1. 1
Stellar Astronomy
Spring_2016
Day-7

3. Course Announcements SW-2 … hop to it.
1st set of Dark Sky observing nights:
Mon. Feb. 8 – 7:30pm at the observatory.
Tues. Feb. 9 is the weather backup if both are cancelled.
Del Square Psi meeting – 5:30pm Wed. 2/3

4. Colored Card Question In what direction is the observer facing?
A) toward the South
B) toward the North
C) toward the East
D) toward the West
Celestial Sphere
Celestial Sphere Rotation
Star A
Star B 1 3 2 4
Figure 2
Horizon

5. Colored Card Question high in the northern sky
Imagine that from your current location you observe a star rising directly in the east. When this star reaches its highest position above the horizon, where will it be?
high in the northern sky
high in the southern sky
high in the western sky
directly overhead

6. Colored Card Question
Where would the observer look to see the star indicated by the arrow?
High in the Northeast
High in the Southeast
High in the Northwest
High in the Southwest
Celestial Sphere
Celestial Sphere Rotation
Star A
Star B 1 3 2 4
Horizon

7. I Realize this is Like Trying to Drink from a Fire Hose

8. Earth’s rotation causes the Sun, Planets, Moon and stars to appear to move when viewed from Earth
Photo: Dr. Spencer Buckner

9. Rotation Figure 1 Figure 2 Celestial Sphere Rotation Star B Star A 1 2
North Star
Star A
Star B 1 3 2 4
Earth’s Equator
Figure 1
Celestial Sphere
Figure 2
Horizon

10. Rotation Figure 1 Figure 2 Celestial Sphere Rotation Star B Star A 1 2
North Star
Star A
Star B 1 3 2 4
Earth’s Equator
Figure 1
Celestial Sphere
Figure 2
Horizon

11. Tutorial: Motion – pg.3 Work with a partner
Read the instructions and questions carefully
Talk to each other and discuss your answers with each another
Come to a consensus answer you both agree on
If you get stuck or are not sure of your answer ask another group
If you get really stuck or don’t understand what the Lecture Tutorial is asking as one of us for help

12. How long did it take to get this picture?

13. Why does the sky change with your location?
As you move away from the pole your horizon moves with you but the locations of the celestial poles and celestial equator remains the same

14. Why does the sky change over the course of a year?
As we orbit the sun the direction opposite the sun changes and we only see the stars when the sun is not up 14

15. Kepler’s First Law: The orbit of a planet about the Sun is an ellipse with the Sun at one focus.

16. Kepler’s Second Law: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time.

17. According to Kepler’s second law, a planet with an orbit like Earth’s would:
move faster when further from the Sun.
move slower when closer to the Sun.
experience a dramatic change in orbital speed from month to month.
experience very little change in orbital speed over the course of the year.
none of the above.

18. a3AU= P2years Kepler’s THIRD LAW
The size of the orbit determines the orbital period
planets that orbit near the Sun orbit with shorter periods than planets that are far from the Sun
a3AU= P2years

19. Kepler’s THIRD LAW The size of the orbit determines the orbital period
planets that orbit near the Sun orbit with shorter periods than planets that are far from the Sun
p = ~ 12 years
p = 1 year

20. Kepler’s THIRD LAW The size of the orbit determines the orbital period
planets that orbit near the Sun orbit with shorter periods than planets that are far from the Sun
MASS DOES NOT MATTER
Both have p = 1 year

21. Which of the following best describes what would happen to a planet’s orbital speed if it’s mass were doubled but it stayed at the same orbital distance?
A. It would orbit half as fast.
B. It would orbit less that half as fast.
C. It would orbit twice as fast.
D. It would orbit more than twice as fast.
E. It would orbit with the same speed.

23. Newton’s First Law of Motion
A body remains at rest or moves in a straight line at a constant speed unless acted upon by an outside (net) force.
A rockets will coast in space along a straight line at constant speed.
A hockey puck glides across the ice at constant speed until it hits something

24. Newton’s Second Law of Motion
(net)Force = mass x acceleration or Fnet = m x a
Acceleration is the rate of change in velocity – or how quickly your motion is changing.
Three accelerators in your car!!

25. Newton’s Third Law of Motion
Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first body.
Don’t need a rocket launch pad!
The Bug and the Windshield – who is having the worse day?

26. Newton’s Laws of Motion & Gravitation
All my favorite Projectiles behave like this!!!
Velocity
Force
Acceleration

27. The gravitational force results in an acceleration.
All objects on Earth fall with the same acceleration known as g.
g = 9.8 m/s2
Fig. 4.1 27

28. Orbits describe one body falling around another.
The less massive object is a satellite of the more massive object.
The two bodies orbit a common center of mass.
For a much smaller satellite, the center of mass is inside the more massive body.
Fig. 4.6 28

29. An astronaut inside an orbiting space shuttle will experience free fall because he is falling around Earth at the same rate as the shuttle.
He is not weightless.
Fig. 4.7 29

30. Uniform circular motion: moving on a circular path at constant speed.
Gravity provides the centripetal force that holds a satellite in its orbit.
Uniform circular motion: moving on a circular path at constant speed.
Still experiencing an acceleration since the direction is constantly changing.
Fig. 4.8 30

31. Circles and ellipses are bound orbits.
Objects with higher orbital speeds can escape bound orbits to be in unbound orbits.
Parabolas and hyperbolas are examples.
Figure 4.10a 31

32. Newton’s Law of Gravitation
Newton’s law of gravitation states: Two bodies attract each other with a force that is directly proportional the product of their masses and is inversely proportional to the square of the distance between them.
What the ….? I thought I understood gravity?

