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EXAM #1
ASTRO 101: PRINCIPLES OF ASTRONOMY
FALL 2010
THURSDAY, SEPTEMBER 30, 2010, 11:00—12:15, PS 130
Use the scantron form No. F-288-PAR-L.
The exam will be closed-book. Calculators may not be used as storage devices for notes and other materials.
This exam has 12 pages. Please check here
if you answered the bonus question on page 12.
This is test form B. Mark the appropriate box on the back of the scantron form.
CHECKLIST: One point deducted for each missing item!
Top of this page: Name & ID
Scantron front: 1: write and bubble 9 digit ID (leave right column blank)
Scantron front: 2: write and bubble name
Scantron back: 3: write and bubble 9 digit ID (leave right column blank)
Scantron back: 4: bubble in test form (see box above)
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PART I (1 pt. each): Complete each sentence or statement.
1. The
distance
the wavelength of the wave.
between any point on a wave and the next similar point is
2. The
of a planet is the ratio of the amount of light reflected by
albedo
that planet to the amount of light received by it.
3. The
is a methodical approach employed by scientists to explore
scientific method
the universe around us in an objective manner.
4. Electromagnetic radiation behaves as though it has properties of both
and waves.
mass
5. The density of a planet is the ratio of its
to its volume.
6. Kepler discovered that the shape of an orbit is a(n)
, as previously believed.
circle
7. Copernicus was the first person to propose a
System.
, not a(n)
ellipse
model for the Solar
heliocentric
8. Right ascension is the celestial analogue of
9. Venus shines by reflected
particles
longitude
.
.
sunlight
10. As seen from the Earth, the Sun and Moon have roughly the same
size.
angular
Possibly helpful words:
absorbs, albedo, angular, circle, color, density, distance, ellipse, emits, frequency, geocentric, heliocentric,
intensity, latitude, longitude, mass, particles, radius, scientific method, sunlight, temperatures, wavelength.
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PART II (10 pts. total): Refer to the diagram and answer the five questions.
Q
D
F
C
E
A
SUN
N
B
This diagram shows the orbit of the Earth from a view above the north pole. The Earth is spinning counterclockwise (as indicated by the short curved arrow) and orbiting the Sun in the counterclockwise direction
(as indicated by the long curved arrow).
1. (2 pts.) What time is it at the position marked A? noon
2. (2 pts.) What time is it at the position marked B? sunrise
3. (2 pts.) Draw where the Moon is when it is in the new phase. Put a circle with the letter N in it.
4. (2 pts.) Draw where the Moon is when it is in the full phase. Put a circle with the letter F in it.
5. (2 pts.) Draw where the Moon is when it is in the first quarter phase. Put a circle with the letter Q in it.
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PART III (5 pts. each): Short answers. Briefly answer each question below and on the following page.
Partial credit will be awarded. Also, up to 1 bonus point each will be awarded for exceptional answers.
1. Explain why the seasons are opposite between the Northern and Southern hemispheres. Draw a diagram
to support your answer.
The single most important thing that causes the seasons on the Earth is the tilt of the Earth’s rotation axis with
respect to its orbital plane. The tilt in the axis leads to a changing pattern of heating from the Sun over the year, as
well as changing day/night lengths.
The reason the seasons are opposite between the northern and southern hemispheres is due to the fact that these
hemispheres are on opposite sides of the equator. When the north pole is inclined towards the Sun so that the Sun
is higher above the horizon at noon in that hemisphere (leading to longer daytime hours and warmer weather), the
south pole must necessarily be inclined away from the Sun. Hence the Sun would be lower above the horizon at noon,
leading to shorter daytime hours and colder weather.
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2. Explain how Galileo’s observations of the phases of Venus provided support for the heliocentric model of
the Solar System. In your answer, be sure to (a) indicate how Venus would look in the geocentric model of
the Solar System; (b) indicate how Venus would look in the heliocentric model of the solar system; and (c)
draw diagrams to support your answer.
Galileo observed that Venus has a full set of phases, much like the Moon. In addition (and quite unlike the Moon),
the angular size of Venus changes with phase. The crescent phases have a larger angular size than the nearly full
phases.
In the geocentric model, Venus orbits the Earth on a sphere that is inside the Sun’s sphere. The path of Venus never
crosses the Sun’s path. Venus is always seen near the Sun on the sky (e.g. just after sunset or just before sunrise, and
never at midnight), so the geocentric model had to have Venus on an epicycle that follows the Sun. Because Venus is
always between us and the Sun in this model, Venus should always appear as a crescent.
In the heliocentric model, Venus orbits inside the Earth’s orbit. As a result, it would always appear close to the Sun
on the sky. When Venus is roughly in between us and the Sun, it is relatively close and hence has a larger angular
size. When Venus is nearly full (the exactly full phase cannot be seen except possibly during a solar eclipse), it is on
the opposite side of the Sun, and hence its angular size would be smaller.
