© 2007 jones and bartlett publishers chapter 2 2-1 thru 2-5 from an earth-centered to a...

© 2007 Jones and Bartlett Publishers

Chapter 22-1 thru 2-5

From an Earth-Centered to a Sun-Centered System

Courtesy of NASA/JPL-Caltech


2-1 Science and Its Ways of Knowing

1. It is not easy to define what science is. Any effort to define it must include its methods, its historical development, its social context, and a clear understanding of its language.

2. It is especially important to recognize the different meanings of the words “fact” and “theory” as they are used in science compared to their everyday meanings.


3. The scientific method is a never-ending cycle of hypothesis, prediction, data gathering, and verification.

4. A scientific hypothesis is an educated guess made in describing the results of an experiment or observation. A hypothesis can be wildly speculative but must be testable.

5. A scientific fact is generally a close agreement by competent observers of a series of observations of the same phenomenon.

6. A scientific law or principle is a scientific hypothesis that has been repeatedly tested and has not been contradicted.


7. A scientific model is a description of a phenomenon, based on observation, experiment, and theoretical considerations. It is not necessarily the truth or reality, but a description that allows prediction of future events.

8. A scientific theory is a synthesis of a large body of information that encompasses well-tested (by repeatable experiments) and verified hypotheses about certain aspects of the natural world. No theory can be proven to be true, but data can prove a theory to be false.

9. In science a scheme is not usually called a theory until its ideas are shown to fit observed data successfully. In every-day language, however, the word “theory” is often used to refer to ideas that are much more fanciful and less secure.


Criteria for Scientific Models

1. The model must fit the data.

2. The model must make predictions that can be tested and be of such nature that it would be possible to disprove the model.

3. The model should be as simple as possible.

Occam’s razor: The principle that the best explanation is the one that requires the fewest unverifiable assumptions.


2-2 From an Earth-Centered to a Sun-Centered System

1. We examine these theories to answer the question of where the Earth fits into the scheme of things, to see how well they match the criteria for a good scientific theory.

2. To understand how astronomy—and science in general—works, we must look at how it progresses with time.


2-3 The Greek Geocentric Model

1. There is a fundamental difference between the contributions to astronomy made by the ancient Greeks and those made by other ancient civilizations.

The Greeks were interested in astronomy because of a pure philosophical desire to understand how the universe works.

They believed in, and looked for, a sense of symmetry, order, and unity in the cosmos.

They took the first steps in creating a unified model of the universe


Parallax is the apparent shifting of nearby objects with respect to distant ones as the position of the observer changes.

2. Aristotle argued that the absence of parallax for the stars in the sky implied that the Earth must be at the center of the solar system.

– This is a valid scientific argument.


4. Stellar parallax was not observed until 1838; the greatest annual shift observed for any star is only 1.5 arcseconds.

5. Even though Aristotle used a correct logical argument, the conclusion was wrong because it was based on incomplete data.

– Parallax is hard to observe because stars are at great distances from us.

6. Aristotle used very good arguments to conclude that:

– the Moon and Earth are spherical,

– the Sun is farther away from earth than the Moon is.


7. Aristotle saw a difference in the “natural” behavior of Earthly objects compared to heavenly objects. He believed that two different sets of rules existed, one for Earthly objects and one for celestial objects.

8. The Greeks’ love of geometry led them to construct a model of the heavens based on spheres, with the Earth at the center.

– To account for the Sun’s apparent motion in the sky, the Sun was located on a sphere around the Earth, inside the celestial sphere of the stars. The axes of the two spheres were tilted with respect to one another.


Figure 2.03: The Greek model located the Sun on a sphere that moves around the stationary Earth inside the celestial sphere of stars.


9. Ptolemy (150 AD) presented the most comprehensive geocentric model, called the Ptolemaic model. Presented in his book called the Almagest, it held sway for more than 1,300 years.

10. Because the heavens were viewed as perfect, the use in the Ptolemaic model of the symmetrical circle to model the motions of celestial objects was thought to be the most reasonable choice.

11. Five planets are visible to the naked eye:

– Mercury,

– Venus,

– Mars,

– Jupiter,

– Saturn.


