Michael Seeds

Dana Backman

Chapter 3The Origin of Modern Astronomy

• How did early astronomers describe Earth’s place in the universe.

• How did Copernicus revise the idea that the Earth was at the center of the Universe

• Why was Galileo condemned by the inquisition.

• How did Isaac Newton change the way people thought about nature and themselves?

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Aristotle 384 -322 bce

Philosophers of the ancient world attempted to deduce truth about the universe by reasoning from first principles.

•A first principle was something that seemed obviously true to everyone and, supposedly, needed no further examination.

• There are three important ideas to note about the ancient universe.

Aristotle’s Universe

• One, ancient philosophers and astronomers accepted without question that:• Heavenly objects must move

on circular paths at constant speeds.

• Earth is motionless at the center of the universe.

Aristotle’s Universe

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The sphere of the Moon (lune) separates the terrestrial region (composed of the concentric shells of the four elements: Earth, Water, Air and Fire) from the celestial region. Beyond the moon are the concentric spheres carrying Mercury, Venus, Sun, Mars, Jupiter, Saturn and the fixed Stars.

• Two, as viewed by you from Earth, the planets seem to follow complicated paths in the sky.– These include episodes of ‘backward’ motion that are

difficult to explain in terms of motion on circular paths at constant speeds.

Aristotle’s Universe

Ptolemy 140 AD

Claudius Ptolemy was a Greco-Roman mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. Born: 100 AD, Alexandria, Egypt

Died: 170 AD

• Third, you can see how Ptolemy created an elaborate geometrical and mathematical model to explain details of the observed motions of the planets, while assuming Earth is motionless at the center of the universe.

Aristotle’s Universe

Ptolemy's universe

Stamp honoring Aristarchus

A proposal of a Heliocentric Universe: Aristarchus of Samos (310-230 BC)

Eratosthenes The Librarian Who Measured the Earth, was one of many illustrious chief librarians of the Great Library at AlexandriaHe was, among other things, a sports historian, who compiled a list of winners of the Olympic Games. Fascinated with geography, he realized the need for a comprehensive book on the subject, and set about writing one. Although the ancient Greeks knew the earth was round, nobody had calculated its circumference. Eratosthenes used geometry to figure it out from sun angles, and his calculation was only off by about 200 miles. 276 BC, Cyrene, Libya Died: 194 BC, Alexandria, Egypt

• Nicolaus Copernicus (originally, Mikolaj Kopernik) was born in 1473 in what is now Poland.– At the time of his birth, and throughout his life, astronomy

was based on Ptolemy’s model of Aristotle’s universe.

At the time of Copernicus birth, and throughout his life, astronomy

was based on Ptolemy’s model of Aristotle’s universe.

• Copernicus’s model was simple and straightforward compared with the multiple off-center circles of the Ptolemaic model.

De Revolutionibus

Figure 3-4 p38

The Copernican universe, as reproduced in his book De Revolutionibus. Earth and all the known planets revolve in separate circular orbits about the sun (Sol) at the center.

The outermost sphere carries the immobile stars of the celestial sphere. Notice the orbit of the moon around Earth (Terra). (b) The model is simple not only in the

arrangement of the planets but also in their motions. Orbital velocities (blue arrows) decrease from that of Mercury, the fastest, to that of Saturn, the slowest.

Tycho Brahe, Johannes Kepler, and Planetary Motion

• As astronomers struggled to understand the place of Earth, they also faced the problem of planetary motion.• How exactly do the planets move?

• That problem was solved by two people:• A nobleman who built a fabulous observatory

• A poor commoner with a talent for mathematics

Tycho Brahe, Johannes Kepler, and Planetary Motion

1546 – 1601 1571 – 1630

• The Danish nobleman Tycho Brahe is remembered, in part, for wearing false noses to hide a dueling scar from his college days.• He was reportedly very proud of

his noble station.

• So, his disfigurement probably did little to improve his lordly disposition.

Tycho lived before the invention of the telescopes.

Tycho Brahe

• In 1572, astronomers were startled to see a new star—now called Tycho’s supernova—appear in the sky.– Aristotle had argued that the heavens were perfect, and

therefore unchanging.

– So, astronomers concluded that the new star had to be nearer than the moon.

– Tycho measured the new star’s position and showed it had to be far beyond the moon and was something changing in the supposedly unchanging starry sphere.

• When Tycho wrote a book about his discovery, the king of Denmark honored him with a generous income and the gift of an island, Hveen.• Tycho built a fabulous observatory on the island.

