Atoms & Light

Part 10 - Galileo

Galileo Galilei (1564 - 1642), an Italian engineer, astronomer and physicist, understood the relationship between mathematics and theoretical physics, noted that the trajectory of a projectile, in the absence of air resistance, was parabolic. He studied gravity and free fall, velocity and inertia, improved the telescope for celestial observation and used it to discover the four largest satellites of Jupiter, Saturn's rings and the phases of Venus.

Between 1595 and 1598, Galileo developed a geometrical and military compass for surveyors and gunners. It provided an accurate way of elevating cannons as well as a method quickly computing the charge of gunpowder for cannonballs of different sizes.

Galileo observed Kepler's supernova in 1604 and concluded that it was a distant star which, showing no detectable diurnal parallax, disproved the Aristotelian assertion that the heavens were unchangeable.

In 1608, Galileo, hearing about the invention of the telescope, made his own with about 3x magnification and later improved his telescopes with up to 30x magnification.

In 1609, Galileo and Englishman Thomas Harriot were two of the first to use telescopes to observe the moons, planets and stars. They found the surface of the Moon was pock marked with craters and not a perfect translucent sphere as supposed. On November 30, Galileo realized that the uneven light was caused by shadows in lunar craters and attempted to estimate the height of the mountains. He found that the Milky Way (the Galaxy) was a uncountable number of densely packed stars rather than the nebulous cloud previously thought. He also located many other stars too faint to be visible with the naked eye and, in 1617, he recorded the double star Mizar in Ursa Major.

In March 1610, Galileo published his observation of the moons of Jupiter which caused a revolution in astronomy. Many astronomers refused to believe there were celestial bodies that did not revolve around the Earth.

From September 1610, Galileo further undermined Ptolemy's theory by observing the full range of Venus's phases, thus confirming Nicolaus Copernicus's prediction that all phases would be visible as Venus rotated around the Sun. It would show its full face the Earth when it was on the opposite side of the Sun and fade to black when it was on the Earth-side of the Sun. The Ptolemaic model became untenable. Galileo thought Saturn a three-bodied system when he first observed Saturn's rings. He was puzzled when he later found two of the bodies had disappeared (as the rings were aligned with Earth). When he next observed the planet in 1616, two of the bodies had reappeared as the rings were again visible.

Galileo observed Neptune in 1612 without realizing that it was a planet. He noted it was an unremarkable dim star, but he did note its motion relative to the stars before losing track of it.

Observation by Galileo and Francesco Sizzi in 1612 showed an apparent annual variation in the movement of sunspots which was another problem for traditional idea of perfect unchanging heavens.

In 1612, Galileo measured the orbital periods of Jupiter's moons and pointed out that this information would make the determination of longitude on Earth possible. The precise time when each of the moons disappeared and reappeared from behind Jupiter could be used as a universal clock visible from most of the Earth's surface. The technique was first successfully tried by Giovanni Domenico Cassini in 1681 and was later used to survey France, and the mid-western United States in 1806. The technique could not be used on a ship at sea, but navigators could place telescopes on shore to find their position accurately.

Galileo proposed that a body would fall with a strictly uniform acceleration only in a vacuum. If it was falling through air, the object would eventually reach a terminal velocity because of air resistance. (During the Apollo 15 mission in 1971, astronaut David Scott proved that Galileo was right. In the vacuum on the Moon, a hammer and a feather fell (accelerated under gravity) at the same speed). 

Ideas about inertia had been proposed earlier, but Galileo stated clearly that objects retain their velocity unless impeded. Which explained why everything did not fall off the moving earth. Galileo also proposed, that there was no absolute motion or absolute rest and the laws of physics were the same in any system that was moving at a constant speed in a straight line, regardless of its particular speed or direction. This was the basic principle of relativity that provided the base for Newton's laws of motion and was central to Einstein's special theory of relativity.

In 1633 Galileo Galilei was convicted, by the Catholic Inquisition, of 'grave suspicion of heresy' for accepting Copernicus's heliocentric theory contrary to the authority of Holy Scripture, and was placed under house arrest for the rest of his life. His offending Dialogue was banned; publication of any of his works was forbidden, including any he might write in the future.

While under house arrest, he wrote, Two New Sciences in which he summarized work he had done some forty years earlier on the two sciences now called kinematics and strength of materials.