PSCI 1414 GENERAL ASTRONOMY LECTURE 3: THE HISTORY OF ASTRONOMY PART 2: THE COPERNICAN REVOLUTION ALEXANDER C. SPAHN

THE COPERNICAN REVOLUTION The Greeks and other ancient peoples developed many important scientific ideas, but what we now think of as science arose during the European Renaissance. Within a half century after the fall of Constantinople in 1453, Polish scientist Nicholas Copernicus began the work that ultimately overturned the Earth-centered Ptolemaic model (geocentric model).

THE COPERNICAN REVOLUTION Copernicus (1473 – 1543) Nicholas Copernicus was born in Poland on February 19, His family was wealthy and he received an education in mathematics, medicine, and law. He began studying astronomy in his late teens. By that time, tables of planetary motion based on the Ptolemaic model had become noticeably inaccurate. However, few people were willing to undertake the difficult calculations required to revise the tables.

THE COPERNICAN REVOLUTION Copernicus (1473 – 1543) In his quest for a better way to predict planetary positions, Copernicus decided to try Aristarchus’s Sun-centered idea, first proposed more than 1700 years earlier. He had read of Aristarchus’s work, and recognized the much simpler explanation for retrograde motion offered by a Sun-centered system. But he went far beyond Aristarchus in working out mathematical details of the model. Through this process, Copernicus discovered simple geometric relationships that allowed him to calculate each planet’s orbital period around the Sun and its relative distance from the Sun in terms of the Earth- Sun distance.

THE COPERNICAN REVOLUTION Copernicus (1473 – 1543) The model’s success in providing a geometric layout for the solar system convinced him that the Sun-centered (Heliocentric) model is correct. Copernicus was hesitant to publish his work, fearing that his suggestion that Earth moved would be considered absurd. However, he discussed his system with other scholars, including high-ranking officials of the Catholic Church, who urged him to publish a book. Copernicus saw the first printed copy of his book, De Revolutionibus Orbium Coelestium (“Concerning the Revolutions of the Heavenly Spheres”), on the day he died—May 24, 1543.

THE COPERNICAN REVOLUTION Copernicus (1473 – 1543) Publication of the book spread the Sun-centered idea widely, and many scholars were drawn to its aesthetic advantages. Nevertheless, the Copernican model gained relatively few converts over the next 50 years, for a good reason: It didn’t work all that well. The primary problem was that while Copernicus had been willing to overturn Earth’s central place in the cosmos, he had held fast to the ancient belief that heavenly motion must occur in perfect circles.

THE COPERNICAN REVOLUTION The heliocentric model accounts for retrograde motion The Earth travels around the Sun more quickly than Mars. Consequently, as Earth overtakes and passes this slower-moving planet, Mars appears for a few months (points 4-6) to fall behind and move backward with respect to the background stars.

CONCEPT CHECK 4.4 In the heliocentric model, could an imaginary observer on the surface of the Sun look out and see planets moving in retrograde motion? No. A planet only appears to move in retrograde motion if seen from another planet if the two planets move at different speeds and pass one another. An imaginary observer on the stationary Sun would only see planets moving in the same direction as they orbit the Sun.

THE COPERNICAN REVOLUTION The heliocentric model helped determined the arrangement of the planets Because Mercury and Venus are always observed fairly near the Sun in the sky, their orbits must be smaller than the Earth’s. Planets in such orbits are called inferior planets. The other visible planets (Mars, Jupiter, and Saturn) are sometimes seen on the side of the celestial sphere opposite the Sun, so these planets appear high above the horizon at midnight when the Sun is far below the horizon. When this happens, Earth must lie between the Sun and those planets. Thus, it was concluded that their orbits were larger than the Earth’s and these planets are referred to as superior planets.

