Announcements - University of Toledotir.astro.utoledo.edu/jdsmith/class/a1010_f10/Lecture7.pdf ·...

AnnouncementsHomework due on Sunday at 11:45pm. Thank your classmate!

You should have finished reading Chapter 3, and started on chapter 4 for next week.

Don’t forget your out of class planetarium show on Friday evenings or Saturday afternoon.

Observing at Brooks Observatory starts next Monday. Come to the 5th floor of this building at 9–10pm. Bring your filled-out blue ticket!

Thursday, September 16, 2010

The Raw Exam Distribution

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0-9 10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89 90-100


GradesYour anonymously coded grades are on the course website:


Buy-Back

Congratulations: as a class, you bought back 387 points!

Remember: Write out the question, the text of the answer you chose, why you chose it, the text of the correct answer, and why.

If you didn’t circle your answers on your exam and take it with you... do so next time!


How to Buy-BackIncorrect:

On #23 I Chose E but I just guessed, I now know it’s B, cause that’s the answer, dude.

Correct:

Question 23: For how many days was this planet in retrograde motion? I Chose E, 32 days, since that’s how long the planet spent in the loop on the sky. Now I understand that it is in retrograde motion only when moving from East to West, which happened from March 31st to April 12th, over the course of 12 days. So the correct answer is B, 12 days.

Also: you get no credit for picking one you already got correct!


The Adjusted Exam Distribution with Buy Back

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0-9 20-29 40-49 60-69 80-89 100-110


Do your homework!

Homework #1 Grade

Exam

#1

Gr

ad

e (

Raw

)


16. The Sun's path, as viewed from Earth’s equator, is highest in the sky on _________.

A. !the winter solsticeB. !the spring and fall equinoxesC. !the day when Earth is closest to the SunD. !the summer solstice

17. What component of Earth’s motion causes the stars to rise earlier on successive nights?

A. its rotation about its axis B. its orbit around the Sun C. the tilt of its rotation axis D. the precession of Earth’s rotation axis

18. You observe a full moon rising in the east. Which image shown below best represents how the Moon will appear when it sets?

19. A planet is moving in normal (“prograde”) motion. Over the course of several nights, how will the planet appear to move relative to the background stars on the celestial sphere?

A. east to west B. west to east C. It will not move at all, as planets do not move with the stars. D. It will move randomly, as planets move differently than the stars.

20. Which of the following statements about the Milky Way Galaxy is not true?

A. One rotation of the galaxy takes about 200 million years.B. Our solar system is located very close to the center of the Milky Way Galaxy.C. It contains between 100 billion and 1 trillion stars.D. The galaxy is about 100,000 light-years in diameter.

ASTR 1010, Section 1, Fall 2010 ! Midterm Exam #1, Page 5/13


30. Tonight, you observe a star located 10 light-years away. When did the light you see

tonight leave the star?

A. yesterday

B. 10 years ago

C. tonight

D. cannot be determined from the information given

31. Lunar eclipses can occur only during a ___________.

A. new moon

B. full moon

C. first quarter moon

D. third quarter moon

32. What effect or effects would be most significant if the Moon's orbital plane were exactly

the same as the ecliptic plane?

A. solar eclipses would be much more frequent

B. solar eclipses would be much rarer

C. total solar eclipses would last much longer

D. both A and C

E. both B and C

33. Which of the following statements does not use the term light-year in an appropriate way?

A. It's about 4 light-years from here to Alpha Centauri.

B. A light-year is about 10 trillion kilometers.

C. It will take me light-years to complete this homework assignment.

D. It will take the Voyager spacecraft about 20,000 years to travel just 1 light-year.

E. The Milky Way Galaxy is about 100,000 light-years in diameter.

34. Which of the following is smallest?

A. size of a typical planet

B. 1 light-second

C. 1 AU

D. size of a typical star


Light would travel 7 times around

Earth in 1 second!


Jupiter is 3 light-seconds in circumference!


25. Which of the following statements does NOT use the term angular size or angular distance correctly?

A. The angular distance between those two houses across the street is 30 degrees.B. The angular size of the Sun is about the same as that of the Moon.C. The angular distance between those two bright stars in the sky is about 2 meters.D. You can use your outstretched hand against the sky to estimate angular sizes and

angular distances.

