Nicolaus Copernicus Biography: Facts and Discoveries The Copernican Planisphere, illustrated in 1661 by Andreas Cellarius. Credit: Public domain In the early 1500s, when virtually everyone believed Earth was the center of the universe, Polish scientist Nicolaus Copernicus proposed that the planets instead revolved around the sun. Although his model wasn't completely correct, it formed a strong foundation for future scientists to build on and improve mankind's understanding of the motion of heavenly bodies. Indeed, other astronomers built on Copernicus’ work and proved that our planet is just one world orbiting one star in a vast cosmos loaded with both, and that we’re far from the center of anything. [See also our overview of Famous Astronomers and great scientists from many fields who have contributed to the rich history of discoveries in astronomy.] Here is a brief biography of Copernicus: Celestial education Born on Feb. 19, 1473, in Poland, Copernicus traveled to Italy at the age of 18 to attend college, where he prepared for a career in the church.

As part of his education, he studied astrology — reading the stars to learn about future events — because at the time it was felt important for priests and doctors. Astronomy, the motion of heavenly bodies, was an important element of this. (Today, astronomy is a recognized science, whereas astrology is not .) Nicolaus Copernicus Credit: Public Domain While attending the University of Bologna, he lived and worked with astronomy professor Domenico Maria de Novara, doing research and helping him make observations of the heavens. When he returned to Poland to take up official duties in Frauenburg, his room in one of the towers surrounding the town boasted an observatory, giving him ample time and opportunity to study the night sky. Copernicus never took orders as a priest, but instead continued to work as a cleric. A new model In Copernicus' lifetime, most believed that Earth held its place at the center of the universe. The sun, the stars, and all of the planets revolved around it. One of the glaring mathematical problems with this model was that the planets, on occasion, would travel backward across the sky over several nights of observation. Astronomers called this retrograde motion .

To account for it, the current model, based on the Greek astronomer and mathematician Ptolemy's view, incorporated a number of circles within circles — epicycles — inside of a planet's path. Some planets required as many as seven circles, creating a cumbersome model many felt was too complicated to have naturally occurred. In 1514, Copernicus distributed a handwritten book to his friends that set out his view of the universe. In it, he proposed that the center of the universe was not Earth, but that the sun lay near it. He also suggested that Earth's rotation accounted for the rise and setting of the sun, the movement of the stars, and that the cycle of seasons was caused by Earth's revolutions around it. Finally, he (correctly) proposed that Earth's motion through space caused the retrograde motion of the planets across the night sky (planets sometimes move in the same directions as stars, slowly across the sky from night to night, but sometimes they move in the opposite, or retrograde, direction).

It wasn't until he lay on his deathbed at the age of 70 that Copernicus published his book, De Revolutionibus Orbium Coelestium ("On the Revolutions of the Heavenly Spheres"). In it, Copernicus established that the planets orbited the sun rather than the Earth. He lay out his model of the solar system and the path of the planets. Refining the work Although Copernicus' model changed the layout of the universe, it still had its faults. For one thing, Copernicus held to the classical idea that the planets traveled in perfect circles. It wasn't until the 1600s that Johannes Kepler proposed the orbits were instead ellipses. As such, Copernicus' model featured the same epicycles that marred in Ptolemy's work, although there were fewer. Copernicus' ideas, published only two months before he died, took nearly a hundred years to seriously take hold. When Galileo Galilei claimed in 1632 that Earth orbited the sun, building upon the Polish astronomer's work, he found himself under house arrest for committing heresy against the Catholic church.

Despite this, the observations of the universe proved the two men correct in their understanding of the motion of celestial bodies. Today, we call the model of the solar system, in which the planets orbit the sun, a heliocentric or Copernican model. — Nola Taylor Redd, SPACE.com Contributor Johannes Kepler Biography Johannes Kepler Biography By Nola Taylor Redd, Space.com Contributor | May 21, 2012 09:03am ET MORE When Johannes Kepler was born in the late sixteenth century, scientists believed that planets in the solar system traveled in circular orbits around the Earth. The occasional problem was solved by the addition of miniature circles test — epicycles — to planetary paths. But Kepler not only adamantly defended the idea that planets orbit the sun, he also revealed that their paths were not perfect circles. His descriptions of planetary motions became known as Kepler’s laws. [See also our overview of Famous Astronomers and great scientists from many fields who have contributed to the rich history of discoveries in astronomy.] Here is a brief biography of Kepler: German astronomer Johannes Kepler used mathematics to calculate the path of the planets, finding that they traveled not in circles, as long expected, but in ellipses.

Credit: Johnnes Kepler Gesammelte Werke , C. H. Beck, 1937 Johannes Kepler Facts Born in December of 1571, young Kepler was a sickly child of poor parents. He was awarded a scholarship to the University of Tübingen, where he studied to become a Lutheran minister. While there, he studied the work of Nicolaus Copernicus , who taught that the planets orbited the sun rather than the Earth, though he had no observational evidence to offer as proof. In 1596, Kepler wrote the first public defense of the Copernican system. This was a dangerous stance, given that in 1539, Martin Luther, founder of the Lutheran church, derided the theory when he first heard it, while the Catholic church deemed such a position heretical in 1615 (they later subjected astronomer Galileo Galilei to house arrest for his publication on the subject). In search of the most detailed notes about the paths of the planets , Kepler contacted astronomer Tycho Brahe.

