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**Johannes Kepler (1571 – 1630)**

Johannes Kepler, a brilliant German mathematician, astronomer and astrologer became famed for his laws of planetary motion. Through observations and mathematics he tested the Copernican theory that the sun was the centre of the universe and not the earth as previously thought. Eventually with trial and error he proved the theory was correct, the planets did indeed travel around the sun.

Using calculations originating from his previous employer Tycho Brahe, as well as his own, he found that planets do not travel around the sun in perfect circles. Infact the orbits are oval – elliptical in shape.

Kepler famously published his three general principles describing the laws of planetary motion around the Sun.

*The First was called the Elliptical Orbit Law:*

The Orbit Laws stated that the motion of each planet is at an ellipse where the Sun is at one of the “two foci”. In plainer English this means that every planet moves in an elliptical shaped orbit with the sun at one focus of the ellipse.

*The Second law is the Equal-Area Law:*

In the Equal-Area Law, a line drawn between a planet and the sun, will cover equal areas over equal time as it continues its orbit.

*The Third law is the Law of Periods:*

This one is a little tricky to explain! The Period law states that the square of the orbital movement of a planet is relative to the cube of the semi-major axis of its orbit. This means the amount of time it takes a planet to orbit the sun is directly proportional to that planets distance from the sun.

Isaac Newton later used Kepler’s laws of gravity motion to help him devise his own theories of universal gravitation.

**Galileo Galilei (1564-1642)**

Galileo, an influential Italian astronomer, physicist and philosopher played a massive role in the Scientific Revolution and made many improvements to the telescope. He is now known as the father of modern observational astronomy, the father of modern physics and also the father of science.

The worlds first telescope is generally credited to lens maker Hans Lippershey in 1608 (though this sometimes disputed). However, without seeing an example, Galilei heard about this “Dutch perspective glass” and subsequently invented a better telescope. Galileo used this telescope to study the planets and made many astronomical discoveries. Carefully observing the sun, he realized that there were dark patches on it, which we now call sunspots. He also used the telescope to confirm the phases of the planet Venus and discovered the the four largest moons of the planet Jupiter (now named the Galilean moons)

Galileo was met with strong opposition from the church for his championing of the Copernican/Heliocentric theory that the sun was at the centre of the solar system. They condemned heliocentric views as “false and contrary to Scripture so he forced to publically withdraw his beliefs. When he later published his work “Dialogue Concerning the Two Chief World Systems” which compared the Copernican system with the older Ptolemaic system, he was found guilty of heresy and after first being sentenced to life imprisonment, spent his final years under house arrest

Galileo continued to write and although he was by now going blind he published ‘Discourses Concerning Two New Sciences’ 1638. A book about kinematics and the strength of materials which later received great praise from Albert Einstein.

Other famous discoveries were the rate at which a pendulum swings depends on its length and not on the distance through which it swings. This led to the development of the law of the pendulum, which was eventually used to help develop clocks. He also invented other instruments such as a thermometer and a military compass.

**Isaac Newton (1643 – 1727)**

Isaac Newton, an English physicist and mathematician is one of greatest scientists of all time. Best known for his Laws of Motions and the Law of Gravitation, his book “The Mathematical Principles of Natural Philosophy” is perhaps the most influential science book ever published which showed that gravity applied to all objects in the universe.

Newton with his theory of gravity, inspired by a falling apple, discovered how the universe was held to together. Nicolaus Copernicus and Johannes Kepler had previously known about gravity but Newton realized Gravity holds the universe together and that there is a gravitational force between all objects and heavenly bodies. There is a force between you and the Sun for example but it is too far away for its influence to be strong. The force between large objects although greater, becomes less the further away an object is.

Newton provided mathematical confirmation for Kepler’s three laws of planetary motion and together with his own gravitation theory helped remove the last doubts that the sun was the centre of the solar system.

*The First Law:*

The law on Inertia means that there is a tendency of all objects to maintain their state of motion unless compelled to change by an external influence.

*The Second Law:*

Newton’s second law of motion states that the acceleration of an object is dependent upon two things, the net force acting upon the object and the mass of the object.

The greater the mass of an entity being accelerated, then the greater the amount of force needed to accelerate it.

*The Third Law*:

Newton’s third law of motion states that for every action there is an equal and opposite reaction so if an object applies a force to a second object, then the second object applies an equal and directly opposite force on the first one.

He also conducted many experiments using light and discovered that white light is composed of all the colours of the spectrum and is given partial credit for the developing the calculus – a method of calculation.

Christiaan Huygens (1629 – 1695)

Huygens, a Dutch mathematician, astronomer and physicist was, after Isaac Newton, the greatest scientist of the 1600s. He made important advancements in many areas and with his powerful telescope, was able to correctly identify the true of the nature of the rings of Saturn, before this they were thought to be arms of Saturn. He also detected its moon Titan.

Responding to Newtons theory of light, Huygens stated that light consists of waves and he also enormously helped with the understanding of wave-particle duality. To describe this complex matter briefly, wave–particle duality is a concept in Quantum physics that matter and light display behaviors of both waves and particles.

In a 1673 book entitled “Horologium Oscillatorium” about his analysis of pendulums, he also suggests important principles of gravity and ‘centrifugal force’. For example, an object moving in a circle acts as if it is experiencing an external influence known as the centrifugal force. Huygens is credited with this discovery.

Inspired by Galileos investigations of pendulums, he learned a great about them and built the very first accurate pendulum clock. He also made many other studies in timekeeping, optics, the calculus and sound perception.

Luan