1_Chapter 04-The Origin of Modern Astronomy

Sejarah Perkembangan AstronomiThe Origin of Modern Astronomy

Penerokaan Angkasa Lepas

The preceding chapters gave you a modern view of Earth. You can now imagine how Earth, the moon, and the sun move through space and how that produces the sights you see in the sky. But how did humanity first realize that we live on a planet moving through space?

That required revolutionary overthrow of an ancient and honored theory of Earth’s place. By the 16th century, many astronomers were uncomfortable with the ancient theory that Earth sat at the center of a spherical universe. In this chapter, you will discover how a Polish astronomer named Nicolaus Copernicus changed the old theory, how a German astronomer named Johannes Kepler discovered the laws of planetary motion, and how the Italian Galileo Galilei changed what we know about nature.

Guidepost

Here you will find answers to four essential questions:

• How did classical philosophers describe Earth’s place in the Universe?

• How did Copernicus revise that ancient theory?

• How did astronomers discover the laws of planetary motion?

• Why was Galileo condemned by the Inquisition?

Guidepost (continued)

This chapter is not just about the history of astronomy. As they struggled to understand Earth and the heavens, the astronomers of the Renaissance invented a new way of understanding nature – a way of thinking that is now called science.

I. The Roots of AstronomyA. ArchaeoastronomyB. The Astronomy of GreeceC. The Ptolemaic Universe

II. The Copernican RevolutionA. Copernicus the RevolutionaryB. Galileo the Defender

III. The Puzzle of Planetary MotionA. Tycho the ObserverB. Tycho Brahe's LegacyC. Kepler the AnalystD. Kepler's Three Laws of Planetary MotionE. The Rudolphine Tables

IV. Modern Astronomy

Outline

Early Astronomy

Astronomy is the science that studies the universe. It includes the observation and interpretation of celestial bodies and phenomena.

The Roots of Astronomy• Already in the stone and bronze ages,

human cultures realized the cyclic nature of motions in the sky. eg Stonehenge (3000 B.C. to 1800 B.C.)

• Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance.

• Those monuments were probably used as calendars or even to predict eclipses. (experts debate their significance)

.

Stonehenge, an Ancient Observatory

Ancient people complete Stonehenge, a monument that mark the directions in which the sun rises and sets on the longest day of the year

Stonehenge

• Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices

Summer solstice

Heelstone

• Constructed: 3000 – 1800 B.C.

• Probably used as calendar

• Salisbury plain in Southern England

Other Examples All Over the World (2)Newgrange, Ireland, built around 3200 B.C.

(5000 years old passage grave) –faces southeast the rising sun

Sunlight shining down a passageway into the central chamber of the mount indicates the day of winter solstice (Dec,21).

• Ancient Egyptian, around 3000 B.C. built the Temple of Isis in Dendera, Egypt to align with the rising point of the bright star Sirius.

• The first appearance of the star in the dawn twilight marked the flooding of the Nile, so it was an important calendrical indicator.

• By dividing the year into 365 days, the ancient Egyptians had created one of the first calendars.

• The Romans borrowed this calender and made changes to it.• With more changes, it eventually became the calender we

know: 11 months having 30 or 31 days, plus one month (February) having 28 or 29 days.

Other Examples All Over the World (3)


Roda Perubatan di Big Horn, Wyoming, balai cerap purba sama seperti takwim utk mengukur hari yang berlalu. Big Horn Medicine Wheel (Wyoming) AD 1450 built by Native Americans.

Individual stones aligned with the rising and setting sun and several bright stars. This indicate to people when it was time to move south for the winter

Other Examples All Around the World (5)

Chaco Canyon, New Mexico

At noon, slit in the rock formation produces a sunlit “dagger” shape, indicating the day of

summer solstice

It may be a symbolic and ceremonial marker that ancient people built to link themselves with the sky.


Caracol (Maya culture, approx. A.D. 900)

Kaum Mayan yang tinggal di selatan Mexico, memerhati peredaran Bulan dan planet Zuhrah dgn. teliti.

