© 2005 pearson education inc., publishing as addison-wesley chapter 3 the science of astronomy

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Chapter 3The Science of Astronomy


Astronomy is the oldest of sciences. In this chapter we will study the history of its development. We will see:

What are the roots of astronomy in ancient observations

Details of Copernican revolution which laid the foundation for nearly all of modern science

What is the nature of modern science and scientific method


3.1 The Ancient Roots of Science

• In what ways do all humans employ scientific thinking?

• How did astronomical observations benefit ancient societies?

• What did ancient civilizations achieve in astronomy?

Our goals for learning:


In what ways do all humans employ scientific thinking?

Scientific thinking is based on everyday ideas of observation and trial-and-error experiments.

Babies experiment all the time, for example they drop the objects (all the time…) and test if they fall. The only difference with scientific thinking is that scientists are trained in scientific language which makes it easier to share their discoveries and employ their collective

wisdom.


Why did ancient people make careful and detailed observations of the sky?

• to satisfy their inherit curiosity • to keeping track of time and seasons:

– for practical purposes, including agriculture– for religious and ceremonial purposes

• as aid to navigation


Ancient people of central Africa (6500 BC) could predict seasons from the orientation of the crescent moon.Question: Can you explain the shape of Sun’s shadow in the middle of wet season?


Modern measures of time come directly from ancient observations of nature: The length of day is the time it takes to Sun to complete a circle over the sky

The length of months is the time it takes the Moon to complete the cycle of phases

The length of year is the time it takes to finish the cycle of seasons.

Seven days of week were named after the seven naked-eye objects which move among the constellations: the Sun, the Moon, and the five planets.


Days of week were named after the Sun, the Moon, and visible planets

(Germanic)


What did ancient civilizations achieve in astronomy?

• daily timekeeping • tracking the seasons and calendar• monitoring lunar cycles• monitoring planets and stars• predicting eclipses• and more…


• In the daytime ancient people would tell time by observing the Sun’s path through the sky.

• Example: Egyptian obelisk, shadows probably used to tell time of day.

• We can trace the origins of modern 24 hour clock to ancient Egypt, some 4,000 year ago. They divided the daylight and darkness into 12 equal parts each (remember they were close to equator, where day and night are of almost equal length). The reason was: they would see 12 distinctive star patterns appearing over the night sky.

Question: compared with the standard hour used today, the hour of ancient Egypt was longer or shorter, during the summer? And during the winter?


Ancient men also marked the seasons by observing the Sun’s path over the year.

England: Stonehenge (completed around 1550 B.C.)


England: Stonehenge (1550 B.C.)

“standing on the right hand side of the Heel Stone and looking between the upright pillars of the grand trilithon the monument would have offered an accurate alignment on the winter solstice sunset”


Mexico: model of the Templo Mayor in the Aztec city of Tenochtitlan (modern day Mexico city).

Sun rose directly through the notch between the temples on equinoxes. Question: in what direction on the horizon does the Sun rise on Equinoxes?


Chaco Canyon, New Mexico: “Sun Dagger” marks summer solstice

Ancient Anasazi people carved a spiral on a vertical cliff face. The Sun’s rays form a dagger of sunlight which pierces the center of the spiral, at noon on the summer solstice.

The ancestral Puebloans were a prehistoric Native American culture centered around the present-day Four Corners area of the Southwest United States. Archaeologists still debate when a distinct culture emerged, but the current consensus suggests their emergence around

1200 B.C.


Many civilizations paid particular attention to lunar phases, often using them as a basis for calendar.

Lunar Calendar:

The months of lunar calendar have 29 or 30 days (so they average 29.5 lunar cycle) making the lunar year 11 days shorter from Sun based calendar. Example: Lunar calendar is still used in Muslim religion and that is why Ramadan (the ninth months) starts 11 days earlier with every subsequent year.

