Physics 9 The Solar System with Prof. D. Tytler September 2000

Homework 1, due Oct 5 in class

Answers

We encourage you to work together, to discuss the topics and solutions, but your answers must use all of your own words, calculations and diagrams.

You must not hand in work copied from someone else. Please write down the code numbers of all in your study group.

The goal of the first two questions is to become familiar with the layout of the text book. Questions 3 ­ 10 encourage you to identify the main ideas in Chapter 1.

1. Examine the parts of the book: table of contents, section headings, figure captions, summary, key terms, review questions, quantitative exercises, additional reading, appendix, units and exponential notation, glossary and index. Explain how you will use each of these parts.

-The table of contents, section headings, and index will all be useful for identifying the topics contained in the text. They are essential for getting the big picture of the whole course, for identifying the relationships of the different topics, and for locating information which will supplement the lecture.

-The summary can give a better idea of what the main point of the text was after reading a chapter. It should also be read before you read the chapter, to become aware of the main points.

-The figure captions are usually very important to understanding what is going on in a figure. Figures summarize much of the material in the book, and visuals are easier to recall than text.

-Review questions and exercises help to ingrain the material in the mind by provoking deeper thought that requires a decent understanding of the basic concepts. They allow you to practice and do some active work.

-Additional reading is of value to the student who wants to know more than the basics of a certain topic or finds the provided explainations confusing. Also, if lecture material extends passed topics in the text, these are possible sources for follow-up lecture reading.

-Key Terms and the glossary will define astronomical syntax and help the student to understand what they read.

-The Appendix will greatly help the student unfamiliar with units and exponential notation, which in turn are essential tools for understanding equations or even just the numbers used to describe the solar system.

2. Make a list of all the parts mentioned above, ranked in order of importance to you. Explain the ranks.

#1 Review Questions=>Without these there is no way to use what has been learned.

#2 Summary=>They contain the essential ideas of each chapter, which you must know.

#3 Key Terms=> Can keep you reminded of what terms have been learned and connects the terms in a chapter. You need the terminology to communicate.

#4 Table of Contents=> Gives the big picture, hence more important than the details which come in the headings and captions.

#5 Headings=>Without these it is difficult to prepare yourself for what lies ahead in the text. Especially useful when reading new text.

#6 Captions=>Can be essential to understanding the figures and the mental picture that forms in the mind.

#7 Index=>Very useful for information location.

#8 Glossary=>Can remind you of word meanings.

#9 Units and exponential notation=>Must understand these to understand the scale and magnitude of objects concerned with science. (Puts things in perspective)

#10 Appendix=>Gives a good explaination of units&exp.

#11 Add. Reading=>Only helps the student willing to look beyond the presented material.

#12 Quantitative Exercises=> we will skip these.

3. Examine chapter 1. What are the two main topics? Explain your choice.

Orbits and Gravity. The first paragraph states the mission of the chapter is to find out, "What moves around what and why." These were answered by describing the currently accepted orbits of the planets and the force of gravity that cause them. In addition, the appearance of the sky is a related major topic. All other information was used to develop these ideas. The history of heliocentric and geocentric models was given as background information to the main topics and sets up a discussion of Kepler’s and Newton’s Laws. When reading science text, try to focus on the new concepts and not on the way it is presented.

4. Why does the sky look different at different times of the year?

The motion of the Earth in its orbit around the Sun causes the seasonal changes seen in the sky thougout the year. We see the part of the sky which is away from the Sun. Other motions: the daily spin of the Earth on its axis, the orbit of the Moon about the Earth and the planets about the Sun also cause changes in the sky we see from Earth, but they are not on the annual time scale (the daily spin), or they effect individual objects in the sky (moon and planets).

5. What is retrograde motion?

Defined as a reversal of normal apparent (not real) direction of movement as seen from the Earth. With respect to the horizon, every celestial body moves East to West, because the Earth is spinning. The motions of the stars relative to one another are too small to see without a telescope, and hence they look fixed, except for the spin and orbit of the Earth.

Planets normally appear to move from West to East with repect to the stars. This motion is caused by the planets' orbits around the sun. Retrograde motion arises when a planet appears to reverse direction, going E to W. All planets show retrograde motion. For planets farther from the Sun than the Earth (superior planets) this happens about once per year, when the Earth overtakes the planet. The planets do not actually reverse their direction, they only appear to do so as the Earth moves through its orbit.

6. Compare and contrast two models for the solar system: Heliocentric and Geocentric. **Note: When asked to compare and contrast, do not simply state the definitions. Questions like these should be answered in your own words with an essay, a full paragraph or two long.

What do both have in common?

2. Ancient ideas

3. Planets move around in orbits

4. Many variations on each theme

5. Circular orbits were used in both, until Kepler.

How they differ?

1. What is at the center? Heliocentric (sun), Geocentric (Earth).

2. What is stationary? Heliocentric (sun), Geocentric (Earth).

How do we know which is correct?

7. Explain the ideas involved in phases.

Phases are the different ways the illuminated (Sun lit) portion of an object appears. Phases change with the relative positions of any one of the Sun, object and the Earth. At any one time, a change in the viewing direction will change the phase of an object. All objects which reflect sunlight (all except the Sun and stars) show phases. If an object is much farther from the Sun than the Earth (Jupiter and beyond), then the phase will be nearly full.

8. Imagine that our Moon was fixed above a given point on the surface of the Earth, at its usual distance form the Earth. Would the moon be visible from anywhere on the Earth? Would it show phases? If so, how would the phases differ from those which we normally observe? You will probably want to make sketches to explain your answers.

First of all, the Moon would only be visible to half the world, and to that half it would be fixed relative to the horizon. It would show the phases of the month every 24 hours. Depending on the position of the Moon, there could be a lunar eclipse every night and a solar eclipse every day.

9. What observational evidence is there for the Heliocentric model?

The observations of Venus can only be explained by the Heliocentric model. The fact that it is seen in full phase and is smaller in full phase than in quarter are inexplicable using the Geocentric model.

10. What is gravity?

Gravity is the force of attraction between all atoms in the universe. Because it is proportional to mass, it is not felt between low mass everyday objects. It is felt between the Earth and everything on it because of the Earth's huge mass.