Pd TM 800 mereka sudah mencipta satu takwim yg lebih tepat drp takwim di Eropah pd waktu itu.

Balai cerap terawal diberi nama Caracol of Chichen Itza, di Yucatan, Mexico. Pergerakan planet Zuhrah di kaji melalui pembukaan di bumbung menara

Other Examples All Over the World (7)Kukulkan built around the year 1050, was used by the Mayans as a calender. It had four stairways, each with 91 steps including the platform on top, the total number of steps is 365, the number of days in a year. You can see this pyramid if you visit Chichen Itza in Mexico

Mayan’s idea of measuring time by Earth moving around the Sun is the basis for our calender today

Other Examples All Around the World (8)

27,000 year old mammoth tusk found at Gontzi, Ukraine. Inscriptions probably describing astronomical events –a record of four cycles of lunar phases. Dapatan ini mecadangkan percubaan manusia untuk mencatat dan merekod kejadian/fenomena astronomi.

Ancient Greek Astronomers (1)• Unfortunately, there are no written documents

about the significance of stone and bronze age monuments.

• Dozens perhaps hundreds Mayan manuscripts –burned by Spanish missionaries. Only four

survived contained astronomical references.

• First preserved written documents about ancient astronomy are from ancient Greek philosophy.

• Greeks tried to understand the motions of the sky and describe them in terms of mathematical (not physical!) models.

Ancient Greek Astronomers (2)Models were generally wrong because they were based on wrong “first principles”, believed to be

“obvious” and not questioned:

1. Geocentric Universe: Earth at the Center of the Universe

2. “Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles

Greeks assumed the Earth was not moving because they did not observe parallaxes in the sky.

Ancient Greek Astronomers (3)• Eudoxus (409 – 356 B.C.):

Model of 27 nested spheres • Aristotle (384 – 322 B.C.),

major authority of philosophy until the late middle ages: Universe can be divided in 2 parts:

1. Imperfect, changeable Earth,2. Perfect Heavens (described by

spheres) –uniform circular motion

• He expanded Eudoxus’ Model to use 55 spheres.• Mengandaikan Bumi adalah pusat alam semesta – Model Geosentrik•Satu abad, selepas Aristotle, Aristarchus mencadangkan bumi berputar pada paksinya dan beredar mengelilingi Matahari.

Ancient Greeks

22.1 Early Astronomy

Geocentric Model• In the ancient Greeks’ geocentric model, the moon, sun, and the

known planets—Mercury, Venus, Mars, and Jupiter—orbit Earth.

Heliocentric Model• In the heliocentric model, Earth and the other planets orbit the

sun.

Geocentric and Heliocentric Models

Sample ProblemCompare the geocentric and heliocentric models of the solar system, using a diagram to illustrate each model. (6%)

GeocentricEarth is the centre of the universe; sun and planetsrevolve around a stationary Earth in circle.Earth is much smaller than outer starSize of cosmos is finiteOnly crescent and new phase of Venus would be seen

Heliocentric

Sun is the centre of the universe; earth and planets

revolve around the sun in circular paths.

Closer planets to the Sun move fasterSize of cosmos is infiniteAll phases of Venus should be visible

[ 2 mark for explanation & 2 mark for each diagram ]

Common Errors and Misconceptions

This question is usually answered well, however, the common mistake students make is reversing the answers.

Quiz Questions

6. Who taught that the Earth is stationary and at the center of the universe with the Sun, the Moon, and the planets moving around Earth in perfect circles?

a. Thales of Miletus (c. 624-547 BC)b. Pythagoras (c. 570-500 BC)c. Eudoxus (409-356 BC)d. Aristotle (384-322 BC)e. Eratosthenes (c. 200 BC)

Astrolabe

300 SMAhli astronomi di Alexandria, Mesir menggunakan astrolab untuk menentukan kedudukan bintang, bulan, Matahari dan planet

Aristotle (384-322)BCPlato’s student

•Aristotle had taught the Earth had to be a sphere because it always casts a round shadow during lunar eclipses.•Eratosthenes (~ 280 B.C.): measure Earth’s radius.

