By Tim Lambert

In the 16th century and the early 17th century there were huge advances in astronomy. Peoples view of the universe changed completely. The first great astronomer was Nicolaus Copernicus. He was born on 19 February 1473 in the Polish town of Torun.

An ancient Greek called Aristarchus (c 310-230 BC) correctly realized that the Earth orbits the Sun. Unfortunately his ideas were rejected and people came to believe that the Earth is the center of the universe. They believed that the Moon, the Sun and the other planets orbit the Earth. Copernicus realized that this is not so. The Sun is the center of the Solar System and the Earth and the other planets orbit it. This is called the Heliocentric system from the Greek word for Sun, Helios. (The older theory that the Earth is the center is called the Ptolemaic System after a Roman astronomer named Ptolemy (c 87-170 AD). Copernicus also realized that the Earth spins on its axis. The axis is tilted. That accounts for the seasons.

However the theory that the Earth is the center of the Universe raised an awkward question. Aristotle taught that objects fall to the Earth because it is at the center of the Universe and it is natural for them to fall towards the center. If Copernicus was right then what caused objects to fall to the Earth? (Remember this was long before the theory of gravity). Furthermore if the Heliocentric theory was true then the Earth was just another planet, part of the Heavens. But people believed the Heavens were constant and unchanging. How could the Earth be part of the Heavens when it was constantly changing? If Copernicus was correct it called people's other beliefs about the world into question.

Copernicus worked out his new theory and wrote a manuscript but he only circulated it among close friends. He needed to make many mathematical calculations before he could publish his new theory. Finally by 1540 he was ready. The theory was published in 1543. The book was called On the Revolutions of the Heavenly Bodies. It was dedicated to Pope Paul III.

Shortly afterwards on 24 may 1543 Copernicus died. However after his death Copernicus became famous throughout Europe.

The next great astronomer of the 16th century was Tycho Brahe. He lived before the telescope was invented yet he made very accurate observations of the positions of stars.

Tycho Brahe was born on Knudstrup on 14 December 1546. (It was then part of Denmark but it is now part of Sweden). On 21 August 1560 he saw a total eclipse of the Sun. Afterwards Tycho fell in love with astronomy and began studying the stars.

In 1562 Tycho went to Leipzig University where he remained until 1565. However in 1563 Tycho observed a conjunction of Jupiter and Saturn. At that time he realized that existing tables predicting the movement of the stars and planets were inaccurate. Afterwards Tycho was determined to produce his own accurate tables.

From 1565 to 1570 Tycho Brahe traveled around Europe and he obtained astronomical instruments. In 1566 he fought a duel with swords and part of his nose was cut off. Afterwards Tycho wore an artificial metal nose.

In 1571 Tycho built an observatory in the southern tip of Sweden. Then on 11 November 1572 an event happened that changed his life. He observed a new star in the constellation Cassiopeia. In the early 16th century educated people believed that the Heavens (beyond the Moon) were eternal and unchanging. The idea that a new star could appear was very unsettling. Tycho proved that the new star was not an atmospheric phenomena. It definitely was beyond the Moon. He published his conclusions in a book called De Nova Stella (Of a new star). This work made Tycho Brahe famous.

In 1576 King Frederick II gave Tycho the island of Hven near Copenhagen and Tycho built an observatory there. (He called the observatory Uraniborg, which means castle of the Heavens). In the following years Tycho continued to make accurate observations of the stars.

In 1577 Tycho observed and a comet and he was able to prove it was further away than the Moon. The Greek philosopher Aristotle thought comets were an atmospheric phenomena but Tycho showed this is not so.

However Tycho was not correct about anything. At the beginning of the 16th century people believed that the Sun, Moon and planets orbited the Earth. In 1543 Copernicus said that the Earth and planets orbit the Sun. In 1588 Tycho proposed an alternative system. He said that the other planets do orbit the Sun but the Sun in turn orbits the Earth.

