Galileo Galilei (15 February 1564 – 8 January 1642) was an Italian polymath. Galileo is a central figure in the transition from natural philosophy to modern science and in the transformation of the scientific Renaissance into a scientific revolution.
Galileo studied speed and velocity, gravity and free fall, the principle of relativity, inertia, projectile motion and also worked in applied science and technology, describing the properties of pendulums and “hydrostatic balances”, inventing the thermoscope and various military compasses, and using the telescope for scientific observations of celestial objects. His contributions to observational astronomy include the telescopic confirmation of the phases of Venus, the discovery of the four largest satellites of Jupiter, the observation of Saturn’s rings (though he could not see them well enough to discern their true nature) and the analysis of sunspots.
In the early 17th century the smartphones weren’t there yet, but people were queueing up for technology nonetheless. Galileo Galilei held the first public demonstration of his telescope from Piazza San Marco, in 1609.
Notables rushed from every corner of Venice to discover the instrument and point it toward distant towns, islands and galleys at sea.
But something different was going on in Galileo’s mind: he was the first to systematically point a telescope towards the heavens, changing the way we see the Universe.
Galileo’s first telescopic observations of the Moon paved the way for discoveries that would cast doubt upon centuries-old beliefs.
Tycho and others had observed the supernova of 1572. Ottavio Brenzoni’s letter of 15 January 1605 to Galileo brought the 1572 supernova and the less bright nova of 1601 to Galileo’s notice. Galileo observed and discussed Kepler’s supernova in 1604. Since these new stars displayed no detectable diurnal parallax, Galileo concluded that they were distant stars, and, therefore, disproved the Aristotelian belief in the immutability of the heavens.
Many contemporary documents describe Galileo’s public demonstration as an incredible and amazing event.
On 7 January 1610, Galileo observed with his telescope what he described at the time as “three fixed stars, totally invisible by their smallness”, all close to Jupiter, and lying on a straight line through it. Observations on subsequent nights showed that the positions of these “stars” relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. On 10 January, Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter. Within a few days, he concluded that they were orbiting Jupiter: he had discovered three of Jupiter’s four largest moons. He discovered the fourth on 13 January. Galileo named the group of four the Medicean stars, in honour of his future patron, Cosimo II de’ Medici, Grand Duke of Tuscany, and Cosimo’s three brothers. Later astronomers, however, renamed them Galilean satellites in honour of their discoverer. These satellites are now called Io, Europa, Ganymede, and Callisto.
From defense to exploration. Although the Senate’s interest in the telescope was motivated by defense purposes, they welcomed Galileo and supported his research—even when it rapidly transcended the material purpose to explore the sky.
From September 1610, Galileo observed that Venus exhibited a full set of phases similar to that of the Moon. The heliocentric model of the solar system developed by Nicolaus Copernicus predicted that all phases would be visible since the orbit of Venus around the Sun would cause its illuminated hemisphere to face the Earth when it was on the opposite side of the Sun and to face away from the Earth when it was on the Earth-side of the Sun. On the other hand, in Ptolemy’s geocentric model it was impossible for any of the planets’ orbits to intersect the spherical shell carrying the Sun. Traditionally, the orbit of Venus was placed entirely on the near side of the Sun, where it could exhibit only crescent and new phases. It was, however, also possible to place it entirely on the far side of the Sun, where it could exhibit only gibbous and full phases. After Galileo’s telescopic observations of the crescent, gibbous and full phases of Venus, therefore, this Ptolemaic model became untenable. Thus in the early 17th century, as a result of his discovery, the great majority of astronomers converted to one of the various geo-heliocentric planetary models, such as the Tychonic, Capellan and Extended Capellan models, each either with or without a daily rotating Earth. These all had the virtue of explaining the phases of Venus without the vice of the ‘refutation’ of full heliocentrism’s prediction of stellar parallax. Galileo’s discovery of the phases of Venus was thus arguably his most empirically practically influential contribution to the two-stage transition from full geocentrism to full heliocentrism via geo-heliocentrism.
Galileo’s observations swept the previous conception of heavens as the realm of perfection away. The Moon was in fact not spherical and, with its scarred surface, hardly perfect.
Galileo made naked-eye and telescopic studies of sunspots. Their existence raised another difficulty with the unchanging perfection of the heavens as posited in orthodox Aristotelian celestial physics. An apparent annual variation in their trajectories, observed by Francesco Sizzi and others in 1612–1613, also provided a powerful argument against both the Ptolemaic system and the geoheliocentric system of Tycho Brahe. A dispute over claimed priority in the discovery of sunspots, and in their interpretation, led Galileo to a long and bitter feud with the Jesuit Christoph Scheiner. In the middle was Mark Welser, to whom Scheiner had announced his discovery, and who asked Galileo for his opinion. In fact, there is little doubt that both of them were beaten by David Fabricius and his son Johannes.
Galileo’s revolutionary thought embodies the spirit of scientific discovery and started here, in Piazza San Marco. The conquest of the sky is among mankind’s efforts to surpass their own limits, where the Moon was the first challenge – and it still is.
The Galileo project
Today, the ASI – Italian Space Agency is a main actor in the GALILEO project, a satellite navigation and tracking system entirely designed for civilian use that can offer a positioning accuracy of less than 10 cm, a precision never before achieved. A system that is not subject to the limitations or interruptions typical of other systems designed for military purposes, starting with the American GPS.
Youth Committee of the Italian National Commission for UNESCO