A wealthy Danish nobleman, Brahe built an observatory in Prague where he tracked the motions of the planets and maintained the most accurate observations of the solar system at the time. In 1600, Brahe invited Kepler to come work with him. Brahe, however, proved to be suspicious and unwilling to share his detailed notes with his assistant. Instead, he assigned Kepler to solve the mystery of Mars, one of the most puzzling problems in astronomy at the time. Ironically, the detailed records of the challenging planet were the tools Kepler needed to understand how the solar system functioned. When Brahe died in 1601, Kepler managed to acquire Brahe's observations before his family could use them to their financial benefit. Kepler's laws The Martian problem, which Kepler said he would solve in eight days, took nearly eight years. Astronomers had long struggled to figure out why Mars appeared to walk backwards across the night sky. No model of the solar system — not even Copernicus' — could account for the retrograde motion. Using Brahe's detailed observations, Kepler realized that the planets traveled in "stretched out" circles known as ellipses.

The sun didn't sit exactly at the center of their orbit, but instead lay off to the side, at one of the two points known as the focus. Some planets, such as Earth, had an orbit that was very close to a circle, but the orbit of Mars was one of the most eccentric, or widely stretched. The fact that planets travel on elliptical paths is known as Kepler's First Law. Mars appeared to move backward when Earth, on an inner orbit, came from behind the red planet, then caught up and passed it. Copernicus had suggested that observations made from a moving Earth (rather than a centrally located one) could be a cause of the retrograde motion, but the perfect circular orbits he posited still required epicycles to account for the paths of the planets. Kepler realized that two planets, traveling on ellipses, would create the appearance of the red planet's backward motion in the night sky. Kepler also struggled with changes in the velocities of the planets. He realized that a planet moved slower when it was farther away from the sun than it did when nearby.

Once he understood that planets traveled in ellipses, he determined that an invisible line connecting the sun to a planet covered an equal amount of area over the same amount of time. He posited this, his Second Law, along with his first, which he published in 1609. Kepler's Third Law was published a decade later, and recognized that the relationship between the period of two planets — the time they take to orbit the sun — is connected to their distance from the sun. Specifically, the square of the ratio of the period of two plants is equal to the cube of the ratio of their radius. While his first two laws focus on the specifics of a single planet's movement, his third is a comparison between the orbit of two planets. Other notable discoveries Though Kepler is best known for his defining laws regarding planetary motion, he made several other notable contributions to science. He was the first to determine that refraction drives vision in the eye, and that using two eyes enables depth perception. He created eyeglasses for both near and farsightedness, and explained how a telescope worked. He described images and magnification, and understood the properties of reflection.

Kepler claimed that gravity was caused by two bodies, rather than one, and as such, the moon was the cause of the motion of tides on the Earth. He suggested that the sun rotates, and created the word 'satellite'. He tried to use his knowledge of the distance Earth travels to measure the distance to the stars. Kepler also calculated the birth year of Christ. In recognition of his contribution to his our understanding of the motion of the planets, NASA named their planet-finding telescope after the German astronomer. Other references: Galileo Unit 2 : Activity 2 : Ptolemaic Model : Copernican Model : Brahe : Kepler : Galileo : Newton : Unit Exam Galileo Galileo Galilei was the first astronomer to use a telescope to study the heavens. Galileo made a number of observations that finally helped convince people that the Sun-centered solar system model (the heliocentric model), as proposed by Copernicus, was correct. These arguments can be divided into two kinds: Those that proved that the Ptolemaic model was incorrect; and those that undermined the broader philosophy of Aristotelianism that included the Ptolemaic model.

We'll first consider some philosophically important observations and then the ones that proved Venus, at least, goes around the Sun and not around Earth. Sun and Moon One of the ideas that made Aristotelianism popular with the church during the middle ages was that the heavens are perfect. This also meant that they were unchanging, because if they change then either they weren't perfect before or they won't be perfect after the change. Galileo discovered spots on the Sun and also saw that the surface of the Moon was rough. People really tried hard to account for these observations without making the heavens imperfect; one suggestion was that over the mountains of the Moon there was a layer of clear crystal so the final surface would be smooth and perfect! Jupiters Moons Galileo saw near Jupiter what he first thought to be stars. When he realized that the stars were actually going around Jupiter, it negated a major argument of the Ptolemaic model. Not only did this mean that the Earth could not be the only center of motion, but also it knocked a hole in another argument. The supporters of the Ptolemaic model argued that if the Earth were moving through space, the Moon would be left behind.

Galileos observations showed that the moons of Jupiter were not being left behind as Jupiter moved. Phases of Venus One observation definitely disproved the Ptolemaic model, although it didn't prove that Copernicus was right (as Tycho Brahe pointed out). This was the observation that Venus has phases, much like our Moon does. To the naked eye, Venus always appears as a bright dot in the sky. With a telescope, however, it is fairly easy to see the phases of Venus. Just as the Moon has phases, Venus too has phases based on the planets position relative to us and the Sun. There was no way for the Ptolemaic model (Earth centered solar system) to account for these phases. They can only occur as Galileo saw them if Venus is circling the Sun, not the Earth. Orbits Version for printing Although the motions of the planets were discussed by the Greeks they believed that the planets revolved round the Earth so are of little interest to us in this article although the method of epicycles is an early application of Fourier series .