Tycho Brahe died on 24 October 1601. He was only 54.

Galileo Galilei was born on 15 February 1564 in Pisa.

In 1581 Galileo started studying medicine at Pisa University. In 1609 Galileo heard of a new invention from Holland. A man named Hans Lippershey (c 1570-1619) had invented the telescope. Galileo made his own telescope and soon improved it.

Using a telescope Galileo was able to see several things invisible to the naked eye. Firstly he could see many stars not visible without a telescope. Secondly the ancient Greeks believed that the Moon was smooth. Looking through a telescope Galileo could see the Moon's surface is actually rough, with mountains and craters. He also discovered 4 small 'moons' orbiting the planet Jupiter. At the time these were astonishing discoveries. Until then nobody knew that any of the other planets, apart from Earth, had 'moons'.

In 1610 Galileo wrote a book called Siderius Nuncius or the Sidereal Messenger. Also that year he was made mathematician and philosopher of the grand duke of Tuscany (at that time Italy was divided into many small states of which Tuscany was one).

At that time astronomers were debating sunspots. A German named Christoph Scheiner claimed that they were satellites of the sun. In 1613 Galileo argued that sunspots are actually on the surface of the sun.

Meanwhile in 1543 a theory by the Polish astronomer Nicolaus Copernicus was published Until his time people believed that the sun, moon and planets orbit the earth. Copernicus argued that the earth and the other planets orbit the sun. At first the church did not have a problem with his theory. However opinion gradually hardened and in 1616 the Copernican theory was declared heretical.

There is a passage in the Old Testament where a prophet named Joshua commanded the sun to stand still in the sky (Joshua 10:12-13). Some scholars said this meant the sun must move. Of course, Joshua knew nothing about Astronomy. To him the sun appeared to move across the sky. Naturally he would command the sun to stand still and to him it would have appeared to stand still. The church's objection to the Copernican theory was based on a misinterpretation of the Bible.

However Galileo was a resolute supporter of the Copernican theory. In 1632 he published a book called Dialogue Concerning the Two Chief World Systems. As a result he was summoned to Rome to be examined by the inquisition. He arrived in January 1633. Galileo was threatened with torture unless he renounced the Copernican theory. Not surprisingly he agreed to do so. Nevertheless he was put under house arrest for the rest of his life.

In 1634 Galileo published a book about mechanics called Dialogue Concerning Two New Sciences. Then in 1637 he noticed that the Moon moves slightly from side to side. Unfortunately in 1638 he went blind.

Galileo died on 8 January 1642 aged 77.

Johannes Kepler was born in Weil der Stadt in southern Germany on 27 December 1571. At first Johannes planned to become a theologian but while at university he became interested in astronomy. (However Kepler remained a devout Christian for the rest of his life). While he was at university Kepler met a man named Michael Maestlin, who believed in the Copernican theory of the universe. Kepler became convinced the Copernican theory was correct and he set out to prove it.

In 1596 Kepler published his book The Cosmic Mystery which defended the Copernican system.

In 1601 Kepler was appointed Imperial Mathematician to the Holy Roman Emperor (who ruled most of central Europe). In 1604 Kepler published his great work The Optical Part of Astronomy in which he studied how lenses work and also how the human eye works. Then in 1604 a new star appeared (today we call it a supernova). In 1606 Kepler described the phenomena in his book On the New Star.

In 1605 while he was trying to calculate the orbit of Mars Kepler realized it moved in an ellipse. Until that time astronomers believed that planets orbited in circles. Kepler realized the orbits of planets are elliptical. He also realized that a planet moves faster when it is closer to the Sun and slower when it is further away from the Sun. He published his observations in a book called New Astronomy in 1609. Johannes Kepler published his work Harmony of the Worlds in 1619.

Between 1618 and 1621 Kepler published his last great work, Epitome of Copernican Astronomy. It was published in 3 volumes.

Johannes Kepler died of fever on 15 November 1630.

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