On the day of summer solstice , sunlight fell to the bottom of a well at Syene (the sun is at the zenith at Syene) but on the same day at Alexandria, the Sun was about ~ 70 south of the zenith i.e. about 1/50 of the circumference of a circle

Calculating Earth’s Circumference

Eratosthenes (~ 280 B.C.):Tinggal di Alexandria (Mesir)Calculation of the Earth’s radius

Angular distance between Syene (Aswan) and Alexandria: ~ 70

Linear distance between Syene and Alexandria: ~ 5,000 stadia

If 5000 stadia = ~ 70 = 1/50 of earth’s circumference,The Earth circumference =250,000 stadia and dividing by 2π, he found that Earth’ radius = 40,000 stadia6 stadia ~ 1 km250,000 stadia =41666 kmEarth Radius ~ 40,000 stadia (probably ~ 4 % too large) – better than any previous radius

estimate by Aristotle.

Penemuan bahawa Bumi tidak rata tetapi melengkung berbentuk glob (sfera) oleh Eratosthenes

He knew that at midday, in the middle of the summer the Sun was exactly overhead as seen from Syene. However at this same moment the Sun was not overhead at the town of Alexandria (utara bandar Syene). From Alexandria, the sun was then 70 from the overhead point and angular distance between Syene and Alexandria is 70. A full circle contains 3600 and 70 is about 1/50 of 360. Therefore if Earth is a globe, its circumference (the distance all round it) would be 50 times the distance from Alexandria to Syene (800 km). Eratosthenes measured this distance and work out the value of the circumference. (800 km x 50 = 40,000 km)

Hari ini kita tahu betapa tepat penghitungan itu. We know today that the circumference of the Earth is about 40, 000 km

Quiz Questions

4. Who accurately determined the size of Earth by considering Sun angles at Syene and Alexandria?

a. Thales of Miletus (c. 624-547 BC)b. Pythagoras (c. 570-500 BC)c. Eudoxus (409-356 BC)d. Aristotle (384-322 BC)e. Eratosthenes (c. 200 BC)

Eratosthenes’s Experiment

(SLIDESHOW MODE ONLY)

Later refinements (2nd century B.C.) • Hipparchus: Placing the Earth away from the centers of the

“perfect spheres”

• Ptolemy: Further refinements, including epicycles

Epicycles

Introduced to explain retrograde (westward) motion of planets

Ptolemy believed that a planet moved in a small circle (epicycle). The center of this small circle was itself moving round(orbit) the Earth in a larger circle (deferent).

Though this explanation was not correct it was generally accepted for some 1700 years

Ptolemy (100-170 TM)

Epicycles

The Ptolemaic model was considered the “standard model” of the Universe until the

Copernican Revolution.

Introduced to explain retrograde (westward)

motion of planets

An equant is the point from which the centre of an epicycle appeared to move at a constant rate.

Ancient Greeks Ptolemy (100-170) TM


Ptolemaic System• Ptolemy created a model of the universe that accounted for the

movement of the planets.

• Retrograde motion is the apparent westward motion of the planets with respect to the stars.

Retrograde Motion

Epicycles

(SLIDESHOW MODE ONLY)

Quiz Questions

3. How did Claudius Ptolemaeus account for the retrograde motion of the planets?

a. Planets slow down, stop, and then reverse their orbital direction around the Sun. b. Inner planets orbit the Sun faster and pass outer planets as they orbit around the Sun.c. Each planet moves on an epicycle, that in turn moves on a deferent that circles around Earth.d. The Sun and Moon orbit Earth, whereas all the other planets orbit the Sun.e. None of the above.

The Birth of Modern Astronomy


Nicolaus Copernicus (1473-1543)• Copernicus concluded that Earth is a planet. He proposed a model

of the solar system with the sun at the center.

The Copernican Revolution

Nicolaus Copernicus (1473 – 1543): Heliocentric Universe (Sun in the Center)The stamp were issued in 1973 to mark the 500th anniversary of his birth

Copernicus’ new (and correct) explanation for retrograde motion of the planets

This made Ptolemy’s epicycles unnecessary.