Some lunar calendars use the fact that 19 years is almost precisely 235 lunar months (every 19 years we get the same lunar phases on almost the same dates). By adding 13th months to 7 years every 19 years a lunar calendar can be kept roughly synchronized to the seasons. Example: The Jewish calendar uses this idea. Since the date of Easter is tied to their festival of Passover, the date of Easter changes from year to year.


Scotland: 4,000-year-old stone circle; Moon rises as shown here every 18.6 years.

The complexity of Moons orbit leads to long term patterns in the Moon’s appearance. For example, the full Moon rises at its most southerly point only

once every 18.6 years.

Question: What other periodicity in Moon-Earth-Sun position of about 18 years you remember from the last class?


Some ancient cultures learned how to predict (mostly lunar) eclipses by recognizing saros cycle.Notably Babylonians and Mayans.


Yucatan, Mexico: Mayan Observatory at Chichen Itza.

Many ancient cultures also made careful observation of planets and stars.

For example, this Mayan observatory has windows suitable for observation of Venus.

The Maya civilization is a Mesoamerican civilization, noted for the only known fully developed written language of the pre-Columbian Americas, as well as its spectacular art, monumental architecture, and sophisticated mathematical and astronomical systems. Many of these reached high development during the Classic period (c. 250 to 900)


Wyoming: Big Horn Medicine Wheel

Medicine wheels are stone structures built by the natives of North America for various spiritual and ritual purposes. Medicine wheels appear all over northern United States and southern Canada, specifically South Dakota, Wyoming, Montana, Alberta, and Saskatchewan. One of the older wheels has been dated to over 4,500 years old.It is not clear what their purpose was, spokes are often aligned with rising and setting of bright stars, and the Sun on equinoxes and solstices.


Polynesians were very skilled in art of celestial navigation as well as knowledgeable about wave patterns around different islands, in order to be able to travel among them.

Question: does the following sentence make sense: when navigating the Polynesians found their latitude with the aid of point stars of the Big Dipper?

South Pacific: a traditional Polynesian navigational instrument.


China: Earliest known records of supernova explosions (1400 B.C.)

Bone or tortoise shell inscription from the 14th century BC.

"On the Xinwei day the new star dwindled."

"On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star."


What have we learned?• In what ways do all humans

employ scientific thinking?– Scientific thinking

involves the same type of trial and error thinking that we use in our everyday lives, but in a carefully organized way

• How did astronomical observations benefit ancient societies?– Keeping track of time and

seasons; navigation


What have we learned?• What did ancient civilizations achieve in

astronomy?– Tell the time of day and year, to track cycles of

the Moon, to observe planets and stars. Many ancient structures aided in astronomical observations.


3.2 Ancient Greek Science

• Why does modern science trace its roots to the Greeks?

• How did the Greeks explain planetary motion?

• How did Islamic scientists preserve and extend Greek science?



As we have seen, elements of modern science were present in many early cultures.

In human history it happened that the first cultures to start to develop elements of modern science were ancient civilizations Mesopotamia and ancient Egypt.

Both civilizations developed calendar (to determine taxpaying days, the dates for religious rituals, proximity of agricultural season…).

In Mesopotamia lunar calendar was in use, and they had to make careful observations to keep it aligned with seasons. This resulted in first “scientific” observations recorded on tables in 747 b.c.. The measurements were carefully repeated and simple arithmetic used in an attempt to explain and predict periodicities in the sky – all important elements of scientific process.


This first attempts culminated in ancient Greece, and today we consider Greek civilization as the starting point of modern thinking.

In 500 B.C. Greece became the center of power and the foundations of Greek philosophy were set. They were influenced by rudimentary scientific methods already developed in Babylonia and ancient Egypt.

In about 330 B.C. Alexander the Great expanded Greek empire throughout Middle East and North Africa spreading already developed culture of Greece and mixing it with the knowledge of other civilizations.

