A Brief History of Astronomy

Galileo to Newton

(1610 to 1666)



The Italian mathematician and astronomer, Galileo Galilei, points the newly invented telescope to the sky and revolutionises astronomy.

Galielo's Telescxope
Galileo's Telescope

On the first night, Galileo sees stars that are invisible to the naked eye. If these stars were being seen for the first time, the 'ancients' could not have known everything! The Milky Way is seen to be a vast collection of stars too numerous to be seen individually.

Observing the Sun, he sees sunspots, imperfections in the 'perfect' Sun. He watches them move across the Sun as it rotates; the first time a celestial body has been observed to rotate on its axis. This leads to the thought that if the large Sun could rotate, why not the smaller Earth.

He sees Venus go through a complete cycle of phases. The planet appears to change its shape like a miniature Moon from full to half to crescent. This could only happen if it was moving around the Sun. If Venus was always between the Sun and the Earth (as Ptolemy thought) it would only exhibit a crescent phase at all times.

The phases of Venus
The phases of Venus

Through the telescope, the Moon appears to have mountains and plains. This showed it to be a world, no different to the Earth.

Jupiter's Moons
Jupiter's Moons
Jupiter's four large moons
Looking at Jupiter Galileo discovers its four large moons, resembling little stars. This proves that not everything is moving directly around the Earth. It is also an indication that it is possible for a body to carry its moons with it as it moves around the Sun. If Jupiter could carry four moons why could the Earth not carry its single moon.

Galileo's observations support the Sun-centred Universe of Copernicus and he advocates this system in all his writings. Unlike most academics of the time who only write in Latin, Galileo writes his books in the local language so that they could be read by everyone. The Catholic Church is angered and forces him to deny that the Earth is moving around the Sun.

Although not the first to perform experiments, Galileo makes it fashionable and disproves some of Aristotle's assertions. By dropping cannon balls from a tower (actually, the Leaning Tower of Pisa) he proves that heavy objects fall to the Earth at the same rate as light objects. He shows that falling bodies accelerate as they fall to Earth. More interestingly, moving bodies could be subject to two separate forces acting independently. This explains how objects could be carried on the Earth even if it was moving.

He also shows that the period of a pendulum swing is constant for a given length. This will eventually lead to accurate timepieces.

He attempts to measure the speed of light but fails due to lack of accurate equipment. Galileo's physics experiments set the stage for Newton's work.

Simon Marius
Simon Marius


Using a telescope, Simon Marius describes a fuzzy patch in the constellation of Andromeda. 300 years later, this object (the Andromeda Galaxy) will dramatically expand humanity's notions about the Universe.

John Kepler
John Kepler


The German mathematician, John Kepler, uses the copious observations of Mars by Tycho Brahe to show that the planets move in elliptical orbits. The circular motion of the ancients is finally removed. This is the first major use of the newly discovered logarithms in a scientific calculation.

In addition, Kepler shows that the closer a planet is to the Sun, the faster it moves. This is the same effect that causes ballet dancers to rotate faster when they bring their arms in. The planets are seen to be following mechanical laws similar to those on the Earth. This is a further blow to the ancient idea of one law for the Earth, another for celestial objects.

Kepler discovers a simple mathematical relationship between the period of a planet to orbit the Sun and its distance from the Sun. The square of the period is proportional to the cube of the distance. This provides a scale for the Solar System. If any single distance in the Solar System could be measured it would be possible to calculate all the others. Saturn, the furthest planet, is shown to be 10 times further from the Sun than the Earth.

Kepler suggests that the Sun somehow pulls the planets around it. He correctly predicts the passage of the planets Mercury and Venus in front of the Sun. These are called transits and they would later help in accurately determining the distance from the Earth to the Sun.

Jeremiah Horrocks
Jeremiah Horrocks


In England, Jeremiah Horrocks observes the first transit of Venus. He suggests that observations of this phenomenon from different parts of the Earth could be used to measure the scale of the Solar System, hence the distance from the Earth to the Sun. Observations of an event or object from two vantage points is called parallax.

He proves that the Moon's orbit around the Earth is an ellipse and suggests that the irregularities in the orbit were due somehow to the Sun. He also suggests that Jupiter and Saturn affect each other's orbits.


Godefroy Wendelin measures the distance between the Earth and the Sun using the method first used by Aristarchus. His result is 60% of the actual figure.

Christian Huygens
Christian Huygens


Giovanni Riccioli discovers a double star with the telescope. This is another example of properties of stars not visible to the naked eye.


Christian Huygens (from the Netherlands) discovers a Moon around Saturn. Since this makes six planets (including the Earth) and six Moons, he declares the Solar System complete! More bizarrely, he finds that Saturn itself is surrounded by a ring.

He discovers a new type of object, the Orion Nebula. This is a fuzzy cloud-like nebulous object amongst the stars. Huygens guesses the distance to the brightest star, Sirius by assuming it is the same luminosity as the Sun. He calculates the distance as being over 25,000 times the distance between the Earth and Sun. This is a very large distance but is actually only one twentieth of the correct distance.


Isaac Newton (England) begins work on his masterpiece, Principia. In this book, he explains the motions of the Earth, Moon, and planets in terms of the same force of gravity that pulls objects (like apples) to the Earth.

Isaac Newton
Isaac Newton

He shows mathematically that two bodies that attract each other gravitationally will orbit each other in an elliptical path (explaining Kepler's results). The more massive body will appear to move less while the less massive body will appear to move more. By studying these motions it is possible to show that the Sun is far more massive than all the planets since they all appear to move around it. The theory allows the motions of the Moon and planets to be calculated from first principles. Most planetary orbits are shown to be almost circular apart from that of Mercury, which is strongly elliptical. Newton also confirmed that the planets affect each other's paths as they orbit the Sun.

His equations of gravity show that all objects should fall to the Earth with the same acceleration, as Galileo found. Newton extends the experimental results of Galileo into his three laws of motion. These explain why we do not feel the rotation of the Earth on its axis or its motion around the Sun. They also explain why the planets did not need to be pushed around the Sun and remove the need for planetary 'crystal spheres'. According to Newton, the period of a pendulum can be used to measure the force of gravity on the surface of the Earth.

Newton also explains the tides. They are caused mainly by the Moon (and to a lesser extent, the Sun). His equations explain why there were two tides every day. The Moon is also shown to be responsible for the Precession of the Equinoxes, discovered by Hipparchus.

For the first time, the laws in the heavens are shown to be the same as the laws on the Earth.

The motions of the Solar System (the Sun and planets) are now understood in detail. The stars, however, are still considered to be lights set on a distant crystal sphere beyond the planets.

Apart from his astronomical discoveries, Newton does important work on optics and mathematics.

With the publication of Newton's work, the Age of Reason is considered to have begun.

[Astronomy History: European]   [Astronomy History: Solar System]

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External Links

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History of Science A large collection of resources looking at the history of astronomy, physics, chemistry and mathematics.