Retrograde (westward) motion of a planet occurs when the Earth passes the planet Mars in the orbit around the Sun.

His idea was not liked by the Church, only after a hundred years after his death, astronomers admit he was right

Copernicus’ new (and correct) explanation for retrograde motion of the planets

(a-c) As earth overtakes Mars, Mars appears to slow its eastward motion. As Earth passes Mars (d) Mars appears to move westward.As Earth draws ahead of Mars (e-g) Mars resumes its eastward motion against the background stars.

Fig 4-9, Pg 59

Quiz Questions

7. How did Nicolaus Copernicus account for the retrograde motion of the planets?

a. Planets slow down, stop, and then reverse their orbital direction around the Earth. b. Inner planets orbit the Sun faster and pass outer planets as they orbit around the Sun.c. Each planet moves on an epicycle, that in turn moves on a deferent that circles around Earth.d. The Sun and Moon orbit Earth, whereas all the other planets orbit the Sun.e. None of the above.

Quiz Questions

8. What feature of Aristotle's model of the universe was included in the model proposed by Copernicus?

a. Earth is stationary and at the center.b. Mercury and Venus move around the Sun.c. Mars, Jupiter, and Saturn move around Earth.d. Uniform circular motion.e. Elliptical orbits.

Quiz Questions

9. Why did the model of the universe proposed by Copernicus gain support soon after its publication?

a. It more accurately predicted the position of planets.b. It gave a better explanation for the phases of the Moon.c. It was a more elegant explanation of retrograde motion.d. The old system of Ptolemy was never very popular.e. It displaced Earth from the center of the universe.



Tycho Brahe (1546-1601) berbangsa Denmark

• Tycho Brahe designed and built instruments to measure the locations of the heavenly bodies. Brahe’s observations, particularly of Mars, were far more precise than any made previously.

• Dari sebuah balai cerap di Hveen, tidak jauh dari pantai Sweden, beliau membuat banyak cerapan tepat tentang bintang dan planet. Hasil ketekunannya, carta bintang yang lebih tepat daripada yang ada sebelum itu dihasilkan.

Tycho Brahe (1546 – 1601)• High precision observations of the

positions of stars and planets

• Evidence against Aristotelian belief of

“perfect”, unchangeable

heavens

• Measurement of the nightly

motion of a “new star” (supernova)

showed no parallax

Tycho Brahe’s Legacy

New World model

• Sun and Moon orbit Earth;

Planets orbit the sun.

• Still geocentric (Earth in the center of the sphere of stars)

Quiz Questions

11. What was the most important contribution of Tycho Brahe to modern astronomy?

a. The invention of the optical telescope.b. The discovery of four moons orbiting Jupiter.c. A model of the universe that was part Aristotelian and part Copernican.d. The study of the Supernova of 1572. e. Twenty years of accurate measurements of planetary positions.



Galileo Galilei (1564 – 1642)• Galileo’s most important contributions were his descriptions of the

behavior of moving objects.

• He developed his own telescope and made important discoveries:

1. Four satellites, or moons, orbit Jupiter.

2. Planets are circular disks, not just points of light.

3. Venus has phases just like the moon.

4. The moon’s surface is not smooth.

5. The sun has sunspots, or dark regions.

The Solar System Model Evolves

Galileo Galilei (1564 – 1642)

• Invented the modern view of science: Transition from a faith-based “science” to an observation-based science.

• Greatly improved on the newly invented telescope technology. (But Galileo did NOT invent the telescope!)

• Was the first to meticulously report telescope observations of the sky to support the Copernican Model of the Universe.

Major Discoveries of Galileo (1)• Moons of Jupiter (4 Galilean moons)

(What he really saw)

• Earth could not be the center of everything

Major Discoveries of Galileo (2)

• Rings of Saturn

(What he really saw)

Major Discoveries of Galileo (3)

• Surface structures on the moon; first estimates of the height of mountains on the moon

• He saw craters and mountains on the Moon

Major Discoveries of Galileo (4)•Sun spots

(proving that the sun is not perfect!)

Major Discoveries of Galileo (5)• Phases of Venus (including “full Venus”),

proving that Venus orbits the sun, not the Earth!