He founded the city of Alexandria, a center of world culture. Its library is known to have had more than half million of books! Unfortunately they were all lost in fire for modern men.


The empire of Alexander the Great

Alexandria

Mesopotamia

Babylonia, named for its capital city, Babylon, was an ancient state in the south part of Mesopotamia (in modern Iraq). The earliest mention of Babylon is dating back to the 23rd century BC. Ancient Egypt was a long-standing civilization in north-eastern Africa. It was concentrated along the middle to lower reaches of the Nile River, reaching its greatest extension during the second millennium BC.


Artist’s reconstruction of Library of Alexandria


1. They tried to explain patterns in nature without resorting to myth or the supernatural. They used explanations based on logic and geometry instead.

2. Greeks were the first people known to make models of nature.

Note: The concept of a model is very important in modern science. It is a conceptual representation of some phenomena, whose purpose is to explain it and predict its future behavior.

Why does modern science trace its roots to the Greeks?


Note: they knew the Earth was round!

Already by about 500 B.C. it was taught by Pythagoras (560-480 B.C.). A century later, Aristotle used the Earths curved shadow on the Moon during the partial lunar eclipse as an evidence for spherical shape of the Earth. They even calculated its circumference!


Special Topic: Eratosthenes measures the Earth (c. 240 BC)

Calculate circumference of Earth:circum. Earth = 5000 360/7 stadia ≈ 250,000 stadia

Measurements:Syene to Alexandria distance ≈ 5000 stadia angle = 7°

Compare to modern value (≈ 40,100 km): Greek stadium ≈ 1/6 km 250,000 stadia ≈ 42,000 km

The Sun was exactly overhead in Syene at noon on summer solstice.

Question: could you guess what is a latitude of Syene?Question: can you guess how Eratosthenes measured the angle of Sun, as viewed from Alexandria?


Greek geocentric model (c. 400 BC).

It was one of the greatest intellectual accomplishments of ancient times.

How did the Greeks explain planetary motion?


Plato (428-348 b.c.)

Aristotle (384-322 b.c.)Plato asserted that all heavenly objects must move in perfect circles and must reside on perfect spheres encircling Earth.

Geocentric model was based on:

1. Philosophical standpoint that the heavens must be geometrically “perfect”

2. Belief that the Earth is at the center of the universe

Aristotle followed Plato’s ideas, arguing forcefully in favor of Earth centered Universe.


But this made it difficult to explain apparent retrograde motion of planets…

Review: Over a period of 10 weeks, Mars appears to stop, back up, then go forward again.Question: Why did Greeks abandon Sun centered model proposed by Aristarchus in 260 b.c. ?


The most sophisticated geocentric model was that of Ptolemy (A.D. 100-170) — the Ptolemaic model:

• Sufficiently accurate to remain in use for 1,500 years. It could correctly predict future planetary motion, up to a few arc degrees (few finger sizes on the sky).

• Arabic translation of Ptolemy’s work named Almagest (“the greatest compilation”)

Ptolemy

The Greeks tried to solve the problem of planetary motion by using additional spheres (“perfect” shape) to explain motion of each planet.


So how does the Ptolemaic model explain retrograde motion?Planets really do go backward in this model..

Elegant as it was, the model had to include a number of complexities in order to correctly explain observations. i.e some of the circles had to be positions slightly off-center from Earth.


Thought QuestionWhich of the following is NOT a fundamental

difference between the geocentric and Sun-centered models of the solar system?

A. Earth is stationary in the geocentric model but moves around Sun in Sun-centered model.

B. Retrograde motion is real (planets really go backward) in geocentric model but only apparent (planets don’t really turn around) in Sun-centered model.

C. Stellar parallax is expected in the Sun-centered model but not in the Earth-centered model.

D. The geocentric model is useless for predicting planetary positions in the sky, while even the earliest Sun-centered models worked almost perfectly.


Which of the following is NOT a fundamental difference between the geocentric and Sun-centered

models of the solar system?