26. Which of the following best describes why we have seasons on Earth?

A. The varying speed of the Earth in its orbit around the Sun gives us summer when we are moving fastest and winter when we are moving slowest.

B. The Earth's elliptical orbit means we are closer to the Sun and therefore receive more intense sunlight at some times of year than at others.

C. The tilt of the Earth's axis causes the northern hemisphere to be closer to the Sun than the southern hemisphere (giving rise to summers in the northern hemisphere and winters in the southern hemisphere), and visa versa.

D. The tilt of the Earth's axis causes different portions of the Earth to receive more or less direct sunlight at different times of year.

27. We cannot see a new moon in our sky because ___________.

A. it is obscured by the Earth's shadowB. a new moon is quite near the Sun in the skyC. no sunlight is illuminating the MoonD. it is above the horizon during the daytime

28. On the scale of the cosmic calendar, in which the history of the universe is compressed to 1 year, how long has human civilization (i.e., since ancient Egypt) existed?

A. about half the yearB. about a monthC. a few secondsD. a few hoursE. less than a millionth of a second

29. Earth is always precisely 1 astronomical unit from the Sun.

A. TrueB. False



http://www.windows2universe.org/physical_science/physics/mechanics/orbit/orbit_shape_interactive.html

25. Which of the following statements does NOT use the term angular size or angular distance correctly?

A. The angular distance between those two houses across the street is 30 degrees.B. The angular size of the Sun is about the same as that of the Moon.C. The angular distance between those two bright stars in the sky is about 2 meters.D. You can use your outstretched hand against the sky to estimate angular sizes and

angular distances.

26. Which of the following best describes why we have seasons on Earth?

A. The varying speed of the Earth in its orbit around the Sun gives us summer when we are moving fastest and winter when we are moving slowest.

B. The Earth's elliptical orbit means we are closer to the Sun and therefore receive more intense sunlight at some times of year than at others.

C. The tilt of the Earth's axis causes the northern hemisphere to be closer to the Sun than the southern hemisphere (giving rise to summers in the northern hemisphere and winters in the southern hemisphere), and visa versa.

D. The tilt of the Earth's axis causes different portions of the Earth to receive more or less direct sunlight at different times of year.

27. We cannot see a new moon in our sky because ___________.

A. it is obscured by the Earth's shadowB. a new moon is quite near the Sun in the skyC. no sunlight is illuminating the MoonD. it is above the horizon during the daytime

28. On the scale of the cosmic calendar, in which the history of the universe is compressed to 1 year, how long has human civilization (i.e., since ancient Egypt) existed?

A. about half the yearB. about a monthC. a few secondsD. a few hoursE. less than a millionth of a second

29. Earth is always precisely 1 astronomical unit from the Sun.

A. TrueB. False



http://localhost/~jdsmith/movies/orbit_shape.swf

http://localhost/~jdsmith/movies/orbit_shape.swf



Last Time

Ancient peoples: structures to mark progression of sun/moon/planets.


Last Time

Greeks: Earth is round, at center of “real” celestial spheres.


Last Time

1500 years of Ptolemaic model before earth was displaced as center of universe.


Last Time

Copernicus: first to gain traction with a sun-centered universe.


Last Time

Brahe/Kepler: measured and refined the model of motions: not circles but ellipses!


Last Time

Galileo: Cemented heliocentric model using a telescope. Moons of jupiter, sunspots, etc.


Last Time

Retrograde motion the key: very complicated “epicycle” models required to account.

Simple to explain when all planets orbit the sun!


Kepler’s Laws of Planetary MotionKepler’s First Law: The orbit of each

planet around the Sun is an ellipse, with the Sun at one focus.


Kepler’s Second Law: As a planet moves around its orbit, a line from the planet to

the Sun sweeps out equal area in equal time.

Kepler’s Laws of Planetary Motion


Kepler’s 2nd LawEqual area law implies that planets move:

– Faster when closer to sun

• Perihelion = planet closest to sun

– Slower when farther from sun

• Aphelion = planet farthest from sun


Kepler’s Third Law: The ratio of the cube of the average distance of the planet from the Sun (a=semimajor axis) to the square of the orbital period (p) is the

same for each planet.

p in units of yearsa in units of Astronomical Units

Kepler’s Laws of Planetary Motion

P2=A3


What is an Astronomical Unit?