The first to propose a system of planetary paths which would set the scene for major advances was Copernicus who in De revolutionibus orbium coelestium (1543), argued that the planets and the Earth moved round the Sun. Although a major breakthrough, Copernicus proposed circular paths for the planets and accurate astronomical observations soon began to show that his proposal was not strictly accurate. You can see a diagram from Astronomia Nova showing Kepler's elliptical path for Mars. However scientists certainly did not accept Kepler 's first two laws with enthusiasm. The first was given a cool reception and was certainly thought to require further work to confirm it. The second of Kepler 's laws suffered an even worse fate in being essentially ignored by scientists for around 80 years. Kepler 's third law, that the squares of the periods of planets are proportional to the cubes of the mean radii of their paths, appeared in Harmonice mundi (1619) and, perhaps surprisingly in view of the above comments, was widely accepted right from the time of its publication. In 1679 Hooke wrote a letter to Newton .

In the letter he explained how he considered planetary motion to be the result of a central force continuously diverting the planet from its path in a straight line. Newton did not answer this directly but explained his own idea that the rotation of the Earth could be proved from the fact that an object dropped from the top of a tower should have a greater tangential velocity than one dropped near the foot of the tower. Newton provided a sketch of the path that the particle would follow, quite incorrectly showing it spiral towards the centre of the Earth. Hooke replied that his theory of planetary motion would lead to the path of the particle being an ellipse so that the particle, were it not for the fact that the Earth was in the way, would return to its original position after traversing the ellipse. Newton, not one to like being corrected, had to admit that his original sketch was incorrect but he "corrected" Hooke 's sketch on the assumption that gravity was constant. Hooke replied to Newton that his own theory involved an inverse square law for gravitational attraction. Many years later Hooke was to claim priority for proposing the inverse square law of gravitation and used this letter to Newton to support his claim.

It is worth emphasising that there is a major step to be made from an inverse square law of force to explain planetary motion and a universal law of gravitation. Certainly the motion of the Moon round the Earth was not seen to necessarily be part of the same laws which govern the motion of the planets round the Sun. Fifty years after these events Newton was to record his own recollections of these events which, although interesting, do not really agree with the known historical facts! [I preserve Newton 's old English.] In the same year I began to think of gravity extending to ye orb of the Moon and (having found out how to estimate the force with wch globe revolving within a sphere presses the surface of a sphere) from Kepler's rule of the periodical times of the Planets being in sesquialternate proportion to their distances from the centres of their Orbs, I deduced that the forces wch keep the Planets in their Orbs must reciprocally as the squares of their distances from the centres about wch they revolve: and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth, and found them answer pretty nearly.

All this was in the two plague years of 1665-1666... In 1684 Wren , Hooke and Halley discussed, at the Royal Society, whether the elliptical shape of planetary orbits was a consequence of an inverse square law of force depending on the distance from the Sun. Halley wrote that Mr Hook said that he had it, but that he would conceale it for some time so that others, triing and failing might know how to value it, when he should make it publick. Later in the same year in August, Halley visited Newton in Cambridge and asked him what orbit a body would follow under an inverse square law of force Sr Isaac replied immediately that it would be an Ellipsis, the Doctor struck with joy and amasement asked him how he knew it, why, said he I have calculated it, whereupon Dr Halley asked him for his calculation without any farther delay, Sr Isaac looked among his papers but could not find it, but he promised him to renew it, and then to send it him. Despite the claims by Newton in the above quote, he had in fact proved this result in 1680 as a direct result of the letters from Hooke .

Newton indeed reworked his proof and sent a nine page paper De motu corporum in gyrum (On the motion of bodies in an orbit) to Halley . It did not state the law of universal gravitation nor Newton 's three laws of motion. All this was to develop over the next couple of years to become the basis for the Principia. Halley was largely responsible for ensuring that the Principia was published. He received Newton 's complete manuscript by April of 1687 but there were many problems not the least being that Newton tried to prevent the publication of the Book III when Hooke claimed priority with the inverse square law of force. In the Principia the problem of two attracting bodies with an inverse square law of force is completely solved (in Propositions 1-17, 57-60 in Book I). Newton argues that an inverse square law must give produce elliptical, parabolic or hyperbolic orbits. A bright comet had appeared on 14 November 1680. It remained visible until 5 December 1680 when it moved too close to the Sun to be observed.

It reappeared two weeks later moving away from the Sun along almost the same path along which it had approached. Newton found good agreement between its orbit and a parabola. He uses the orbit of this comet, and comets in general, to support his inverse square law of gravitation in the Principia. You can see a diagram of the orbit of the comet of 1680 from the Principia. In the Principia Newton also deduced Kepler 's third law. He looked briefly (in Propositions 65 and 66) at the problem of three bodies. However Newton later said that an exact solution for three bodies exceeds, if I am not mistaken, the force of any human mind. It is important at this stage to examine the problems which now arose. Newton had completely solved the theoretical problem of the motion of two point masses under an inverse square law of attraction. For more than two point masses only approximations to the motion of the bodies could be found and this line of research led to a large effort by mathematicians to develop methods to attack this three body problem. However, the problem of the actual motion of the planets and moons in the solar system was highly complicated by other considerations.