A. Earth is stationary in the geocentric model but moves around Sun in Sun-centered model.

B. Retrograde motion is real (planets really go backward) in geocentric model but only apparent (planets don’t really turn around) in Sun-centered model.

C. Stellar parallax is expected in the Sun-centered model but not in the Earth-centered model.

D. The geocentric model is useless for predicting planetary positions in the sky, while even the earliest Sun-centered models worked almost perfectly.


How did Islamic scientists preserve and extend Greek science?

• Much of Greek knowledge was lost with the destruction of the Library of Alexandria.

• While European civilization was in the period of Dark Ages (roughly 500-1000 a.d.), Muslim world preserved and enhanced the knowledge they received from the Greeks

• Al-Mamun’s House of Wisdom in Baghdad was a great center of learning around A.D. 800. Founded in spirit of openness and tolerance it employed Jews, Christians and Muslims which translated Greek texts and developed algebra and many new astronomical instruments. Most of official names of stars and constellations come from this period.


•The Islamic word was in contact with Hindu Scholars from India, and through them received lots of Hindu as well as Chinese influence which was then spread to Byzantine Empire (the eastern part of the former Roman Empire).

• With the fall of Constantinople (Istanbul) in 1453, Eastern scholars headed west to Europe, carrying knowledge that helped ignite the European Renaissance.


What have we learned?•Why does modern science trace its roots to the Greeks?

They developed models of nature and emphasized that the predictions of models should agree with observations.

•How did the Greeks explain planetary motion?

The Ptolemaic model had each planet move on a small circle whose center moves around Earth on a larger circle.


What have we learned?

• How did Islamic scientists preserve and extend Greek science?

While Europe was in its Dark Ages, Islamic scientists preserved and extended Greek science, later helping to ignite the European Renaissance.


3.3 The Copernican Revolution – the European Renaissance

• How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea?

• What are Kepler’s three laws of planetary motion?

• How did Galileo solidify the Copernican revolution?


“Halfway through the thirteenth century, after Greek manuscripts of Arabic science had been translated into Latin for study in European universities, knowledge of astronomy had spread throughout Europe. The Renaissance blossomed in the next two centuries, ushering in a new era in picturing the physical world, ending the dominance of ecclesiastical concerns. The protestant reformation (1517) had challenged the authority of church hierarchy with "Sola Scriptura" ( ie Only the Scriptures). In the atmosphere of this new intellectual freedom of thought, Copernicus came up with a simplified geometrical system of looking at the universe.”


How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea?

Copernicus (1473-1543): • born in Poland and received first class education

• studied astronomy based on Ptolemaic predictions which by that time were noticeably inaccurate. (Question: Why it became inaccurate in time? )

• proposed Sun-centered model, but was reluctant to publish it (published 1543, on the day he died)


• model was no more accurate than Ptolemaic model in predicting planetary positions, because he still believed that heavenly motion must occur in perfect circles.

•As a result he had to add circles upon circles as Ptolemy, and his idea won relatively few converts.

But . . .


Tycho Brahe (1546-1601)

• Danish nobleman

• Observed nova (“new star”, actually explosion of distant star) in 1563 and proved that it is farther than the Moon (couldn’t measure its parallax). This was in disagreement with Aristotelian view that star sky is eternal and unchangeable.

• In 1577 observed a comet and proved it is also in realm of heavens (Aristotle thought it a phenomena of the atmosphere)

• Danish king help him built unparalleled observatory for naked eye observations (the telescope was not yet invented! ). Tycho Brahe’s observatory.


•Compiled the most accurate (up to one arc minute, size of a fingernail on the sky) naked eye measurements ever made of planetary positions (one

of the first men to recognize the importance of precision measurement).

•Still could not detect stellar parallax, and thus still thought Earth must be at center of Solar system (but recognized that other planets go around Sun)

• In 1600 hired Kepler, who used his observations to discover the truth about planetary motion.