It is Earth’s distance from the Sun (or more technically, it is the Earth’s semi-major axis)

1 A.U. = 1.5 x 1011 m

499 light seconds!


Does the third law work for the Earth?

Earth: P = 1 year, a = 1 A.U.

P2 = a3 so 12 = 13 or 1 = 1

It works!


Graphical Version of Kepler’s Third LawThe farther you are from the Sun, the

longer your orbital period

The farther you are from the Sun, the

slower you orbit the Sun

p2 = a3


One year of time (Earth is green)


An asteroid orbits the sun at an average distance of 4 A.U. What is it’s orbital period?

A) 1 yearB) 4 yearsC) 8 yearsD) 64 years

A BC D




A BC D

✪




A BC D

✪

A = 4 A.U.P2 = A3 = 43 = 64P = 8 years!


Consider a planet orbiting the Sun. If the mass of the planet doubled but the planet stayed at the same orbital distance, then the planet would take

A) more than twice as long to orbit the Sun.

B) exactly twice as long to orbit the Sun. C) the same amount of time to orbit the

Sun.D) exactly half as long to orbit the Sun.

A BC D


Consider a planet orbiting the Sun. If the mass of the planet doubled but the planet stayed at the same orbital distance, then the planet would take

A) more than twice as long to orbit the Sun.

B) exactly twice as long to orbit the Sun. C) the same amount of time to orbit the

Sun.D) exactly half as long to orbit the Sun.

A BC D

✪


A3 = P2


If a small weather satellite and the large International Space Station are orbiting Earth at the same altitude above Earth’s surface, which object takes longer to orbit once around Earth?

A) the large space station B) the small weather satellite C) the same amount of time to orbit the

Sun.

A BC D


If a small weather satellite and the large International Space Station are orbiting Earth at the same altitude above Earth’s surface, which object takes longer to orbit once around Earth?

A) the large space station B) the small weather satellite C) the same amount of time to orbit the

Sun.

A BC D

✪


Which of the following best describes what would happen if Mercury and Jupiter were to switch places in their orbits about the Sun?

A) Jupiter, the larger planet, would have a shorter orbital period than before.

B) Mercury, the smaller planet, would have a shorter orbital period than before.

C) Neither of the two planets would have any change in their orbital periods.

A BC D


Which of the following best describes what would happen if Mercury and Jupiter were to switch places in their orbits about the Sun?

A) Jupiter, the larger planet, would have a shorter orbital period than before.

B) Mercury, the smaller planet, would have a shorter orbital period than before.

C) Neither of the two planets would have any change in their orbital periods.

A BC D

✪


Where do Kepler’s laws work?

Everywhere!


Why do Kepler’s Laws work everywhere?

Gravity ... which brings us to...

Sir Isaac Newton


Gravity, Energy, and Motion


Describing Motion

speed = rate at which an object moves. Distance/Timeexample: 10 m/s

velocity = speed + direction.example: 10 m/s due West

acceleration = change in the speed or directionExample: 10 m/s2


The Acceleration Due to Gravity

All falling objects accelerate at the same rate (not counting air resistance) independent of mass.

On Earth, g ≈ 10 m/s2: speed increases 10 m/s with each falling second

10 m/s per second or 10 m/s2


If you drop a hammer and a feather at the same time on the moon:

A) the hammer will hit the ground firstB) the feather will hit the ground firstC) they will hit the ground at the same

time

A BC D


If you drop a hammer and a feather at the same time on the moon:

A) the hammer will hit the ground firstB) the feather will hit the ground firstC) they will hit the ground at the same

time

A BC D

✪


Gravity

The acceleration due to gravity is the same for any object on the surface of the earth.

Hammer and feather fall at the same rate (neglecting air resistance).


For Next Time

Start Reading Chapter 4.

Remember: First week of Evening observing at Brooks Observatory starts Monday.

HW#2 Due Sunday Evening


Announcements - University of Toledotir.astro.utoledo.edu/jdsmith/class/a1010_f10/Lecture7.pdf ·...

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