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Part IV (2 pts. each): Multiple choice. Mark the best answer on the scantron sheet.
1. The orbits of the major planets all lie in nearly the same plane resulting in a disk-like structure for the
solar system. This disk-like structure is believed to exist because
a.
b.
c.
d.
e.
Jupiter’s gravity was great enough to pull all of the other planets into the plane of its orbit.
the Sun’s magnetic field slowed down the rotation of the solar nebula.
the original solar nebulae had a disk-like structure.
a bi-polar flow from the young Sun cleared all material out of the nebula except that in the disk.
planetesimals settled into the plane of rotation of the solar nebula.
2. The
a.
b.
c.
d.
e.
is the point on the celestial sphere directly above any observer.
south celestial pole
north celestial pole
celestial equator
asterism
zenith
3. Imagine you are in the middle of the Pacific Ocean. If the south celestial pole appears on your horizon,
what is your latitude?
a.
b.
c.
d.
e.
90◦ S
45◦ S
The latitude of the observer can not be determined from the information given.
0◦
90◦ N
4. Which of the following numbers has the same meaning as 4,670.2?
a.
b.
c.
d.
4.6702 × 103
4.6702 × 101
4.6702 × 102
4.6 × 103
5. A black hole is an object with a surface gravity so intense that nothing, not even light can escape.
According to the law of universal gravitation, what would happen to the Earth if the Sun were to somehow
suddenly change into to black hole with the same mass?
a.
b.
c.
d.
e.
The Earth would slowly spiral towards the black hole.
The Earth would quickly be sucked into the black hole.
The Earth would move away from the black hole in a straight line.
The Earth would slowly spiral away from the black hole.
The Earth’s orbit would not change.
6. If the Earth were to rotate faster on its axis
a.
b.
c.
d.
e.
the summer season would be longer.
the summer season would be shorter.
the day would be shorter.
the day would be longer.
none of the above.
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7. Tycho Brahe’s greatest contribution to astronomy was
a.
b.
c.
d.
e.
his discovery of three laws of motion
his 20 years of careful observations of the planets.
his telescopic observations.
his discovery of the moons of Jupiter.
his model of the universe.
8. You have two balls of equal size and smoothness, and you can ignore air resistance. One is heavy, the
other much lighter. You hold one in each hand at the same height above the ground. You release them
at the same time. What will happen?
a. The heavier one will hit the ground first.
b. The lighter one will hit the ground first.
c. They will hit the ground at the same time.
9. Compared with visible-light photons, a radio-wave photon has
a.
b.
c.
d.
the same energy.
less energy.
a shorter wavelength.
more energy.
10. If you double the frequency of a photon, its energy
a.
b.
c.
d.
e.
is reduced by a factor of 2.
increases by a factor of 4.
stays the same.
increases by a factor of 2.
is reduced by a factor of 4.
11. The objective of most radio telescopes is similar to the objective of a reflecting optical telescope in which
way?
a.
b.
c.
d.
e.
They are both concave in shape.
They have nothing in common!
They are both made of metal.
They are typically the same or similar size.
They are both made of glass.
12. The phase of the Moon on a particular night is determined by
a.
b.
c.
d.
e.
the season of the year.
where the Earth’s shadow hits the Moon.
the distance from the Earth to the Moon.
the relative position of the Sun, Earth, and Moon.
the speed of the Moon in its orbit.
13. How much total surface of the Moon is illuminated by the Sun when it is at quarter phase?
a.
b.
c.
d.
e.
one half
three quarters
one quarter
almost none
all of it
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14. Which of the following statements is correct for eclipses in the Sun-Earth-Moon system?
a.
b.
c.
d.
e.
An eclipse of the Sun occurs only at new Moon.
An eclipse of the Sun occurs only at first-quarter Moon.
An eclipse of the Sun occurs only at full Moon.
An eclipse of the Moon occurs only at new Moon.
An eclipse of the Sun will be visible from some place on Earth every month.
15. If it is just after sunset, and the Moon is in its full phase, the Moon will be
a.
b.
c.
d.
directly overhead.
just setting.
not visible.
just rising.
16. The low average density of Saturn suggests
a.
b.
c.
d.
e.
its core is iron.
it is very cool.
it contains large quantities of hydrogen and helium.
it is very hot.
it is hollow.
17. Newton concluded that some force had to act on the Moon because
a.
b.
c.
d.
e.
a force is needed to
a force is needed
the moon moved at
a force is needed to
all of the above
pull the moon outward.
to pull the moon away from straight-line motion.
a constant velocity.
keep the moon in motion.
18. Imagine that the Earth’s orbit were changed to be a perfect circle about the Sun so that the distance to
the Sun never changed. How would this affect the seasons?
a.
b.
c.
d.