Johannes Kepler(1571-1630)

•German astronomer

•Kepler first tried to match Tycho’s observations with circular orbits

• Particularly difficult was to find a model for an orbit for Mars, which was the hardest to explain with circular orbit. But an 8 arcminute

discrepancy led him eventually to ellipses…

If I had believed that we could ignore these eight minutes [of arc], I wouldhave patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy.

His decision to trust data over his preconceived beliefs is important point in the history of science.


An ellipse looks like an elongated circle

What is an Ellipse?

He realized he has to abandon the idea of perfect circle.


Eccentricity of an Ellipse


Kepler’s First Law: The orbit of each planet around the Sun is an ellipse with the Sun at one focus.

What are Kepler’s three laws of planetary motion?

Once he abandoned perfect circles in favor of ellipse, Kepler soon came up with model which could predict planetary motion with far greater accuracy.

(average distance from the sun)


Kepler’s Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal times

means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun.


More distant planets orbit the Sun at slower average speeds, obeying the relationship

p2 = a3

p = orbital period in years a = avg. distance from Sun in AU

Kepler’s Third Law


Kepler’s 3rd Law


Graphical version of Kepler’s Third Law

Question: suppose a comet has an orbit that brings it quite close to the Sun at its perihelion and beyond Mars at its aphelion, but with an average distance of 1 AU. How long does it take for comet to complete each orbit of the Sun? Would it spend most of its time close to the Sun or far from the Sun?


Thought Question: An asteroid orbits the Sun at an average distance a = 4 AU. How long does it take to orbit the Sun?

A. 4 years

B. 8 years

C. 16 years

D. 64 years

Hint: Remember that p2 = a3


An asteroid orbits the Sun at an average distance a = 4 AU. How long does it take to orbit the Sun?

A. 4 yearsB. 8 yearsC. 16 yearsD. 64 years

We need to find p so that p2 = a3 Since a = 4, a3 = 43 = 64Therefore p = 8, p2 = 82 = 64


How did Galileo solidify the Copernican revolution?Galileo (1564-1642) overcame major objections to Copernican view. Threekey objections rooted in Aristotelian view were:

1. Earth could not be moving because objects in air would be left behind.

2. Non-circular orbits are not “perfect” as heavens should be.

3. If Earth were really orbiting Sun,we’d detect stellar parallax.


Galileo’s experiments showed that objects in air would stay with a moving Earth.

Overcoming the first objection (nature of motion):

• Aristotle thought that all objects naturally come to rest.• Galileo showed that objects will stay in motion unlessa force acts to slow them down (Newton’s first law of motion).

For example, if you get up from your seat in the plane, you still keep on moving with a plane.

Question: Why did Aristotle think that all objects would come to rest? What was his mistake as a scientist?


Overcoming the second objection (heavenly perfection):

• Tycho’s observations of comet and supernova already challenged this idea.

• Using his telescope, Galileo saw:

sunspots on Sun (“imperfections”)

mountains and valleys on the Moon (proving it is not a perfect sphere)

Note: Galileo did not invent the telescope, it was invented in 1608 by Hans Lippershey. Galileo built his telescope in 1609, turning it into scientific instrument.


• Tycho thought he had measured stellar distances, so lack of parallax seemed to rule out an orbiting Earth.

• Galileo argued that stars must be much farther than Tycho thought — in part by using his telescope to see the Milky Way is countless individual stars.

If stars were much farther away, then lack of detectable parallax was no longer so troubling.

Overcoming the third objection (parallax):


Galileo also saw four moons orbiting Jupiter, proving that not all objects orbit the Earth…


… and his observations of phases of Venus proved that it orbits the Sun and not Earth.


Galileo Galilei (1564-1642)

The Catholic Church ordered Galileo to recant his claim that Earth orbits the Sun in 1633

His book on the subject was removed from the Church’s index of banned books in 1824

Galileo was formally vindicated by the Church in 1992



• How did Copernicus, Tycho and Kepler challenge the Earth-centered idea?

