Download 1 Chapter 2 - University of Minnesota

Chapter 2:
11. Jupiter
a. 4 to 8 meters between the “terrestrial planets,” but
55 meters from Mars to Jupiter.
b. Outer planets are relatively widely spaced.
c. More massive than all other planets combined.
d. Has at least 16 moons.
e. Even has rings, but not a big deal, like those of Saturn.
f. Composition of Jupiter is like that of the sun.
g. Not quite big enough to be a star.
h. No solid surface, but metallic hydrogen in core.
i. Rotates in 10 hours, which generates so much centrifugal
force that it is visibly oblate.
j. Horizontal bands and Great Red Spot.
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k. “Galilean satellites.”
Chapter 2:
12. Saturn
a. 65 meters further on to Saturn, or ¾ length of football field.
b. Giant, with composition like the sun, just like Jupiter.
c. Rings can be seen through relatively small telescope.
d. Largest moon, Titan, has atmosphere of mostly Nitrogen,
but with no Oxygen.
e. Titan, because of thick atmosphere, appears essentially
featureless.
f. Titan’s atmospheric pressure is similar to that of Earth.
g. But Titan is really cold.
h. If there is life on Titan, it is probably pretty weird.
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Chapter 2:
14. Neptune
a. 160 meters further on from Uranus to Neptune.
b. At least 10 moons.
c. Dark rings, that are difficult to photograph.
d. Size and composition very similar to Uranus, but its color
is more blue.
e. Careful observations of Uranus showed its orbit to be
slightly inconsistent with Newton’s theory. In the early
1840s, John Adams, English, aged 24, computed from the
theory the location of a additional planet that could explain
the inconsistency.
f. Leverrier, a French astronomer, made same calculation in
1846, which led to the discovery of Neptune, just 1º away
from its predicted position. He also thought a new planet
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caused peculiarities of Mercury’s orbit, but this was wrong.
MOON
Model
Diameter
Model
Distance
Io
0.36 mm
4.2 cm
Europa
0.31 mm
6.7 cm
Ganymede
0.53 mm
10.7 cm
Callisto
0.48 mm
18.8 cm
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Chapter 2:
13. Uranus
a. 144 meters further on from Saturn to Uranus, about the
length of 1½ football fields.
b. Much smaller than Jupiter or Saturn; much larger than
Earth.
c. Like Jupiter and Saturn, made largely of H and He.
d. Methane in atmosphere gives Uranus its pale green color.
e. Rings and at least 15 moons.
f. Rotation axis lies nearly in the orbital plane.
Result of some cataclysmic collision as it was forming?
g. Not discovered until 1781, because it moves so slowly
against the background stars.
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Chapter 2:
15. Pluto
a. Average distance of Pluto from the grapefruit-sized sun in
our model is more than ½ km.
b. Pluto is made mostly of ices, unlike the gaseous planets
Jupiter, Saturn, Uranus, and Neptune.
c. Pluto is the smallest planet.
d. Single moon, Charon, is nearly as large as Pluto.
e. Orbit of Pluto is inclined, and crosses the orbit of Neptune,
so that for 20 years of its 248-year orbit it is closer to the
sun than Neptune.
f. Discovered in 1930. Thought it must exist because of
peculiarities in orbit of Uranus, but these were actually
measurement errors.
g. Pluto may just be one of many large icy bodies in outer
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solar system, much like comets.
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Chapter 2:
16. Comets
a. detect comets when their orbits perturbed and they enter
the inner solar system.
b. From the number we see, we think there are about a trillion
of them out there.
c. In 1994, a comet struck the planet Jupiter.
d. It could happen to us. Really?
e. It is possible that much of the water now on the earth
arrived through collisions with comets.
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Chapter 2:
17. The nearest star, a three-star system, Alpha Centauri
g. Imagine trying to see a pinhead-sized planet like the earth
orbiting Alpha Centauri at a distance of 15 meters from a
viewing location 4,400 km away!
h. No wonder we have not yet discovered planets like Earth
orbiting other stars.
i. Planets as large as Jupiter perturb the motion of the stars
they orbit, and we think we have detected such
perturbations in several cases. (People tend to argue about
this kind of thing.)
j. One possible justification for the construction and launch
of a bigger telescope to follow the Hubble Space Telescope
in orbit would be the hope of direct detection of planets
around other nearby stars.
It could happen. (Remember to vote to pay for it.)
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Chapter 2:
20. Possible reactions to the enormity of the universe:
a. Myron Lane’s reaction.
b. If somebody built all this just for us, they certainly went to
a lot of needless work.
c. If we are so insignificant, why bother to do well in this
course. What can it matter?
d. It is a great achievement of mankind to be able to sort out,
even to this very limited degree, something fashioned on a
scale so utterly disparate from that of the human body.
