WORKS AND INSTRUMENTS OF GALILEO

Sidereus Nuncius

Florence National Museum

This book was published in March, 1610. Here Galileo described his discovery of 4 Jupiter's satellites. He first noticed three and later four "starlets" close to the planet. They seemed to follow Jupiter in its motion, and their relative positions changed. "On January 7, 1610,  Jupiter with 3 stars could be seen with the telescope. None of the stars could be seen without the instrument"   They could not be fixed stars, so the only possible conclusion was that they were Jupiter's satellites: "...four wandering stars around Jupiter, just like the Moon around the Earth". This conclusion represented a proof contrary to Ptolemaic cosmology, which did not consider other centers of the motion, besides the Earth. The Earth was the center of the celestial spheres. The astronomer dedicated his discovery to Cosimo II de' Medici, who was at the time grand duke of Tuscany, and this can also be read on the book's cover.

Dialog concerning the two chief world systems

Library of the Bishop's Seminary in Padova

In 1623, Maffeo Barberini became Pope Urban VIII. He was considered a patron of artists and scientists. Galileo tried to raise the Copernican question again, and the Pope gave him permission to write a dialog, where he could discuss the principles of the theory. However he was asked to treat it as a simple mathematical hypothesis, and not to argue in its favor. Galileo worked at the Dialog  until 1630. The writing is divided into four days, during which the Copernican Salviati (standing for Galileo himself) and the Aristotelian Simplicio argue and present the two theories. A third character, Sagredo, often comes into the discussion to support Salviati. During the first three days, the three people consider the terrestrial motion and a few celestial phenomena,  which seem to invalidate the Aristotelian cosmology. The fourth day is dedicated to tides, which is the phenomenon that more than others convinced Galileo of the validity of the Copernican theory. The phenomenon was explained, wrongly, as the combination of the annual revolution and the daily rotation motions. Galileo did not take into account the gravitational attraction of the Moon. The Dialog presented some conclusions supporting the Copernican theory. When Galileo submitted his work to the Church's judgment, Pope Urban VIII managed against its publication, and called the attention of the Inquisition. Galileo was tried and forced to abjure.

Discourse concerning two new sciences of mechanics and local motions

In this work, written between 1633 and 1636, Galileo treats the resistance of materials and some topics concerning Dynamics. The work is structured, like the "Dialog", in four days, during which the same characters (Salviati, Simplicio and Sagredo) discuss several subjects in Physics. The first two days are conceived as a real dialog, and many experiments are presented. In the last two days, some Dynamics theorems are treated with a mathematical formalism. Here Galileo is clever and unveils the paradigms that form the basis of the "daily" Physics. For example, he compares the speed of sound to that of light, he explains the free fall of bodies on an inclined plane, he compares the acoustic vibrations to the musical intervals, and the free motion to the forced motion of bodies (for example, that of projectiles). He always tried to find the common ratio of phenomena, coupling the insight on physical phenomena to  the exactness of the mathematical description.

Hydrostatic balance

In ancient times, precious metals were weighed both in the air and by immersion in water. In the latter way, the weight relative to an equal volume of water was determined, i.e. the specific gravity.
At the age of 22, Galileo wrote a small treatise where he explained how to make the measure more accurate and quantitative. The device that he designed (called hydrostatic balance) was made by a lever mechanism. The weight was fixed to an arm, which was wrapped with a metallic wire. The displacement of the weight could be measured very accurately, by counting how many wire loops it passed.
Galileo built his balance many years later, in 1608.
 
 

Thermoscope

Thermoscope (Museum of Historical Science, Florence)

In the early seventeenth century, no method was known to measure the amount of heat of a body. Many ones knew that if the air is heated, then it expands. The thermoscope was designed by Galileo in the early 1600, and was made by a small bottle whose neck was long and thin. It was set upside down into a basin filled with water. When the bottle was heated, the air inside expanded, and the water level in the neck dropped. When the bottle was cooled, the opposite phenomenon was observed. During the following years, this device was improved first by Galileo and the by his friends Santorio Santorio and Gianfrancesco Sagredo. A numerical scale was provided, sot the first air thermometer was invented. At that same time, and independently, other European investigators designed similar devices. Around 1630, thermometers filled with fluids were perfected, but we had to wait the nineteenth century to have a universal scale of temperatures, based on two base points (ice melting and water boiling).  D.G. Fahrenheit and A. Celsius devised the most widely used scales.

Telescope

The  telescope has been one of the most important instruments for the scientific revolution in 1600, and it had a key role in the success of the Copernican system. Since ancient times, it was known that a few transparent objects can magnify or reduce the apparent images. However, the lenses spread in Europe only at the end of 1200. They were mostly used as glasses, to correct for sight defects.
Even though it was maybe known earlier, the telescope firstly appeared in 1608 in Holland.  H. Lipperhey and J. Metius designed the first projects. It enlarged the images by a factor three or four.
The news of this invention soon spread in Europe, and it was made and used in 1609 by several scientists, for their
astronomical observations. Galileo was not the inventor of the telescope, neither was the first to use it in astronomical research. However, he carried out  the first key astronomical discoveries, which made the instrument famous. He built a telescope which enlarged images by a factor of eight, and which was presented to Venetian Senate in August, 1609. Later, using an improved 20 enlargements instruments, he observed the Moon and discovered Jupiter's satellites.
Later on, other investigators built powerful instruments, and carried out independent observations, such as those of the Sun spots.
 
 

A typical Galileian telescope, such as that used by the scientist to observe Jupiter's satellites, is made by two concentric tubes. Two lenses are fixed to the extremes: a flat/convex objective (the lens facing the object), whose focal distance is  75-100 cm, and a flat/concave eye-piece  (the lens facing the eye), whose focal distance is about 5 cm. The eye-piece tube can be adjusted to focus the target.