Answers to Idea 1 Study Questions -- 7 Ideas Sec. 005

1. Several possible answers:

Ancient explanations tended to explain natural phenomena in terms of mythical human-like beings

Ancient explanations did not have to explain every detail that was observed (for example, quantitative explanations of why the planets "wander" on the sky as they do). Modern "scientific" explanations must explain all details.

2. "Physics" is the science dealing with space, time, energy, and matter. Since this is more or less everything all of science deals with, one can say that physics (along with chemistry and geophysics) deals with the inanimate world - though more and more physicists are helping to explain living things as well ("biophysics"). Thus the first definition is more generally useful.

3. An experiment is some procedure (some set of things that you do) that is designed to help answer a specific question (not always one in physics, of course). Some examples:

The "ball whacking" demo in class that crudely attempted to answer the question "is heat related to motion?".

Trying a new ingredient in a recipe to find out what effect it has on the flavor.

4. Science is simply the study of nature throughout the Universe- it is that simple.

Technology uses the knowledge of scientists to try to solve some practical problem facing people. Of course, sometimes either the problem didn't really exist or the cure is worse than the problem!

5. Think of the number as 112000000000.0 (put a decimal point in). Then count the number of spaces you have to move the decimal point to the left to get it next to the first digit (1) - I get 12 times. Thus 112000000000.0 = 1.12 x 1012. If this is too complicated try it first with, say, 130: 130=130.0=1.13*100=1.13x102.

6. Lots of possible answers. We just had one in class - the solar system is really big from a human standpoint (it takes our best spacecraft many years to travel a few billion miles) but it is quite tiny compared to the size of, say, the galaxy. Thus the "bigness" of the solar totally depends on what you are trying to do or study.

7. Various things, all having to do with circles:

Stars moving in circles, rising on one horizon and setting on the other

Probably planets moving in circles but not quite as regular circles as the stars

(probably because some of the planets may not be visible - why?)

The moon, also moving in a circle

8. This term actually has a couple of meanings. For us it meant the vast apparent sphere in the heavens that seems to spin about the Earth every night. Of course, we know now that there is no actual "sphere" - the stars and so on are all at different distances from the Earth.

9. Retrograde motion is the apparent "backwards" motion of planets as compared to the stars. Ptolemy incorrectly explained it in terms of epicycles. Copernicus correctly noted that a heliocentric model naturally explains it.

10. If you watched the stars and planets night after night you would notice that all of the stars behave in the same way (they are so far away that they never seem to move by themselves - all of their apparent motion is due to that of the Earth) but the planets "wander" over the sky, sometimes even going backwards.

11. geo - earth centered. helio - sun centered.

12. Circles and spheres because they posses complete symmetry - now matter how you rotate a sphere it looks exactly the same.

13. (a) Philosophical reasons - heavens are perfect so the Earth cannot be in them and so on.

(b) If the Earth go around the Sun why don't dropped objects fall behind us?

(c) Why don't things fly off a rotating Earth? (They didn't properly understand gravity)

(d) If the earth is going around the sun why don't we see parallax when the earth is at opposite points in its orbit? (see question 24).

14. Aristotle put lots (50 some) spheres in the sky all interconnected in complex ways and all rotating around the Earth. They were "driven" by some mystical outer sphere called the "Prime Mover". Since this did not explain everything that was observed (see how even these old astronomers were moving towards a scientific approach?) Ptolemy made it even MORE complicated with lots of "devices" or tricks like epicycles etc. etc.

15. For example, the apparent change in size of the moon and the changing brightness of the planets during the year (which requires the distance from the earth to the planet to change with time - impossible in Aristotle's simple nested sphere Universe).

16. Ptolemy used various devices to dress up Aristotle's model. For example, he moved the Earth off center the (still perfectly circular) orbit of the sun and planets - he called this trick an "eccentric" Similarly, he added "circles on circles" - the planets are sun and whatnot moved on small orbits superimposed on large ones - these were "epicycles".

