1795-Week-5 Galileo Time

Galileo: Overview

  • Time Period: 1564-1642

  • Nationality: Italian

  • Professions: Astronomer, physicist, engineer, philosopher, mathematician

  • Key Contributions: Improved the telescope, established a combined observational and mathematical approach to science, laid foundations for the scientific method

Galileo’s Historical Context

  • Conflict with the Church:

    • Faced challenges for supporting Copernicus's heliocentric model, contrary to the Church's Aristotelian geocentric view.

    • Significant Observations: The moons of Jupiter, phases of Venus, and imperfections of celestial bodies like the Moon challenge the perfection of the heavens as described by Aristotle.

Contributions to Physics

Understanding Motion

  • Key Question: Which object falls faster: a heavy ball or a light ball?

    • Traditional belief (Aristotle): Heavier objects fall faster due to an inherent urgency to return to their natural place: the Earth.

Critique of Aristotelian Physics

  • Aristotle’s Model:

    • Earth-centered (geocentric) universe with four earthly elements: earth, water, air, fire; and a fifth celestial element (quintessence).

    • Motion was explained as a return to natural places.

Galileo’s Experiments

  1. Projectile Motion:

    • Contradicted Aristotle’s view using observational and mathematical approaches.

    • Demonstrated through the experiment with an inked ball rolling down a curved incline that projectile motion is parabolic.

  2. Inclined Plane:

    • Created artificial hills to study motion without external friction.

    • Determined that the distance covered is related to time in a specific mathematical way: distance = time².

Introducing Time into Physics

  • Measurement Methods:

    • Used pendulums, water clocks, and internal time counts (musical training) to measure motions accurately.

  • Key Finding:

    • The relationship between the distance covered by a falling object and the time taken was consistent despite changing angles of incline.

Establishing the Law of Inertia

  • Thought Experiment:

    • Conducted using inclined planes.

    • Concluded that in the absence of friction, a ball would rise to the same height it started from, leading to the discovery of inertia.


    • Galileo's Law of Inertia: An object in motion remains in motion unless acted upon by an external force.

Major Publications

  1. Sidereus Nuncius (1610):

    • Observations of celestial bodies, including the moons of Jupiter.

  2. Il Saggiatore (1623):

    • Discussion on physical phenomena and philosophy of science.

  3. Dialogue on the Two Chief Systems of the World (1632):

    • A debate on geocentric vs heliocentric models, cleverly framing arguments that support heliocentrism.

  4. Discorsi (1638):

    • Kinematic analysis of motion and material strength.

Conflict with the Church

  • 1616 Inquisition:

    • Galileo questioned for supporting the heliocentric model; Church declared geocentrism as doctrine.

  • Publication & Recantation:

    • After the publication of 'Dialogue,' faced trial in 1633, recanted, and spent life under house arrest.

Legacy and Conclusions

  • Influence on Science:

    • Galileo is recognized for establishing experimental methods in science and influencing Newton’s laws of motion.

  • Papal Acknowledgment (1992):

    • Pope John Paul II recognized the Church's error in prosecuting Galileo.

  • Galileo's Quote:

    • "Natural philosophy is written in this enormous book... However, it cannot be understood without first learning to understand the language, and to recognize the characters in which it is written. It is written in a mathematical language."

Key Astronomical Observations

  • The Moon has mountains and craters, Jupiter has four moons, and Venus goes through phases.

  • Concept of Perfect Celestial Bodies Defeated:

    • Discovered the Sun had spots, questioning the perfection of celestial bodies.

  • The Milky Way:

    • Composed of numerous stars, suggesting an expansive universe.