Even if the Earth - Moon system were considered as a two body problem, theoretically solved in the Principia, the orbits would not be simple ellipses. Neither the Earth nor the Moon is a perfect sphere so does not behave as a point mass. This was to lead to the development of mechanics of rigid bodies, but even this would not give a completely accurate picture of the two body problem since tidal forces mean that neither the Earth nor Moon is rigid. The observational data used by Newton in the Principia was provided by the Royal Greenwich Observatory. However modern scholars such as Richard Westfall claim that Newton sometimes adjusted his calculations to fit his theories. Certainly the observational evidence could not be used to prove the inverse square law of gravitation. Many problems relating observation to theory existed at the time of the Principia and more would arise. Halley used Newton 's method and found almost parabolic orbits for a number of comets. When he computed the orbits for three comets which had appeared in 1537, 1607 and one Halley observed himself in 1682, he found that the characteristics of the orbits were almost identical.

Halley deduced they were the same comet and later was able to identify it with one which had appeared in 1456 and 1378. He computed an elliptical orbit for the comet and he noticed that Jupiter and Saturn were perturbing the orbit slightly between each return of the comet. Taking the perturbations into account Halley predicted the comet would return and reach perihelion (the point nearest the Sun) on 13 April 1759. He gave an error of one month on either side of this date. The comet was actually first seen again in December 1758 reaching perihelion on 12 March 1759. In 1713 a second edition of the Principia, edited by Roger Cotes , appeared. Cotes wrote a preface defending the theory of gravitation given in the Principia. Cotes was himself to provide the next mathematical steps by finding the derivatives of the trigonometric functions, results published after his death. Euler developed methods of integrating linear differential equations in 1739 and made known Cotes ' work on trigonometric functions.

He drew up lunar tables in 1744, clearly already studying gravitational attraction in the Earth, Moon, Sun system. Clairaut and d'Alembert were also studying perturbations of the Moon and, in 1747, Clairaut proposed adding a 1/r4 term to the gravitational law to explain the observed motion of the perihelion, the point in the orbit of the Moon where it is closest to the Earth. However by the end of 1748 Clairaut had discovered that a more accurate application of the inverse square law came close to explaining the orbit. He published his version in 1752 and, two years later, d'Alembert published his calculations going to more terms in his approximation than Clairaut . In fact this work was of importance in having Newton 's inverse square law of force accepted in Continental Europe. The Earth's axis of rotation precesses, that is the direction of the axis of rotation itself rotates in a circle with a period of about 26000 years. Precession is caused by the gravitational attraction of the Sun on the equatorial bulge of the Earth, the bulge being predicted by Newton .

Cassini made a measurement of an arc of longitude in 1712 but obtained a result which wrongly suggested that the Earth was elongated at the poles. In 1736 Maupertuis obtained the correct result verifying Newton 's predictions. However, this illustrates the problems encountered by mathematicians at this time with basic data about bodies in the solar system, even the Earth, being highly inaccurate. There is a small periodic effect called nutation superimposed on precession caused by the motion of the perihelion of the Moon. This superimposed effect has a period of 18.6 years and was first observed by Bradley in 1730 but not announced until 18 years later when he had observed the full cycle. D'Alembert quickly showed that Bradley's observed period was deducible from the inverse square law and Euler further clarified this with further work on the mechanics of rigid bodies during the 1750's. The problem of the orbits of Jupiter and Saturn had troubled astronomers and mathematicians from Kepler 's first theory of elliptical orbits.

The Paris Académie des Sciences offered Prizes for work on this topic in 1748, 1750 and 1752. In 1748 Euler 's studies of the perturbation of Saturn's orbit won him the Prize. His work for the 1752 Prize, however, contains many mathematical errors and was not published until 17 years later. It did contain significant ideas, however, which were independently discovered since Euler 's work was not known. Lagrange won the Académie des Sciences Prize in 1764 for a work on the libration of the Moon. This is a periodic movement in the axis the Moon pointing towards the Earth which allows, over a period of time, more than 50% of the surface of the Moon to be seen. He also won the Académie des Sciences of 1766 for work on the orbits of the moons of Jupiter where he gave a mathematical analysis to explain an observed inequality in the sequence of eclipses of the moons.

Euler , from 1760 onwards, seems to be the first to study the general problem of three bodies under mutual gravitation (rather than looking at bodies in the solar system) although at first he only considered the restricted three body problem when one of the bodies has negligible mass. When one body has negligible mass it is assumed that the motions of the other two can be solved as a two body problem, the body of negligible mass having no effect on the other two. Then the problem is to determine the motion of the third body attracted to the other two bodies which orbit each other. Even in this form the problem does not lead to exact solutions. Euler , however, found a particular solution with all three bodies in a straight line. The first comet to have an elliptical orbit calculated which was far from a parabola was observed by Messier in 1769. The elliptical orbit was computed by Lexell who correctly realised that the small elliptical orbit had been produced by perturbations by Jupiter. The comet made no reappearance and again Lexell correctly deduced that Jupiter had changed the orbit so much that it was thrown far away from the Sun.