Tycho Brahe

• Tycho lived before the invention of the telescopes.

• So, his observatory was equipped with wonderful instruments for measuring the positions of the sun, moon, and planets using the naked eye and peep sights. • For 20 years, Tycho and his

assistants measured the positions of the stars and planets.

• After the death of the Danish king, Tycho moved to Prague.

• Here, he became the Imperial Mathematician to the Holy Roman Emperor Rudolph II. • Tyco hired a few assistants, including a German school

teacher named Johannes Kepler.

• Kepler worked with Mars orbit (off from perfect circle by error by eight minutes of arc (8/60 of a degree))

Kepler’s Three Laws of Planetary Motion

• Based on observations made by Tycho Brahe

• Newton later demonstrated that these laws were consequences of the gravitational force between any two objects together with Newton’s laws of motion

Section 7.6

Kepler’s First Law

• All planets move in elliptical orbits with the Sun at one focus.

– Any object bound to another by an inverse square law will move in an elliptical path

– Second focus is empty

Section 7.6

Kepler’s Second Law

• A line drawn from the Sun to any planet will sweep out equal areas in equal times

– Area from A to B and C to D are the same

– The planet moves more slowly when farther from the Sun (A to B)

– The planet moves more quickly when closest to the Sun (C to D) Section 7.6

• Kepler’s third law states that the two quantities, orbital period and semi-major axis, are related.– Orbital period squared is proportional to the semi-

major axis cubed.

Kepler’s Three Laws of Planetary Motion

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T rGM

Fig. 4-10b, p. 54

3rd Law: A planets orbital period squared is proportional to its average

distance to the sun cubed

Kepler’s Laws, cont.

• Based on observations made by Brahe

• It is important to notice that Kepler’s three laws are empirical.

• That is, they describe a phenomenon without explaining why it occurs.

• Newton later demonstrated that these laws were consequences of the gravitational force between any two objects together with Newton’s laws of motion

born in the Italian city of Pisa in and studied medicine at the university there.

• Most people know two ‘facts’about Galileo.

• Both are wrong.– He did not invent the telescope.

– He was not condemned by the Inquisition for believing that Earth moved around the sun.

– Galileo remained under house arrest, despite many medical problems and a deteriorating state of health, until his death in 1642. The Church finally accepted that Galileo might be right in 1983.

Galileo Galilei

• Galileo did not invent the telescope.• It was apparently invented around 1608 by lens makers in

Holland.

• Galileo, hearing descriptions in the fall of 1609, was able to build working telescopes in his workshop.

• It was the telescope that drove Galileo to publicly defend the heliocentric model.

• What Galileo saw through his telescopes was so amazing he rushed a small book into print, Sidereus Nuncius (The Starry Messenger).

• In the book, he reported two major discoveries about the solar system.

Telescopic Observations

• First, the moon was not perfect.– It had mountains and valleys on its surface.

– Galileo used the shadows to calculate the height of the mountains.

– Aristotle’s philosophy held that the moon was perfect.

– Galileo, however, showed that it was not only imperfect but was even a world like Earth.

• Second, Galileo’s telescope revealed four new ‘planets’ circling Jupiter.– Today, these ‘planets’ are

known to be the Galilean moons of Jupiter.

• The moons of Jupiter supported the Copernican model over the Ptolemaic model.– Critics of Copernicus had said

Earth could not move—because the moon would be

• Also, Aristotle’s philosophy included the belief that all heavenly motion was centered on Earth.– Galileo showed that Jupiter's moons revolve around Jupiter.

– So, there could be centers of motion other than Earth.

Telescopic Observations

• In the years of further exploration with his telescope, Galileo made additional fundamental discoveries.

• When he observed Venus, he saw that it was going through phases like those of the moon.

• The Inquisition (formally named the Congregation of the Holy Office) banned books relevant to the Copernican hypothesis.

• When Galileo visited Rome again in 1616, Cardinal Bellarmine interviewed him privately and ordered him to cease public debate about models of the universe.

• Galileo appears to have mostly followed the order.

• Nevertheless, Galileo began to write a massive defense of Copernicus’s model, completing it in 1629.– After some delay, Galileo’s book was approved by both the

local censor in Florence and the head censor of the Vatican in Rome.

– It was printed in 1632.

• The book is called Dialogo Dei Due Massimi Sistemi (Dialogue Concerning the Two Chief World Systems).

• It confronts the ancient astronomy of Aristotle and Ptolemy with the Copernican model.