12. The Ptolemiac model fit the fact that the planets

– Sometimes stop their eastward motion among the stars and move westward for a while.

– This is called retrograde motion.

13. The planets always stay near the ecliptic.

– In addition, Mercury and Venus never appear very far from the position of the Sun in the sky. Thus their elongation (the angle in the sky from an object to the Sun) is small.

14. Any model for the planets must explain these observations.


A Model of Planetary Motion: Epicycles

1. Ptolemy’s geocentric model was able to explain the planetary motions using epicycles.

An epicycle is the circular orbit of a planet, the center of which revolves around the Earth in another circle.

Fig. 2-5

Figure 2.05: Mars's motion on its epicycle results in a looping path.


2. The model retained the idea of perfect heavenly circles and uniform speeds.

– The model explained why the planets never move far from the ecliptic, but treated Mercury and Venus as special cases in order to explain their small elongations.

Figure 2.06: In the Ptolemaic model, the centers of Mercury's and Venus's epicycles stay between the Earth and the Sun.


3. Ptolemy’s model meets the first two criteria of a good scientific model fairly well.

• The model must fit the data.

• The model must make predictions that can be tested and be of such nature that it would be possible to disprove it.

but it is much less successful with the third.

• The model should be aesthetically pleasing— simple, neat, and elegant.

4. Ptolemaic model did fit the data, so we must judge it as an acceptable model even though it lacked that certain neatness we would like.


Question 1

How did Aristotle justify his theories on the spherical Earth and geocentric universe?


2-4 Aristarchus's Heliocentric Model

1. 400 years before Ptolemy, around 280 BC, the Greek philosopher Aristarchus proposed a moving-Earth solution to explain celestial motions.

– He introduced the concept of a spinning Earth and the first heliocentric model, 1800 years before Copernicus

2. Even though Aristarchus could not explain the lack of observable parallax at his time (Aristotle’s argument), he believed that the Sun was at the center of the solar system because it was much bigger in size than the Earth.


3. With powerful and simple arguments based on observations he concluded:

– The Sun was about 20 times farther from the Earth than the Moon is.

– He showed that the Earth is 3 times larger than the Moon in diameter, and the Sun is about 20 times larger than the Moon in diameter.

– This implies the Sun is about 7 times larger than the Earth in diameter.

4. Aristarchus was the first to create a map of the solar system. He simply did not have the scale for it.


2. By comparing shadows at noon during summer solstice at two different locations he understood that the Sun must be directly overhead (at the zenith) in Syene but that the Sun’s direction was off the vertical by 7 in Alexandria.

Measuring the Size of the Earth

1. Eratosthenes (276--195 BC) was the first person to clearly understand the Earth’s shape and approximate size.

Figure 2.09


3. He realized that the 7 difference was due to the Earth’s curvature.

Therefore the Earth’s circumference was about 360/7 50 times the distance between the two cities.

Knowing this distance he was able to find the Earth’s diameter.

His calculation was very close to the correct value.


4. Combining the calculations of Aristarchus and Eratosthenes, the ancient Greeks had for the first time measurements of the radii of Earth, Moon, and Sun and their relative distances.

We had to wait until 1769 AD to observe the actual value of the astronomical unit and thus the true dimensions of the solar system.

5. The important point here is not the accuracy of the measurements but the power of simple logical arguments that allowed the ancient Greeks to have a very good sense of the solar system more than 2000 years ago.


Question 2

Even though the Greek’s calculations weren’t exact, what was significant about their work in regard to the Earth and Solar System?


2-5 The Marriage of Aristotle and Christianity

1. In the 13th century St. Thomas Aquinas blended the natural philosophy of Aristotle and Ptolemy’s work with Christian beliefs.

2. A central, unmoving Earth fit perfectly with Christian thinking and a literal interpretation of the Bible.

3. People during the Middle Ages placed a great reliance on authority, especially authorities of the past.


Question 3

Why would St. Thomas Aquinas and other 13th century scholars want to rely on such old work, why didn’t they just establish their belief with their own work?

© 2007 jones and bartlett publishers chapter 2 2-1 thru 2-5 from an earth-centered to a...

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