The Académie des Sciences Prize of 1772 for work on the orbit of the Moon was jointly won by Lagrange and Euler . Lagrange submitted Essai sur le problème des trois corps in which he showed that Euler 's restricted three body solution held for the general three body problem. He also found another solution where the three bodies were at the vertices of an equilateral triangle. Lagrange considers his solutions do not apply to the solar system but we now know the both the Earth and Jupiter have asteroids sharing their orbits in the equilateral triangle solution configuration discovered by Lagrange. For Jupiter these bodies are called Trojan planets, the first to be discovered being Achilles in 1908. The Trojan planets move 60 in front and 60 behind Jupiter at what are now called the Lagrangian points. You can see the known asteroids . This picture shows the positions of nearly 6000 asteroids whose orbits are now known. The effect of the Lagrangian points is readily seen. Jupiter is the outermost planet depicted in the picture. Stanford SOLAR Center -- Ask A Solar Physicist FAQs - Answer Who discovered that the Sun was a star?

This answer is courtesy of Louis Strous of the National Solar Observatory, Sacramento Peak, NM. Many people's work was needed to prove that the Sun is a star. The first person we know of to suggest that the Sun is a star up close (or, conversely, that stars are Suns far away) was Anaxagoras, around 450 BC. It was again suggested by Aristarchus of Samos, but this idea did not catch on. About 1800 years later, around AD 1590, Giordano Bruno suggested the same thing, and was burnt at the stake for it. Through the work of Galileo, Kepler, and Copernicus during the 16th and 17th centuries the nature of the solar system and the Sun's place in it became clear, and finally in the 19th century the distances to stars and other things about them could be measured by various people. Only then was it proved that the Sun is a star. For most of human history, almost all people have thought that the Earth was in the center of a giant sphere (or ball, called the "celestial sphere") with the stars stuck to the inside of the sphere.

The planets, Sun, and Moon were thought to move between the sphere of stars and the Earth, and to be different from both the Earth and the stars. Anaxagoras, who lived in Athens, Greece, around 450 BC (about 2450 years ago), thought that the Sun and stars were fiery stones, that the stars were too far away for their heat to be felt, and that the Sun was perhaps more than a few hundred miles in size. With that Anaxagoras was, as far as we know, the first one to suggest that the Sun is a star. His ideas were met with disapproval and he was finally imprisoned for impiety, because his ideas did not fit the prejudices of the time. Aristarchus of Samos (Samos is a Greek island in the Aegean Sea) lived from about 310 to 230 BC, about 2250 years ago. He measured the size and distance of the Sun and, though his observations were inaccurate, found that the Sun is much larger than the Earth.

Aristarchus then suggested that the small Earth orbits around the big Sun rather than the other way around, and he also suspected that stars were nothing but distant suns, but his ideas were rejected and later forgotten, and he, too, was threatened for suggesting such things. Aristarchus and Anaxagoras had no way of actually measuring the sizes of or distances to stars (except the Sun), so they had no proof for their ideas. Claudius Ptolemaeus (commonly called Ptolemy by speakers of English) of Alexandria (a Greek city in what is now Egypt) around AD 140 (about 1860 years ago) described a geocentric (= earth-centered) model of the universe, with the Earth in the center of the Universe, the Sun as one of the wanderers ("planetes" in Greek) that move relative to the stars, and the stars fixed to the outermost celestial sphere. In this model, the stars and the Sun were completely different.

The universe described in this book (which book came to be known as the Almagest) was accepted as the truth by practically everybody for the next 14 centuries, mostly because it was endorsed by the Roman Catholic Church, which became very powerful during that time. This model described fairly accurately how planets move, but not why they moved in just that way, and it lumped the Sun together with the planets rather than with the stars. Mikolaj Kopernik (known as Nicholas Copernicus outside of his native Poland) lived from 1473 to 1543. In 1543, just before he died, he published a book called "De revolutionibus orbium celestium" in which he proposed a heliocentric (= sun-centered) solar system with the Sun in the center and the Earth merely one of the planets orbiting the Sun, just like the other ones. This model was simpler than Ptolemy's geocentric model, though either one could be used to predict planetary motion. The model of Copernicus set the Sun apart from the planets, but did not say anything about the stars.

Copernicus waited as long as possible before publishing this book because he was afraid the Church would not approve of it. At first, most opposition to his ideas actually came from Protestants, not Catholics. Martin Luther, one of the main early figures in protestantism, declared loudly that Copernicus was a fool for "setting the Earth in motion". Giordano Bruno, an Italian philosopher, lived from 1548 to 1600. He decided that if the Earth is a planet just like the others, then it does not make sense to divide the Universe into a sphere of fixed stars and a solar system. He said that the Sun is a star, that the Universe is infinitely large, and that there are many worlds. He was condemned by both the Roman Catholic and Reformed Churches for this as well as other things and was burnt alive in Rome in 1600 for heresy (claiming something that does not fit the ideas accepted by the Church). Galileo Galilei, an Italian scientist, lived from 1564 to 1642. In 1610, he was the first person we know of to use the newly invented telescope to look at the stars and planets.