33. Newton’s Law of Gravitation
To figure out the gravitational force just multiply the mass of the two things together then divide by the distance they are apart (squared). m1 d m2

34. Tutorial: Newton’s Law and Gravity – pg.29
Work with a partner
Read the instructions and questions carefully
Talk to each other and discuss your answers with each another
Come to a consensus answer you both agree on
If you get stuck or are not sure of your answer ask another group
If you get really stuck or don’t understand what the Lecture Tutorial is asking as one of us for help

41. Lab This Week What you need to know:
Hydrogen Energy Levels
What you need to know:
You get to explore the possible energy transitions for Hydrogen.
Reading ahead in Chapter 5 will help.

43. Tutorial: Newton’s Law and Gravity – pg.29
Work with a partner
Read the instructions and questions carefully
Talk to each other and discuss your answers with each another
Come to a consensus answer you both agree on
If you get stuck or are not sure of your answer ask another group
If you get really stuck or don’t understand what the Lecture Tutorial is asking as one of us for help

44. Concept Quiz—Earth’s Position
Assume Earth were moved to a distance from the Sun twice that of what it is now. How would that change the gravitational force it would experience from the Sun?
It would be half as strong.
It would be one-fourth as strong.
It would be twice as strong.
It would be four times as strong.
It would not change.
Answer: B
Explanation: Because of the inverse square law of gravity with regard to distance, twice as far means one-fourth as strong. 44

45. MATH TOOLS 4.1
The gravitational acceleration at the surface of Earth, g, can be solved for by using the formula for the gravitational force and Newton’s second law.
The m cancels.
g is the same for all
objects at the same R. 45

46. Newton’s Law of Gravitation
g ~ 10 m/s2 “the acceleration of gravity” & g x m is your weight!
Newton’s law of gravitation states: Two bodies attract each other with a force that is directly proportional the product of their masses and is inversely proportional to the square of the distance between them.

47. Concept Quiz—Gravity and Weight
Your weight equals the force between you and Earth.
Suppose you weigh 600 newtons. The force you exert on Earth is:
600 newtons.
much smaller than 600 newtons because your mass is much less than Earth’s.
exactly zero, since only massive objects have gravity.
Answer: A
Explanation: Forces come in pairs of equal size. These act on both bodies. 47

48. Concept Quiz—Earth and Moon
Earth and the Moon have a gravitational force between them. The mass of the Moon is 1.2 percent of that of the Earth. Which statement is incorrect?
The force on the Moon is much larger than that on Earth.
The forces are equal size, even though the masses are different.
The Moon has a larger acceleration than Earth.
Answer: A
Explanation: Statement A is the only incorrect statement. Forces come in pairs of equal size (but opposite direction); therefore, statement B is correct. Statement C is also correct. The forces are equal, but the Earth’s mass is much larger; therefore, it will experience a much smaller acceleration than does the Moon. 48

49. Gravity works on every part of every body.
CONNECTIONS 4.1
Fig. 4.5
Gravity works on every part of every body.
Therefore, self-gravity exists within a planet.
This produces internal forces, which hold the planet together. 49

50. There’s a special case: spherically symmetric bodies.
CONNECTIONS 4.1
Fig. 4.5
There’s a special case: spherically symmetric bodies.
Force from a spherically symmetric body is the same as from a point mass at the center. 50

51. You can solve for the period by noting that
MATH TOOLS 4.2
The velocity of an object traveling in a circular orbit can be found by equating the gravitational force and the resulting centripetal force.
This yields:
You can solve for the period by noting that
This yields Kepler’s third law: 51

52. Therefore, Earth’s escape velocity is
MATH TOOLS 4.3
In order to leave a planet’s surface, an object must achieve a velocity greater than the planet’s escape velocity.
Therefore, Earth’s escape velocity is 52

53. Newton derived Kepler’s laws from his law of gravity.
Physical laws explain Kepler’s empirical results:
Distant planets orbit more slowly; the harmonic law and the law of equal areas result.
Newton’s laws were tested by Kepler’s observations. 53

54. CONNECTIONS 4.2
The gravitational interaction of three bodies leads to Lagrangian equilibrium points.
These are special orbital resonances where the object at that point orbits in lockstep.
SOHO is near L1.
Fig. 4.18 54

55. Lab This Week Blackbody Curves and UBV Filters What you need to know:
A Blackbody is a perfect emitter.
Stars are NOT blackbodies.
The Stefan-Boltzmann Law (Chapter 5): F = sT4