Quiz Questions

17. Galileo's discovery of four moons orbiting Jupiter showed that planetary bodies could move and carry moons. This supports the model of the universe presented by

a. Aristotleb. Claudius Ptolemaeusc. Nicolaus Copernicusd. Both a and b above.e. All of the above.

Major Discoveries of Galileo

• Galileo did not hide his opinions. He wrote them down in a great book. • He was made to say publicly that he no longer believed in Copernicus ideas.• He was kept under close guard until he died in 1642



Johannes Kepler (1571-1630)• Kepler discovered three laws of planetary motion:

1. Orbits of the planets are elliptical.

2. Planets revolve around the sun at varying speed. Planets moves faster when closest to the Sun and slowest when it is furthest away.

3. There is a proportional relationship between a planet’s orbital period and its distance to the sun.



Johannes Kepler (1571-1630) a German, assistant to Tycho Brahe

• An ellipse is an oval-shaped path.

• An astronomical unit (AU) is the average distance between Earth and the sun; it is about 150 million kilometers.

Planet Revolution

Johannes Kepler (1571 – 1630)

• Used the precise observational tables of Tycho Brahe to study planetary motion mathematically.

1. Circular motion

2. Uniform motion

• Planets move around the sun on elliptical paths, with non-uniform velocities.

• Found a consistent description by abandoning both

Kepler’s Laws of Planetary Motion

1.The orbits of the planets are ellipses with the sun at one focus.

Eccentricity e = c/a

c

Drawing an ellipseFix two pins into a piece of board and join them with a loose thread as shown. Place a pencil through the thread and trace out a curve. The curve will be an ellipse and the pins will mark the foci of the ellipse.

Eccentricities of Ellipses

e = 0.02 e = 0.1 e = 0.2

e = 0.4 e = 0.6

1) 2) 3)

4) 5)

Eccentricity measures how much the ellipse deviates from being a circle

Eccentricities of Planetary OrbitsOrbits of planets are virtually indistinguishable from circles:

Earth: e = 0.0167Most extreme example:

Pluto: e = 0.248

Planetary Orbits (2)2. A line from a planet to the sun sweeps

over equal areas in equal intervals of time.

Planet moves fastest when closest to the Sun (perihelion)

Planet moves slowestwhen farthest from the Sun (aphelion)

The planet goes from A’ to B’ and from A to B in the same amount of time

Planetary Orbits (3)3. A planet’s orbital period (P) squared is

proportional to its average distance from the sun (a) cubed:

Py2 = aAU

3 (Py = period in years; aAU = distance in AU)

• Earth orbital period is over 365 days, Mercury 88 days and Neptune over 164 years

If the distance of a planet from the Sun is 4 AU, what is its orbital period? 8 years


Sir Isaac Newton (1642- 1727)• Although others had theorized the existence of gravitational force,

Newton was the first to formulate and test the law of universal gravitation.

Universal Gravitation• Gravitational force decreases with distance.

• The greater the mass of an object, the greater is its gravitational force.

Motion and Gravity• Kepler’s laws explain how the planets are

observed to move.• Newton (1642-1727) a physicist and

mathematician formulated laws that explain why the planets move as they do.

• Newton’s law of motion state:1. A body stays in a state of rest or uniform motion

unless an outside force acts on it.2. The net force, F on a body is equal to its mass, m

multiplied by its acceleration, a. The formula is F=ma

Motion and Gravity3. Whenever one body exerts a force on a second

body, the second body exerts an equal and opposite force on the first body.

Newton’s law of gravity, F=Gm1m2

d² G = gravitational constant

F F

d

m2m1

Gravity’s Influence on Orbits

• Orbital motion

Inward motiondue to force ofgravity

The combination of the forward motion of a planet and inward motion due to of sun’s gravitational force keeps the planet traveling in its orbit around the sun

Quiz Questions

15. What does Kepler's second law indicate about the orbital speed of a planet?

a. The orbital speed of each planet is constant.b. A planet moves at its slowest when it is closest to the Sun.c. A planet moves at its fastest when it is closest to the Sun.d. The orbital speed of a planet varies in no predictable way.e. None of the above.