He discovered the satellites of Jupiter, which showed that Ptolemy's and the Church's idea that there was only one center of orbits in the Universe (namely, the Earth) was incorrect. Based on his observations, Galilei argued for the heliocentric model of Copernicus. He noticed that stars look like little points even when seen through a telescope, and concluded that stars must be very far away indeed. In part because Bruno (a convicted heretic) supported them, the ideas of Copernicus were condemned by the Catholic Church in 1616, and Galileo was tried and convicted of heresy in 1633. He was forced to publicly deny the ideas of Copernicus, and was held under house arrest until he died in 1642. In 1979 a reinvestigation of this conviction was started by the Church and finally the conviction was overturned, about 340 years after Galileo's death. A famous story, but perhaps untrue, has Galileo mutter (of the Earth) "And yet she moves!" on his death-bed. Yet, Galileo, like Bruno and Aristarchus before him, had no proof that the Sun and stars are alike.

Johannes Kepler of Germany lived from 1571 to 1630. He studied the positions of planets very carefully and from that determined three Laws of planetary motion that firmly put the Sun in the center of the solar system with the planets orbiting the Sun. It was now clear that the Sun is not a planet, though why these laws of planetary motion should be the way the are was still unclear. Christiaan Huygens of Holland lived from 1629 to 1695. He determined the distance to the star Sirius, assuming that that star was as bright as the Sun and appeared faint only because of its great distance. He found that the distance to Sirius must be very great. At this time, then, the idea that the Sun is a star was considered seriously by scientists. Isaac Newton, an English scientist, lived from 1642 to 1727. In 1665 he realized that it was gravity that held the solar system together. Another famous story, probably untrue, has this thought pop into Newton's head when an apple falls on his head while he sits under an apple tree, watching the Moon.

Newton then determined the formula that describes how gravity works and showed that this explains the orbits and motion of the planets around the Sun and of moons around planets, and therefore also Kepler's three Laws of planetary motion. The motion of the planets and moons were now explained by a single formula: Newton's Law of Gravity. People speculated that this same law might be valid all through the universe. Finally, in 1838, Friedrich Bessel for the first time measured the distance to a star without any assumptions about the nature of stars and found it to be enormous. Distances to other stars followed soon, and then people could calculate the true brightnesses of stars, corrected for their distance to us, and discovered them to be about as bright as the Sun. When other things about the Sun were also found to be like those of stars, such as its surface temperature and chemical composition, then the proof was finally here that the Sun is a star. The Sun is now classified as a G2V star: a main-sequence dwarf star of moderate temperature.

You'll notice that during most of the history described above, people have been persecuted for suggesting things that did not fit the prejudices of the time, even when (or perhaps because) they presented proof that those prejudices were incorrect. Things are better nowadays: you may still be laughed or shouted at for suggesting ideas that are different from current beliefs, but you will no longer be burnt at the stake for them. Sources/Further reading: Planetary Motion: The History of an Idea That Launched the Scientific Revolution : Feature Articles The Science: Orbital Mechanics Kepler’s Laws of Planetary Motion While Copernicus rightly observed that the planets revolve around the Sun, it was Kepler who correctly defined their orbits. At the age of 27, Kepler became the assistant of a wealthy astronomer, Tycho Brahe, who asked him to define the orbit of Mars. Brahe had collected a lifetime of astronomical observations, which, on his death, passed into Kepler’s hands.

(Brahe, who had his own Earth-centered model of the Universe, withheld the bulk of his observations from Kepler at least in part because he did not want Kepler to use them to prove Copernican theory correct.) Using these observations, Kepler found that the orbits of the planets followed three laws. Brahe believed in a model of the Universe with the Sun (rayed disk) orbiting the Earth (black dot), but the other planets ( symbols ) orbiting the Sun. In an attempt to prove his theory, Brahe compiled extensive astronomical records, which Kepler eventually used to prove heliocentrism and to calculate the orbital laws. [Adapted from Tycho Brahe, Astronomiae instauratae progymnasmata (“Introductory exercises toward the restoration of astronomy.”)] Like many philosophers of his era, Kepler had a mystical belief that the circle was the Universe’s perfect shape, and that as a manifestation of Divine order, the planets’ orbits must be circular. For many years, he struggled to make Brahe’s observations of the motions of Mars match up with a circular orbit.

Eventually, however, Kepler noticed that an imaginary line drawn from a planet to the Sun swept out an equal area of space in equal times, regardless of where the planet was in its orbit. If you draw a triangle out from the Sun to a planet’s position at one point in time and its position at a fixed time later—say, 5 hours, or 2 days—the area of that triangle is always the same, anywhere in the orbit. For all these triangles to have the same area, the planet must move more quickly when it is near the Sun, but more slowly when it is farthest from the Sun. This discovery (which became Kepler’s second law of orbital motion) led to the realization of what became Kepler’s first law: that the planets move in an ellipse (a squashed circle) with the Sun at one focus point, offset from the center. Through Brahe’s astronomical measurements and Kepler’s own drawings of the geometrical relationship between the Sun and Mars in various parts of the planet’s orbit, Kepler discovered that planets moved faster when they were closer to the Sun. From this realization, he concluded that the orbit of Mars was elliptical, not circular.

