical approach to the question of the motion. He soon began to critically analyze the Aristotelian physics that was taught to him, by means of direct experiments concerning the targets of his study.
It is believed that Galileo began studying the motion of the pendulum in 1581, after watching the oscillation of a lamp inside the cathedral of Pisa, where he carried out his university education. He realized that the oscillation period of a pendulum is independent from its amplitude, a phenomenon that is named"isochronism" of the pendulum. He also tried to find the relations between the length and the weight of the pendulum, and its period. Actually, a pendulum is strictly isochronous only if its oscillations are small. This was discovered by Huygens a few decades later.
A pendulum could then be used as an instrument to measure time intervals, and it was for example used in medicine, in order to measure heart beats.
Many years later, in 1641, Galileo proposed to use the pendulum in order to set the pace of clocks, and sketched a preliminary project. However, he was already old and blind, and he could not carry it out. The clock pendulum was built in 1657, by Christiaan Huygens.
 

Motion of bodies

Galileo studied Aristotelian physics at the university of Pisa, but he soon started to critically analyze it. Aristotelian philosophers had a qualitative approach towards the physical world, which was described in general terms, and their ideas were never verified experimentally. Conversely, the scientist tried to develop a quantitative and mathematical investigation.
One of the subjects to his investigations was the motion of material bodies, and in particular those in free fall. According to the Aristotelian physics, the motion of a body is determined by the forces that act upon it. In the case of a free falling body, these forces are its weight and the air resistance. According to this view, when a body is dropped from a given height, it would reach the ground faster if its weight is larger.
Galileo began a critical investigation of this hypothesis, and Giuseppe Moletti and Benedetto Varchi did so before him. They verified that two bodies made of the same stuff, and with different weights, reach the ground at the same time when dropped from the same height.
In the beginning, the scientist thought that bodies fall with some characteristic uniform speed, which depended on an intrinsic property called specific gravity. When teaching Mathematics at the university of Pisa (1589 to 1592), he began the description of this theory in the book "De Motu", which actually was never published.
During the next twenty years, Galileo made other experiments, and reached the conclusion that all bodies in vacuum fall with uniform acceleration. It does not depend on the nature of their composition, their weight or their shape. The distance traveled during the fall is proportional to the squared time interval that they take.
 
 
 
 

Motion of projectiles

In his "Dialog on Two New Sciences", Galileo faces the problem of the motion of projectiles. Before Galileo, it was believed that if a body is launched horizontally, it moves so until its "impetus" is lost, and afterwards it drops towards the ground along an unknown curved trajectory.
Galileo realized that projectiles are subject not only to the force that pushes them along the horizontal direction, but also to the gravitational force attracting them downwards. The first component acts as an inertial force, so the horizontal distance is proportional to the time taken to cover it. The second component instead causes a constantly accelerated motion, so the vertical distance is proportional to the squared time. Galileo demonstrated that the combination of the two components results in a parabolic motion.
 

The theory of tides

Galileo tried to explain the phenomenon of tides in a purely dynamical way, within the Copernican theory of the motion of celestial bodies.
Just like the motion of water inside a vessel is affected by the motion of the vessel itself, so the motion of the oceans is affected by the motion of the Earth, according to Galileo.
He thought that during its compound motion (rotation plus revolution), the Earth is subject to decelerations and accelerations of its rotation motion, whose period is 12 hours. Due to its own inertia, the seas would rise when "left behind" by the underlying Earth, and vice versa.
This theory is not correct. The actual cause of tides is the gravitational attraction of the Moon upon the Earth. In any case, even if he was sometimes wrong, Galileo tried to explain, through the observations and the Mathematics, the natural phenomena. He did not accept an aprioristic interpretation such as the Aristotelian theory did. This was a step forward in the definition of modern science.