Ch. 2Discovering the Universe for Yourself

1

2.1 Patterns in the Night Sky

• What does the universe look like from Earth?

• Why do stars rise and set?

Our goals for learning:

2

What does the universe look like from Earth?

With the naked eye, we can see more than 2,000 stars as well as the Milky Way.

3

Constellations

88 constellations fill the entire sky.

A constellation is a region of the sky.

4

Often you see this type of diagram....

5

Consider the dome of the sky over our heads….

mixing bowl

6

inverted mixing bowl ….

Consider the dome of the sky over our heads….

The Celestial Sphere!!

7

QuizThe brightest stars in a constellation…

A. All belong to the same star cluster.B. All lie at about the same distance from

Earth.C. May actually be quite far away from each

other.

8

The Celestial Sphere

Stars at different distances all appear to lie on the celestial sphere.

Ecliptic is Sun’s apparent path through the celestial sphere.

9

Earth orbits the Sun (revolves) once every year:

• at an average distance of 1 AU ≈ 150 million km.• with Earth’s axis tilted by 23.5º (pointing to Polaris)• and rotating in the same direction it orbits, counter-clockwise as viewed from above the North Pole.

10

The Celestial SphereThe 88 official constellations cover the celestial sphere.

Imagining a spinning Celestial Sphere surrounding Earth aids in thinking about the position and motion of the sky

11

The Milky WayA band of light making a circle around the celestial sphere.

What is it?Our view into the plane of our galaxy.

12

The Milky Way

13

Credit:Richard Bell

http://www.richardbell.net/

Location: TexasExposure: 60 min28 mm lens

14

The Local SkyAn object’s altitude (above horizon) and direction (along horizon) specifies its location in your local sky

15

The Local Sky

Zenith: The point directly overhead

Horizon: All points 90° away from zenith

Meridian: Line passing through zenith and connecting N and S points on horizon

16

Here’s the Celestial Sphere....

Celestial Sphere

Celestial Sphere Rotation

Celestial Sphere Rotation

Celestial Sphere

Star A

Star B

1

1

3

2

2

4

4

3

Figure 2

Horizon

17

Celestial Sphere

Celestial Sphere Rotation

Celestial Sphere Rotation

Celestial Sphere

Star A

Star B

1

1

3

2

2

4

4

3

Figure 2

Horizon

Is the horizon shown a real physical horizon, or an imaginary plane that extends from the observer and Earth out to the stars?

Can the observer shown see an object located below the horizon?

Is there a star that is in an unobservable position?

When a star travels from being below the observer’s horizon to being above the observer’s horizon, is that star rising or setting?

18

Lecture-Tutorial (LT):Position (pp. 1-2)

• Work with a partner!

• Read the instructions and questions carefully.

• Discuss the concepts and your answers with one 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 LT is asking, ask for help.

19

In what direction is the

observer facing?

1. toward the South2. toward the North3. toward the East4. toward the West

Celestial Sphere

Celestial Sphere Rotation

Celestial Sphere Rotation

Celestial Sphere

Star A

Star B

1

1

3

2

2

4

4

3

Figure 2

Horizon

20

Where would the observer

look to see the star indicated by

the arrow?A. High in the NortheastB. High in the SoutheastC. High in the Northwest D. High in the Southwest

Celestial Sphere

Celestial Sphere Rotation

Celestial Sphere Rotation

Celestial Sphere

Star A

Star B

1

1

3

2

2

4

4

3

Figure 2

Horizon

21

We measure the sky using angles

22

Angular Measurements• Full circle = 360º• 1º = 60ʹ′ (arcminutes) • 1ʹ′ = 60ʺ″ (arcseconds)

23

Quiz The angular size of your finger at arm’s length is

about 1°. How many arcseconds is this?

A. 60 arcsecondsB. 600 arcsecondsC. 60 × 60 = 3,600 arcseconds

24

Lecture-Tutorial (LT):Motion (pp. 3-6)

• Work with a partner!

• Read the instructions and questions carefully.

• Discuss the concepts and your answers with one 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 LT is asking, ask for help.

25