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Chapter 2:
17. The nearest star, a three-star system, Alpha Centauri
a. Alpha Centauri is about 4.4 light-years away.
b. A light-year is about 10 trillion km.
c. On our 1 to 10 billion scale, a light-year is therefore about
1000 km.
d. On this scale, Alpha Centauri is about 4,400 km away from
our grapefruit-sized sun, which is about the distance from
New York to Los Angeles.
e. Alpha Centauri B is dimmer than Alpha Centauri A and
separated from it by only about the same distance as Pluto
is separated from our sun, so at their great distance from us,
they look like a single point of light.
f. Proxima Centauri, the third star in this system is separated
from the other two by about 1/10 of a light-year, or 100 km
on our scale. It is too dim to see without a telescope. 8
Chapter 2:
18. Now let’s reduce the scale of our model by an additional factor
of a billion, so that it becomes a 1 to 10 billion billion, or a
1 to 1019 scale.
19. On this new scale, each light-year becomes 1 mm.
a. The distance to Alpha Centauri is now just 4.4 mm,
and the stars themselves are only about as big as individual
atoms.
b. Now the Milky Way, which is 100,000 light-years across,
becomes 100 meters on this scale, about the length of a
football field. Our location, as it were, is about at the 20
yard line.
c. The Milky Way is thought to contain around 100 billion
stars. That’s really a lot.
d. We think there are about 50 to 100 billion galaxies out
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there. That’s really a lot too.
Chapter 2:
21. Remember that in astronomy, space and time are inextricably
linked by the speed of light.
a. Looking out in space is also, necessarily, looking back in
time.
b. The person who wrote the book thinks the universe is 12
billion years old (this estimate tends to vary, but this
number should be good to within a factor of two).
c. Representing the course of this 12 billion years by a single
year, with creation (or whatever) at January 1, human
beings appeared on the earth at about 9 PM on New Year’s
Eve.
d. Conclusion: we haven’t been around very long, as things
go.
e. Possible conclusion: we may not be the last word in
beings.
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Review Questions you should be able to answer:
Chapter 4:
12, 14, 16, 17, 18, 20, 21
Chapter 5:
4 (sort of), 8 (sort of)
Chapter 6:
1, 2, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16, 18, 19 (sort of), 20
(for 19, know that measurements of the period and the distance of an orbiting object of relatively
The Cosmic Perspective
Chapter VI
with bits of chapter III and IV
“The Copernican Revolution
and the Development of the
Scientific Method”
small mass can be used to calculate the mass of the object being orbited)
Chapter 7:
2, 3, 4, 5, 9, 10, 12, 13, 14, 15, 17, 18
Chapter S2:
None of the questions. This chapter is unimportant.
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Outline of Lecture on Copernican Revolution:
1. Source of word “revolution” with its present meaning.
2. Why did people care where the planets were?
a. Agriculture.
b. Auguries.
c. Cyclic behavior suggests some law, difficult to divine.
3. Why the sun, stars, and planets all should revolve around us.
a. We are the center of everything, of course.
b. Most observed motions immediately explained.
c. Seasons caused by inclination and speed of sun’s motion.
d. Moon’s phases come out fine.
e. But there are small, nagging problems.
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Notes compiled by Paul Woodward
University of Minnesota
Dept. of Astronomy
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Review from Chapter 2:
f. The rotation of the earth causes the sky to appear to move.
g. The earth is like an enormous gyroscope.
1) Except for a truly negligible precession, the direction of the earth’s
rotation axis remains fixed in space, providing us with an invaluably
constant frame of reference.
2) The earth’s rotation axis is used to define the north and south
celestial poles, and also the celestial equator.
3) The north-south line passing through our location on the earth’s
surface, the meridian, is easily determined and helps us to construct
latitudes for the celestial coordinate system.
4) Longitudes are harder, because the earth is rotating. We can’t use
the location of Greenwich, or any other city.
5) We can define a circle, the ecliptic, on the celestial sphere which
marks the apparent path of the sun over each year.
6) The two points of intersection of the ecliptic and the celestial
equator, the 2 equinoxes, can serve to define longitudes.
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A reminder of the real cause of the seasons:
h. Effects of earth’s revolution about the sun:
1) Earth’s orbit nearly a circle.
2) Earth’s rotation axis inclined by 23.5º to its orbital plane.
3) At summer solstice, N. hemisphere most nearly facing sun.
4) At winter solstice, S. hemisphere most nearly facing sun.
5) At equinoxes, neither hemisphere is facing toward the sun.
i. Effects of sun’s apparent revolution about the earth:
1) Sun’s orbit assumed a circle, the most perfect curve.
2) All the effects listed above work just as well.
3) The same is true for the phases of the moon.
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This picture was meant to illustrate something else, but it shows how the earth is illuminated by the sun at a solstice.
A person at a northern latitude (left-most yellow circle) is carried by the earth’s rotation in sunlight for most of the day.
A person at a southern latitude (right-most yellow circle) is carried by the earth’s rotation in darkness most of the day.
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Outline of Lecture on Copernican Revolution:
4. Problems with the geocentric model:
a. Retrograde motion of some (the outer) planets.
Mars, Jupiter, and Saturn occasionally slow down relative
to the stars and actually go backwards for a time.
b. Phases of Venus (an objection more notable after the
invention of the telescope).
Full cycle of phases, correlated with Venus’ apparent size.
5. Retrograde motion “explained.”
a. Explained by epicyclic motion.
b. Imperfect, but still circles, just more of them.
c. This idea came from the Greek, Hipparchus, and was
elaborated by Ptolemy, who lived in Alexandria in the
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second century AD.
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