We would no longer experience a difference between the seasons.
We would still experience seasons, but the difference would be much MORE noticeable.
We would still experience seasons, but the difference would be much LESS noticeable.
We would continue to experience the seasons in the same way we do now.
19. Blue light has higher frequency than red light. Thus, blue light has
a.
b.
c.
d.
higher energy and longer wavelength than red light.
lower energy and longer wavelength than red light.
higher energy and shorter wavelength than red light.
lower energy and shorter wavelength than red light.
20. A long, thin cloud that stretched from directly overhead to the western horizon would have an angular
size of
a.
b.
c.
d.
180◦
90◦
360◦
45◦
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21. The Earth’s shadow falling on the Moon is the reason we see
a.
b.
c.
d.
lunar eclipses
solar eclipses
the phases of the Moon
the Earth’s shadow cannot fall on the Moon
22. Which of the following statements regarding energy is false?
a. A faster moving body has more energy of motion than does a slower moving body with the same
mass.
b. Gasoline has potential energy.
c. Energy is the ability to do work.
d. Energy can be destroyed.
e. Energy can in principle be created from ordinary matter.
23. During December, the daytime hours in San Francisco (approximately 400 miles north of San Diego) are
a.
b.
c.
d.
sometimes shorter, sometimes longer, depending on the Moon’s phase.
shorter than in San Diego.
longer than in San Diego.
the same as in San Diego.
24. Stars in the same constellation
a.
b.
c.
d.
e.
may actually be very far away from each other.
must be part of the same cluster of stars in space.
must have been discovered at about the same time.
will have similar brightnesses.
probably formed at the same time.
25. A calculation of how long it takes a planet to orbit the Sun would be most closely related to
a.
b.
c.
d.
Kepler’s third law
Newton’s first law of motion
Kepler’s first law
Kepler’s second law
26. The Earth is closer to the Sun in January than it is in July. Which of the following statements, in accord
with Kepler’s Second Law, is correct?
a.
b.
c.
d.
The Earth travels faster in its orbit around the Sun in January than in July.
The orbital speed of the Earth is the same in January as in July.
The Earth travels faster in its orbit around the Sun in July than in January.
Kepler’s Second Law does not apply here.
27. Venus is never seen at midnight because
a.
b.
c.
d.
e.
it is visible only at sunset.
it will be in its new phase then.
it is closer to the Sun than is Earth.
the Sun is blocking the view.
the Earth is closer to the Sun than is Venus.
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28. The apparent visual magnitude of a star is a measure of the star’s
a.
b.
c.
d.
e.
color.
distance.
brightness.
size.
temperature.
29. The force due to gravity between two objects depends on
I.
II.
III.
IV.
a.
b.
c.
d.
e.
the
the
the
the
mass of each object.
distance each object is from Earth.
distance between the two objects.
speed of light.
I, II, & III
I & II
I & III
I, II, III, & IV
II & IV
30. A good scientific theory should
a.
b.
c.
d.
e.
be able to be proven correct.
make predictions that can be tested.
be based equally on intuition and experiment.
not predict new results, only explain prior data.
be based primarily on philosophical considerations.
31. The distance traveled by light in an interval of time is equal to the speed of light multiplied by
a.
b.
c.
d.
e.
the wavelength of the light in meters.
the time.
the mass of the light.
the energy of the light.
the frequency of the light.
32. Imagine that you are building a scale model of the Earth and the Moon. You are going to use a 12-inch
basketball to represent the Earth and a 3-inch tennis ball to represent the Moon. To maintain the proper
distance scale, about how far from the surface of the basketball should the tennis ball be placed?
a.
b.
c.
d.
e.
300 feet
6 inches (1/2 foot)
30 feet
36 inches (3 feet)
4 inches (1/3 foot)
33. Which of the following features of an optical telescope determines its light gathering power?
a.
b.
c.
d.
e.
The focal length of the objective.
The diameter of the primary mirror.
The diameter of the secondary mirror.
The length of the telescope tube.
The focal length of the eyepiece.
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34. In space, the velocity of radio waves is about
a.
b.
c.
d.
e.
3 km/s.
300 km/s.
300,000 km/s.
300 m/s.
3 m/s.
35. You observe a full Moon rising in the east. How will it appear in six hours?
a)
b)
c)
d)
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Bonus Question (2 pts.): Explain in detail the precession of the equinoxes. Draw diagrams to support
your answer.
Possibly helpful formula:
L ∝ 4πR2 T 4
where L is the luminosity, R is the radius, and T is the temperature.
λmax ∝ 1/T
where λmax is the wavelength corresponding to the peak emission, and T is the temperature.
c = νλ
where c is the speed of light, ν is the frequency, and λ is the wavelength.
E = hν
where E is the photon energy, and h is a constant.
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