17. Please see the book, figure 2.5c (page 32).

18. Too bloody complicated and it his use of eccentrics and equants (see the book) violated, in Copernicus's eyes, the age-old rule of perfect circles. (So he was led to the right heliocentric model for the wrong reason!)

19. Copernicus simply put the sun in the center, the Earth and the other planets went around the sun (he even got the order of the planets right), and the moon around the Earth - just fine. Unfortunately, he was still using perfect circles…

20. See the book, page 36 figure 2.7b.

21. A man of rather substantial principle who, amongst other things, believed in an infinite Universe where the Earth was not center of everything. For his various (rather modern beliefs) he was imprisoned, tortured, and burned at the stake in 1600.

22. Brahe was a hell of an observational astronomer BEFORE the age of telescopes. He measured the positions of the objects in the sky with obscene precision. This data was a fundamental reason Kepler was able to discover that the planets (and comets and everything that moves in an orbit around something else) move in ellipses and not only in perfect circles. (A circle is indeed the simplest ellipse - one with no ovalness. Kepler realized that everything generally moves in the ovally kind of ellipse).

23. The observation (by Galileo and others) of exploding stars (supernovae) that suddenly appeared in the heavens convinced them that the heavens were NOT the unchanging ideal place everyone had believed they were for 2000 years. Their observations of comets zipping in from the outer planets, whipping around the sun, and leaving on elliptic orbits showed them that (a) not everything goes in circles (comet orbits are REALLY ovally) and (b) if there are "spheres" holding the planets and sun like Aristotle said, the comet is somehow able to go straight through them!

24. Parallax is the apparent change in position of something when you view it from two different places. Remember the demo - put your thumb at arm's length in front of your face and look at it first through one eye and then the other - your thumb seems to move. If you moved your thumb farther and farther away (don't try this, it will hurt!), the parallax will seem less and less. Now every six months, the Earth is at opposite ends of its yearly orbit around the sun. If we look at the same star now and six months from now (sort of like blinking your eyes but with your eyes separated by the diameter of Earth's orbit, almost 200 million miles), why doesn't the star seem to move? The fact that the old astronomers did NOT see movement was used as an attack on heliocentric theories. Now we know that the star DOES move, just very little (we have much better instruments now to measure it). The reason there is so little parallax is that the stars are much further away than the old folks thought.

25. Some examples:

(a) Moons of Jupiter - something in the solar system definitely goes around something other than the Earth.

(b) The roughness of the moon - objects in the heavens are not perfect after all.

(c) Same with sunspots.

26. He was imprisoned in his home under house arrest for 11 years until his death. He was not even generally allowed to go to church or see doctors.


28. (a) Planets move in ellipses with the sun at one of the focuses.

(b) Planets sweep out equal areas in equal times (in other words, they go faster when they are near the sun, essentially because they are "falling" under the force of gravity more quickly when they are near the sun).

(c) Roughly speaking, the size of a planets orbit is directly related to how long it takes to orbit the sun (one "year" for the planet).

29. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.

30. Jupiter - lots of gas, especially hydrogen. Saturn is close in size and of similar composition.

31. The really thick atmosphere of Venus traps heat from the sun under it - the "greenhouse effect". The same thing seems to be happening on the Earth due to the junk humans dump into the atmosphere.

32. Almost all hydrogen (90%) and helium.

33. It can become a huge red giant (engulfing and destroying nearby planets) and then run out of fuel and quietly die and cool off OR it can collapse at the end of its life and become a supernovae, one of the most powerful explosions in the Universe. The latter happens if the star is relatively massive.

34. We don't know. The evidence from Martian meteorites is not really clear and only more exploration (which is underway) will tell…

35. The huge expansion of space and time that is generally believed to have given birth to our Universe something like 13 billion years ago.

36. The Kuiper belt is a bunch of relatively small objects outside the orbit of Neptune. The belief is that these things are made up of the primordial substance that the earth, Sun, and the rest of the solar system formed of 5 billion years ago. A space mission is proposed that would explore the belt.