[Adapted from Johannes Kepler, Epitome astronomia Copernicanae (“Epitome of Copernican Astronomy.”)] Kepler’s third law shows that there is a precise mathematical relationship between a planet’s distance from the Sun and the amount of time it takes revolve around the Sun. It was this law that inspired Newton, who came up with three laws of his own to explain why the planets move as they do. Newton’s Laws of Motion If Kepler’s laws define the motion of the planets, Newton’s laws define motion. Thinking on Kepler’s laws, Newton realized that all motion, whether it was the orbit of the Moon around the Earth or an apple falling from a tree, followed the same basic principles. “To the same natural effects,” he wrote, “we must, as far as possible, assign the same causes.” Previous Aristotelian thinking, physicist Stephen Hawking has written, assigned different causes to different types of motion. By unifying all motion, Newton shifted the scientific perspective to a search for large, unifying patterns in nature. Newton outlined his laws in Philosophiae Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy,”) published in 1687. Law I.

Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed theron. In essence, a moving object won’t change speed or direction, nor will a still object start moving, unless some outside force acts on it. The law is regularly summed up in one word: inertia. Law II. The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. Newton’s second law is most recognizable in its mathematical form, the iconic equation: F=ma. The strength of the force (F) is defined by how much it changes the motion (acceleration, a) of an object with some mass (m). Law III. To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. As Newton himself described: “If you press a stone with your finger, the finger is also pressed by the stone.” Gravity Within the pages of Principia, Newton also presented his law of universal gravitation as a case study of his laws of motion.

All matter exerts a force, which he called gravity, that pulls all other matter towards its center. The strength of the force depends on the mass of the object: the Sun has more gravity than Earth, which in turn has more gravity than an apple. Also, the force weakens with distance. Objects far from the Sun won’t be influenced by its gravity. Isaac Newton demonstrated his universal law of gravitation by showing that a comet visible during 1680 and 1681 followed the path of a parabola. [Adapted from Isaac Newton, 1687. Philosophiae Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy.”)] Newton’s laws of motion and gravity explained Earth’s annual journey around the Sun. Earth would move straight forward through the universe, but the Sun exerts a constant pull on our planet. This force bends Earth’s path toward the Sun, pulling the planet into an elliptical (almost circular) orbit. His theories also made it possible to explain and predict the tides. The rise and fall of ocean water levels are created by the gravitational pull of the Moon as it orbits Earth.

Einstein and Relativity The ideas outlined in Newton’s laws of motion and universal gravitation stood unchallenged for nearly 220 years until Albert Einstein presented his theory of special relativity in 1905. Newton’s theory depended on the assumption that mass, time, and distance are constant regardless of where you measure them. The theory of relativity treats time, space, and mass as fluid things, defined by an observer’s frame of reference. All of us moving through the universe on the Earth are in a single frame of reference, but an astronaut in a fast-moving spaceship would be in a different reference frame. Within a single frame of reference, the laws of classical physics, including Newton’s laws, hold true. But Newton’s laws can’t explain the differences in motion, mass, distance, and time that result when objects are observed from two very different frames of reference. To describe motion in these situations, scientists must rely on Einstein’s theory of relativity. At slow speeds and at large scales, however, the differences in time, length, and mass predicted by relativity are small enough that they appear to be constant, and Newton’s laws still work.

In general, few things are moving at speeds fast enough for us to notice relativity. For large, slow-moving satellites, Newton’s laws still define orbits. We can still use them to launch Earth-observing satellites and predict their motion. We can use them to reach the Moon, Mars, and other places beyond Earth. For this reason, many scientists see Einstein’s laws of general and special relativity not as a replacement of Newton’s laws of motion and universal gravitation, but as the full culmination of his idea. References heliocentric The Heliocentric Theory: Copernicus, Galileo, Kepler, Newton by Tom Irvine, February 17, 2006 Introduction The conclusion that the "Earth circles the Sun," was reached and publicized by Copernicus, Galileo, Kepler, Newton, and Halley. This is the "heliocentric theory." To appreciate the work of these men, one must also consider the role of ancient Greek philosophers and the Roman Catholic Church. Aristotle Aristotle (384 - 322 BC), a Greek philosopher, taught that the Earth was the center of the Universe. He thus taught that the Sun, the Moon, and the planets thus orbited the Earth.

This is the "geocentric theory." Aristotle set forth this theory in a book called, "On the Heavens." On the other hand, another Greek named Aristarchus of Samos in the 3rd century BC placed the Earth and other planets in motion around the central Sun. This idea, however, was rejected by the people in favor of Aristotle's geocentric theory. Aristarchus of Samos Aristarchus (310 - 230 BC) was both a mathematician and astronomer. He the first to propose a sun-centered universe. Archimedes (287 - 212 BC) critized this theory in The Sand-Reckoner. Ptolemy Claudius Ptolemy (85-165 AD) was a later philosopher who lived in Alexandria. He wrote that the Earth was motionless because constant gales would sweep across it if it were in motion. Ptolemy also devised a complex system of "epicycles" to account for the apparent retrograde motion of the planets. Retrograde means backwards. Consider two objects rotating about a common point with the inner object traveling at a faster angular speed than the outer object.