• Copernicus created a sun-centered model; Tycho provided the data needed to improve this model; Kepler found a model that fit Tycho’s data.


What have we learned?• Kepler’s three laws of

planetary motion:

1. The orbit of each planet is an ellipse with the Sun at one focus

2. As a planet moves around its orbit it sweeps our equal areas in equal times

3. More distance planets orbit the Sun at slower average speeds: p2 = a3

Kepler’s second law



• What was Galileo’s role in the Copernican revolution?

• His experiments and observations overcame the remaining objections to the Sun-centered solar system


3.4 The Nature of Science

• How can we distinguish science from nonscience?

• What is a scientific theory?



What have we learned from history? We have seen how models were formulated and tested against observation. The significance of detailed unbiased observations And how models were modified and replaced when they failed those test

Also, common mistakes as: Failure to question current beliefs, such as that orbits have to be circles.


How can we distinguish science from non-science?

• Defining science can be surprisingly difficult.

• Science from the Latin scientia, meaning “knowledge.”

• But not all knowledge comes from science…


The idealized scientific method

• Based on proposing and testing hypotheses

• hypothesis = educated guess


But science rarely proceeds in this idealized way… For example:

• Sometimes we start by “just looking” then coming up with possible explanations.

• Sometimes we follow our intuition rather than a particular line of evidence.

Even though each scientist is more or less subjective, science is collective effort and each idea is tested independently many times, so science becomes objective as a whole.


Hallmarks of Science: #1

Modern science seeks explanations for observed phenomena that rely solely on natural causes.

(A scientific model cannot include divine intervention)

It is hard to define what science is, there are some characteristics which is shared by all scientific thought.



Science progresses through the creation and testing of models of nature that explain the observations as simply as possible.

Or "All things being equal, the simplest solution tends to be the best one.“, a principle attributed to the 14th-century English logician and Franciscan friar William of Ockham.

Question: can you give one example from today’s class which proves this point?



A scientific model must make testable predictions about natural phenomena that would force us to revise or abandon the model if the predictions do not agree with observations.


What is a scientific theory?• The word theory has a different meaning in science

than in everyday life.• In science, a theory is NOT the same as a hypothesis,

rather:• A scientific theory must:

Explain a wide variety of observations with a few simple principles, AND

Must be supported by a large, compelling body of evidence.Must NOT have failed any crucial test of its validity.

Question: can you think of any example of scientific theory?


Thought QuestionDarwin’s theory of evolution meets all the criteria of

a scientific theory. This means:

A. Scientific opinion is about evenly split as to whether evolution really happened.

B. Scientific opinion runs about 90% in favor of the theory of evolution and about 10% opposed.

C. After more than 100 years of putting Darwin’s theory to the test, the theory stands stronger than ever, having successfully met every scientific challenge to its validity.

D. There is no longer any doubt that the theory of evolution is absolutely true.


Darwin’s theory of evolution meets all the criteria of a scientific theory. This means:

A. Scientific opinion is about evenly split as to whether evolution really happened.

B. Scientific opinion runs about 90% in favor of the theory of evolution and about 10% opposed.

C. After more than 100 years of putting Darwin’s theory to the test, the theory stands stronger than ever, having successfully met every scientific challenge to its validity.

D. There is no longer any doubt that the theory of evolution is absolutely true.


Question: What do you think, did Einstein’s theory of relativity prove that Newton theory of gravity is wrong?



• How can we distinguish science from non-science?• Science: seeks explanations that rely solely on

natural causes; progresses through the creation and testing of models of nature; models must make testable predictions

• What is a scientific theory?• A model that explains a wide variety of

observations in terms of a few general principles and that has survived repeated and varied testing

© 2005 pearson education inc., publishing as addison-wesley chapter 3 the science of astronomy

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