Galileo Galilei (1564 – 1642) was an Italian astronomer and scientist who launched the scientific revolution and is widely considered the father of modern science. He was a prolific inventor who is credited with several inventions including a hydrostatic balance, a military compass and a forerunner of the modern thermometer. Galileo is the first known person who studied the skies in detail with a telescope. He made numerous significant discoveries in astronomy including the Phases of Venus and the four largest moons of Jupiter. His contributions to science include the Galilean Invariance and discovery of Isochronism in pendulums. Know why Galileo is considered the father of modern science by studying his 10 major accomplishments and their relevance.


#1 He invented a hydrostatic balance

Based on Archimedes principle, weighing precious metals in air and then in water to determine their purity was a common practice among jewelers in Europe. In 1586, at the age of 22, Galileo developed a better method, which he described in a treatise titled La Bilancetta (The Little Balance). In Galileo’s hydrostatic balance, the part of the arm on which the counter weight was hung was wrapped with metal wire. The amount by which the counterweight had to be moved when weighing in water could then be determined very accurately by counting the number of turns of the wire. Thus the proportion of metals, like gold to silver, in the object could be read off directly.

Diagram of Galiloe's hydrostatic balance
Diagram of Galiloe’s hydrostatic balance


#2 Galileo invented a forerunner to the modern thermometer

A thermoscope is a device that shows changes in temperature. It consists of a tube in which the liquid rises and falls as the temperature changes. The modern thermometer gradually evolved from the thermoscope. Galileo discovered the principle on which the thermoscope was based and went on to invent the device in 1593. Galileo’s thermoscope consisted of a vase filled with water, attached to a thin vertical pipe with a large empty glass ball at the top. Changes in temperature of the glass ball exerted vacuum pressure on the water below causing it to rise or fall in the thin vertical pipe.

Thermoscope of Galileo Galilei
Thermoscope of Galileo Galilei at Musee des Arts et Metiers in Paris


#3 He is credited with the invention of an improved military compass

Between 1595 and 1598, Galileo invented an improved military compass or sector. Galileo’s military compass had many useful scales engraved on its legs due to which it could be used for a variety of purposes. It gave gunners a new and safer way of elevating cannons accurately. In addition, it also allowed them to compute the amount of gunpowder needed based on the size and material of the cannonball. As a geometric instrument, the sector facilitated various mathematical calculations, like computation of the area of any polygon or circular sector. It became the major calculating instrument in use from the end of the sixteenth century till the nineteenth century.

Military compass of Galileo
Galileo’s geometrical and military compass


#4 Galileo discovered that pendulums were isochronous

Galileo is the first known person to study in detail the properties of pendulums. Beginning in 1602, he began his studies of the pendulum which led him to note that the swings of the pendulum always take the same amount of time, independent of the amplitude. Thus a pendulum is isochronous, i.e. the oscillation period of a given pendulum is nearly constant, regardless of the angle of its swing. Isochronism is the reason pendulums proved to be so useful for timekeeping. The pendulum clock later became the world’s standard timekeeper till the 1930s. Galileo also found that the period of oscillation of a pendulum is independent of the mass of the bob, and proportional to the square root of the length of the pendulum.

1636 Portrait of Galileo Galilei
Portrait of Galileo Galilei (1636) by Justus Sustermans


#5 He is considered as the father of observational astronomy

The first practical refracting telescopes appeared in the Netherlands around the year 1608. Galileo Galilei heard of the invention and constructed his own version of the instrument, which is now known as a Galilean telescope. Galileo’s best telescope magnified objects about 30 times and, despite its flaws, was good enough for Galileo to explore the sky and make some of the most important discoveries in the history of astronomy. Galileo was the first known person to study, in detail, the skies with a telescope and he is widely regarded as the “father of observational astronomy”.

Galilean Telescope
Photo of a Galilean Telescope


#6 Galileo proved the heliocentric model through his discovery of the Phases of Venus

The Phases of Venus are the different variations of lighting seen on the planet’s surface similar to the moon’s phases. They result from the planet’s orbit around the Sun being inside the Earth’s orbit. The first known observations of the planetary phases of Venus were done by Galileo with his telescope in late 1610. The discovery of Venus’s phases was monumental as it essentially ruled out Ptolemy’s geocentric model which placed the earth at the center of the universe; and was compatible only with the heliocentric model of Nicolaus Copernicus with sun at the center of the solar system.

Phases of Venus
Phases of Venus and evolution of its apparent diameter


#7 He discovered the four largest moons of Jupiter

In January 1610, Galileo Galilei discovered the four largest moons of Jupiter – Io, Europa, Ganymede, and Callisto. Though Galileo named the four moons Medicean stars in honor of his future patron Cosimo II de’ Medici and his 3 brothers, they were later renamed Galilean moons in his honor. Galileo’s observations of the satellites of Jupiter caused a revolution in astronomy as it contradicted the principles of Aristotelian cosmology, which held that all heavenly bodies should circle the Earth. Galileo Galilei was also one of the first Europeans to observe the phenomena of sunspots; and the first to discover that the moon had craters, mountains and valleys, and was not a translucent and perfect sphere as supposed till then.

Galilean Moons
Jupiter and the Galilean Moons (from top to bottom) – Io, Europa, Ganymede and Callisto


#8 His book The Assayer is considered a pioneering work of the scientific method

Prior to Galileo Galilei, scientific inquiry relied heavily on ipse dixit (“he himself said it”), a method in which arguments were justified solely on the basis of authority of an individual, most prominently the ancient Greek philosopher Aristotle. Study of science was also deeply influenced by religious beliefs. In October 1623, Galileo’s book The Assayer (Il Saggiatore) was published. In it Galileo famously wrote that mathematics is the language of science and the only means to achieve lasting truth in physics. The Assayer is considered one of the pioneering works of the scientific method. It attacked theories based on Aristotle’s authority; and promoted experimentation, and mathematical formulation of scientific ideas.


#9 Galileo stated the basic principle of relativity

Cover of Two New Sciences
Cover of Galileo’s final book Two New Sciences

In his famous 1632 work Dialogue Concerning the Two Chief World Systems, Galileo put forward the basic principle of relativity, that the laws of physics are the same in any system moving at a constant velocity. This principle, known as Galilean Invariance, provided the basic framework for Newton’s laws of motion and is central to Einstein’s special theory of relativity. Galileo’s final book, The Discourses and Mathematical Demonstrations Relating to Two New Sciences, was published in 1638. It summarized his work on the two sciences now known as kinematics and strength of materials. It included his law of falling bodies, which said that the acceleration of falling bodies was constant and independent of mass; and his studies of parabolic path of projectiles as a combination of two motions, constant speed and uniform acceleration.

#10 He is considered the father of modern science

Galileo played a key role in the Copernican Revolution by advocating the heliocentric model of the Solar System, which was ultimately established beyond doubt by Isaac Newton. He is considered the greatest scientist of his age who, more than anyone else, launched the scientific revolution through his telescopic discoveries, theories, emphasis on experimentation and mathematical approach to science. His last work, Two New Sciences, is considered his finest and regarded as an influential landmark in the history of science. Due to his immense contribution Galileo Galilei has been called the father of modern science by many people including Albert Einstein and Stephen Hawking.


  1. So Galileo confirmed Copernicus’ theory in a way that the earth was not flat, as everybody believed up to then!


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