As the inner objects passes by the outer object, the outer object will appear to move backwards. This effect is an optical illusion. In Ptolemy's epicycle system, each planet revolved around the earth in a large circle by making a series of smaller circles, to account for this effect. Ptolemy wrote these theories in book called "Almagest," around 140 AD. Thomas Aquinas The theories of Aristotle and Ptolemy were adopted by the Roman Catholic Church, which played an important role in presenting these theories to the Europeans. Thomas Aquinas (1225-1274), a priest and philosopher, used the writings of these men to show that the heavens were "God-ordained and man-centered." The Church leaders felt that they had the duty of teaching the people about the universe, which declared the glory of God. Nicholas Copernicus Nicholas Copernicus (1473 - 1543) studied to become a priest in the University of Bologna in 1497. His religious training included astronomy, since one of his duties would be to warn the people about the future by what happened in the heavens.

Domenico Maria Novara was an astronomy professor who taught Copernicus. Domenico and Copernicus spent many nights studying the heavens together. They freely discussed ways for improving and simplifying Ptolemy's system of planetary motion. Copernicus set out to improve the system devised by Ptolemy. He realized that the rising and setting of the Sun, Moon, and stars could be accounted for by a daily revolution of the Earth. Also, he found that if he put the Sun at the center of the planet's orbits he could simplify the number of epicycles from 80 in Ptolemy's system to a mere 34. Although epicycles do not exist, Copernicus' intuition was correct.

His idea that the Earth and planets orbited about the sun became know as the "heliocentric theory." He wrote about it in his book "De Revolutionibus, " which translates to "Concerning the Revolutions." Copernicus defended his placement of the Sun at the center of the Universe by asking, "For who would place this lamp of a very beautiful temple in another or better place than this, wherefrom it can illuminate everything at the same time?" Martin Luther, Philipp Melancthon, Andrew Osiander, Tommasco Caccini and other religious leaders were quick to denounce Copernicus' heliocentric theory. Galileo Galilei Galileo Galilei (1564 - 1642) lived in Italy. He used the newly-invented telescope to make his own observations. He studied mountains and craters on the Earth's moon, the phases of Venus, and the moons of Jupiter. Particularly he noted that Venus at times appears to be a crescent, just as the Earth's moon does. All of these findings supported Copernicus' heliocentric theory. Galileo wrote about his observations and thus angered the Roman Catholic Church. The Church eventually placed him under house arrest.

The Inquisition was the tribunal of the Roman Catholic Church at this time. The Inquisition made Galileo kneel before them and confess that the heliocentric theory was false. A modern author name Preserved Smith wrote, "Though the implications of the new science were not worked out immediately, it began to be suspected that if the theories were true, man had lost his birthright as the creature for whose sake all else existed, and had been reduced to the position of a puny and local spectator of infinite forces unresponsive to his wishes and unmindful of his purposes." Tycho Brahe Tycho Brahe (1546-1601) was a Danish astronomer who made measurements of the planet and stars. His measurements were the most accurate that had yet been made. Tycho began his observations in Denmark but later moved to Prague to continue his work. Tycho proposed a system in which all of the planets except for Earth orbited about the Sun. He claimed that the Sun still orbited about the Earth, however. Johannes Kepler Johannes Kepler (1571 - 1630) was a German astronomer. Kepler was invited to live in Prague by Tycho Brahe.

Tycho died a year after Kepler's arrival. Kepler inherited a wealth of astronomical data from Tycho. Kepler used this data to draw conclusions about the orbits of the planets. Kepler's Three Laws can be used to describe the motion of the Planets: 1. The Planets move in orbits about the Sun that are ellipses 2 . The planets move such that the line between the Sun and the Planet sweeps out the same area in the same area in the same time no matter where in the orbit. 3. The square of the period of the orbit of a planet is proportional to the mean distance from the Sun cubed. Isaac Newton Isaac Newton (1642 - 1727) lived in England. Newton derived the law of gravitation between two masses. Since the Sun was the most massive object in the planetary system, all of the planets would naturally be attracted to it and revolve around it, in the same manner as the Moon revolves around the Earth. Newton eventually wrote about gravitation and the heliocentric theory in Principia Mathematica in 1687, at the prompting of another famous astronomer, Edmund Halley (1656-1742) .

Halley used Newton's equations to predict that a comet seen in 1682 would return in 1758. The return of Halley's comet gave final proof to the heliocentric theory. Roman Catholic Church In 1992, the Roman Catholic Church finally repealed the ruling of the Inquisition against Galileo. The Church gave a pardon to Galileo and admitted that the heliocentric theory was correct. This pardon came 350 years after Galileo's death. Bibliography

Question: Which Polish-born astronomer proposed that the planets orbit the sun, and was later proved right?
Answer: Copernicus