Quiz Questions

14. Which of the "First Principles of Ancient Astronomy" did Kepler's laws contradict?

a. Earth is at the center of the universe.b. The heavens are perfect and Earth is imperfect.c. All heavenly motion is uniform and circular.d. Both a and b above.e. Both a and c above.

Historical Overview

archaeoastronomyeccentricuniform circular motiongeocentric universeparallaxretrograde motionepicycledeferentequantheliocentric universeparadigmellipsesemimajor axis, aeccentricity, ehypothesistheorynatural law

New Terms

1. Historian of science Thomas Kuhn has said that De Revolutionibus was a revolution-making book but not a revolutionary book. How was it an old-fashioned, classical book?

2. Why might Tycho Brahe have hesitated to hire Kepler? Why do you suppose he appointed Kepler his scientific heir?

3. How does the modern controversy over creationism and evolution reflect two ways of knowing about the physical world?

Discussion Questions

Quiz Questions

1. Why are Stonehenge and The Big Horn Medicine Wheel thought to be ancient astronomical observatories?

a. Petroglyphs at each site describe how they were used to make observations.b. Ancient Greek writings list the important discoveries made at each of these two sites.c. Stones at each site aligned with significant rising and setting positions.d. Both a and c above.e. All of the above.

Quiz Questions

2. Plato proposed that all heavenly motion is

a. constantly changingb. circularc. uniformd. Answers a and b above.e. Answers b and c above.

Quiz Questions

5. One of the first principles of ancient astronomy is that the heavens beyond _____ are perfect, and the Earth is corrupt.

a. the atmosphereb. the Sunc. the Moond. Saturne. Pluto

Quiz Questions

10. When Tycho observed the new star of 1572, he could detect no parallax. Why did that result undermine belief in the Ptolemaic system?

a. This star is closer than the Moon, and thus stars are not all at the same distance.b. This star is closer than the Moon, and thus smaller than other stars.c. This star is farther away than the Moon, and thus the heavens are perfect and unchanging.d. This star is farther away than the Moon, and thus the heavens are not perfect and unchanging.e. This star is planet-like.

Quiz Questions

12. How was Tycho Brahe able to make more accurate astronomical measurements than had been made before his time?

a. He used a telescope to magnify the image and spacing of celestial objects.b. He designed and used large devices to measure small angles. c. His island observatory was hundreds of miles offshore, under very dark skies.d. His observatory was at high elevation and thus above much of Earth's atmosphere.e. All of the above.

Quiz Questions

13. How did Kepler's first law of planetary motion alter the Copernican system?

a. It changed the perfect circles to ellipses.b. It added epicycles to the perfect circles.c. It placed the Sun at one focus of each orbit.d. Answers a and c above.e. Answers b and c above.

Quiz Questions

16. If the semimajor axis of a planet is 4 AU, what is its orbital period?

a. 4 years.b. 8 years.c. 16 years.d. 64 years.e. It cannot be determined from the given information.

Quiz Questions

18. What phases of Venus are predicted by the Ptolemaic system?

a. New and Crescent phases only.b. Quarter and Gibbous phases only.c. Gibbous and Full phases only.d. Crescent and Gibbous phases only.e. New, Crescent, Quarter, Gibbous, and Full phases.

Quiz Questions

19. What phases of Venus were observed by Galileo?

a. New and Crescent phases only.b. Quarter and Gibbous phases only.c. Gibbous and Full phases only.d. Crescent and Gibbous phases only.e. New, Crescent, Quarter, Gibbous, and Full phases.

Quiz Questions

20. The phases of Venus observed by Galileo support the model of the universe presented by

a. Aristotleb. Claudius Ptolemaeusc. Nicolaus Copernicusd. Both a and b above.e. All of the above.

Answers

1. c2. e3. c4. e5. c6. d7. b8. d9. c10. d

11. e12. b13. d14. e15. c16. b17. c18. a19. e20. c

1_Chapter 04-The Origin of Modern Astronomy

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