THE COPERNICAN REVOLUTION The heliocentric model explains why planets appear in different pars of the sky on different dates Elongation – The angle between the Sun and a planet as viewed from Earth Greatest eastern elongation – When a planet’s position in the sky is as far east of the Sun as possible Greatest western elongation - When a planet’s position in the sky is as far east of the Sun as possible Inferior conjunction – When an inferior planet is between the Earth and the Sun, moving into the morning sky Superior conjunction – When an inferior planet is on the opposite side of the Sun, moving into the evening sky Opposition – When a superior planet lies directly behind the Earth from the Sun and is brightest in the night sky Conjunction – When a superior planet lies on the opposite side of the Sun and is only up in daytime

THE COPERNICAN REVOLUTION When and where in the sky a planet can be seen from Earth depends on the size of its orbit and its location on that orbit. The inferior planets cycle between being visible in the west after sunset and in the east before sunrise.

CONCEPT CHECK 4.5 How many times is Mars at inferior conjunction during one orbit around the Sun? Mars has an orbit around the Sun that is larger than Earth’s orbit. As a result, Mars never moves to a position between the Earth and the Sun, so Mars never is at inferior conjunction.

THE COPERNICAN REVOLUTION Copernicus found a correspondence between the time a planet takes to complete one orbit – that is, its period – and the size of the orbit. Synodic period – the time that elapses between two successive identical configurations as seen from Earth (for example: from one opposition to the next or from one conjunction to the next) Sidereal period – the true orbital period of a planet, the time it takes the planet to complete one full orbit of the Sun relative to the stars The heliocentric model showed that there are rules relating the motion of one planet to another

CONCEPT CHECK 4.7 Why is Jupiter’s sidereal period longer than its synodic period? Jupiter moves slowly and does not move very far in the time it takes for Earth to pass by Jupiter, move around the Sun, and pass by Jupiter again, giving Jupiter a synodic period similar to the Earth’s orbital period of one year. However, slow moving Jupiter takes more than a decade to move around the Sun back to its original position, giving it a large sidereal period.

THE COPERNICAN REVOLUTION Copernicus devised a straightforward geometric method of determining the relative distances using trigonometry. His answers turned out to be remarkably close to modern values. The distances in this table are given in terms of the astronomical unit (AU), which is the average distance of the Earth from the Sun. 1 AU = x 10 8 km The heliocentric model led to the determination of the relative distances of the planets from the Sun

THE COPERNICAN REVOLUTION By comparing the two tables, it is clear that the farther a planet is from the Sun, the longer it takes to travel around its orbit. This is so for two reasons: 1)The larger the orbit, the farther a planet must travel to complete an orbit 2)The larger the orbit, the slower a planet moves

THE COPERNICAN REVOLUTION Tycho ( ) Part of the difficulty faced by astronomers who sought to improve either the Ptolemaic or the Copernican system was a lack of quality data. The telescope had not yet been invented, and existing naked-eye observations were not very accurate. Better data were needed, and they were provided by the Danish nobleman Tycho Brahe. In 1563, Tycho decided to observe a widely anticipated alignment of Jupiter and Saturn. To his surprise, the alignment occurred nearly 2 days later than the date Copernicus had predicted. Resolving to improve the state of astronomical prediction, he set about compiling careful observations of stellar and planetary positions in the sky.

THE COPERNICAN REVOLUTION Tycho ( ) Tycho’s fame grew after he observed what he called a nova, meaning “new star,” in By measuring its parallax and comparing it to the parallax of the Moon, he proved that the nova was much farther away than the Moon (Today, we know that Tycho saw a supernova—the explosion of a distant star). Parallax – A phenomenon in which the apparent position of an object changes because of the motion of the observer

THE COPERNICAN REVOLUTION Tycho ( ) King Frederick II of Denmark decided to sponsor Tycho’s ongoing work, providing him with money to build an unparalleled observatory for naked eye observations. Over a period of three decades, Tycho and his assistants compiled naked-eye observations accurate to within less than 1 arcminute— less than the thickness of a fingernail viewed at arm’s length. Because the telescope was invented shortly after his death, Tycho’s data remain the best set of naked-eye observations ever made.

FOR NEXT TIME… Read box 4.2, section 4.5, and the first several sections of chapter 2. Homework 2: Chapter 4 Questions 6, 8, 9, 12 (Due Friday by 4:00 p.m. in my office)