Dialogo and Trial

• Galileo wrote the book as a debate among three friends.• Salviati is a swift-tongued defender of Copernicus.

• Sagredo is intelligent but largely uninformed.

• Simplicio is a dim-witted defender of Ptolemy.

• The book was a clear defense of Copernicus.

• Galileo placed the pope’s argument in the mouth of Simplicio.

– The pope took offense and ordered Galileo to face the Inquisition.

Dialogo and Trial

• However, Galileo’s trial did not center on his belief in Copernicus’s model. – Dialogo had been approved by two censors.

Rather, the trial centered on a record of the meeting in 1616 between Galileo and Cardinal Bellarmine that included the statement that Galileo was “not to hold, teach, or defend in any way” the principles of Copernicus.

The Inquisition condemned Galileo not primarily for heresy but for disobeying the orders given him in 1616.

• Although he was sentenced to life imprisonment, he was actually confined at his villa for the next 10 years—perhaps through the intervention of the pope.– He died there in 1642, 99 years after the death of

Copernicus.

Dialogo and Trial

• To understand the trial, you must recognize that it was the result of a conflict between two ways of understanding the universe.

• Since the Middle Ages, scholars had taught that the only path to true understanding was through religious faith.

St. Augustine (AD 354-430) wrote “Credo ut intelligame,” which can be translated as, “Believe in order to understand.”

• Galileo and other scientists of the Renaissance, however, used their own observations to try to understand the universe.

• When their observations contradicted Scripture, they assumed their observations of reality were correct. – Galileo paraphrased Cardinal Baronius in saying, “The Bible tells us how to go to heaven, not how the heavens go.”

The significance of Galileo’s trial is about the birth of modern science as a new way to understand the universe

Figure 3-11 p47

• Galileo died in January 1642.

• Some 11 months later, on Christmas day 1642, Isaac Newton was born in the English village of Woolsthorpe.

Isaac Newton, Gravity, and Orbits

• Newton studied optics,

• developed three laws of motion,

• probed the nature of gravity, and

• invented differential calculus.

The publication of his work in his book Principia in 1687 placed the fields of physics and astronomy on a new firm base.

Table 3-2 p48

Difference between weight and mass

• Weight is not an inherent property of an object

– mass is an inherent property

• Weight depends upon location

Newton’s Law of Universal Gravitation

• Every particle in the Universe attracts every other particle with a force that is directly proportional to the product of the masses and inversely proportional to the square of the distance between them.

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3. Finally, notice the difference between closed orbits and open orbits. If you want to leave the

Earth and never return, you must give your spaceship a high enough velocity so it will follow

an open orbit

Orbital Motion

• 1. An object orbiting the Earth is actually falling (being accelerated toward the Earth’s center. The object continually misses the Earth because of its orbital velocity

• 2. Also, two objects orbiting each other actually revolve about their center of mass.

Why doesn’t the moon fall into the Earth ?

spectroscopic binary - Google Video#

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Figure 3-13a p52

Figure 3-13b p52

Figure 3-13c p52

Lunar Ranging Beams of laser light are fired through three telescopes at the Observatoire de la Cote d’Azur, France. The light is then reflected back by reflectors placed on the Moon by Apollo astronauts. From the time it takes the light to reach the Moon and return to Earth, astronomers can determine the distance to the Moon to within 2cm. They have determined it is spiraling away from Earth at a rate of 3.8 cm per year.

At present, many species of cyanobacteria are found in places with environmental conditions serious for most living things, for example, hypersaline seas like Hamelin Pool, frozen lakes in the Antarctic Region and hot springs at Yellowstone in U.S. Since the cyanobacteria forming the stromatolites are only active during the day and they leave a daily trace of sediment, the number of layers counted in the stromatolites is indicating the number of days in the period for which the stromatolites has been built. The photosynthetic bacteria proliferate well on the part exposed to the sun, then the stromatolites grow towards the same direction, following annual motion of the sun across the skies. This annual motion changes with the seasons, in consequence the growth axis of the stromatolite colums changes. As stated above, stromatolites record daily, monthly and annual incidents. By analyzing the annual cycles and counting the number of sediment layers of the stromatolite fossils lived about 850 million years ago, it has been estimated that one year period consisted of about 435 days at that time.

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Learning to look 1.

• What three astronomical objects are represented on this excerpted image from a postage stamp? What are the two rings?

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• Why can the satellite TV dish shown by bolted in place and used 24 hrs. a day without adjustment?

Learning to look 2.

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Learning to look 3.

• Why is it a little misleading to say that this astronaut is weightless?