Until the early 1500s, it was thought that the Earth was the centre of the Universe. Copernicus was the first astronomer to challenge this view and suggest in 1543 that the Earth and the other planets actually orbited the Sun, but he still believed that the Sun was the centre of the Universe. He actually developed his ideas around 1510, but they were only published openly 33 years later in his book “De Revolutionibus Orbium Coelestium.” The idea of a Suncentred, or “heliocentric” universe was only accepted after the work of Kepler and Galileo.
Following observation of the orbit of Mars, Johannes Kepler discovered that the planets all move on elliptical orbits, and developed his three laws, published between 1609 and 1621:
Kepler’s First Law:
The orbit of each of the planets is an ellipse, with the Sun at one focus.
Kepler’s Second Law:
The radius vector joining the planet to the Sun sweeps out equal areas in equal times.
This means that the planet orbits, its speed changes. It moves fastest when it is closest to the Sun (at “perihelion”) and slowest when it is furthest away (at “aphelion”).
Kepler’s Third Law:
The square of the orbital period of the planet in years is proportional to the cube of the semimajor axis of the planet’s orbit.
This simply relates average distance from the Sun to orbital period.
These laws showed that the planets did indeed orbit the Sun, supporting the Copernican universe.
Galileo built his own telescopes after hearing of their invention in 1609. He used them to observe our Moon, the planets, the moons of Jupiter and stars in our galaxy. His observations confirmed Copernicus’ view of the Universe, which he publicly supported in his book “Discourses and Mathematical Demonstrations on Two New Sciences.” Following its publication in 1632, he was tried for heresy by the Catholic church and confined to house arrest for the rest of his life.
In 1687 the English physicist Isaac Newton put forward his Law of Gravitation that two bodies attract each other with a force that depends on their masses and separation.
F  = 

Where G is a constant known as the gravitational constant. The law was published in Newton’s book “Philosophiae Naturalis Principia Mathematica” (“Principia” for short). It is one of the most important scientific works ever written.
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Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001