1. Our goals for learning:
Spaceship Earth
Our goals for learning:
How is Earth moving in our solar system?
How is our solar system moving in the Milky Way Galaxy?
How do galaxies move within the universe?
Are we ever sitting still?

2. How is Earth moving in our solar system?
Contrary to our perception, we are not “sitting still.”
We are moving with Earth in several ways, and at surprisingly fast speeds.
The Earth rotates around its axis once every day.
Our first motion is ROTATION.
Point out that most of us are moving in circles around the axis at speeds far faster than commercial jets travel, which is why jets cannot keep up with the Sun when going opposite Earth’s rotation…

3. Earth orbits the Sun (revolves) once every year:
at an average distance of 1 AU ≈ 150 million kilometers.
with Earth’s axis tilted by 23.5º (pointing to Polaris)
It rotates in the same direction it orbits, counterclockwise as viewed from above the North Pole.
Our second motion is ORBIT. Point out the surprisingly high speed of over 100,000 km/hr.

4. How is our Sun moving in in the Milky Way Galaxy?
Our Sun moves randomly relative to the other stars in the local solar neighborhood…
typical relative speeds of more than 70,000 km/hr
but stars are so far away that we cannot easily notice their motion
… and orbits the galaxy every 230 million years.
Our third and fourth motions are MOTION WITH THE LOCAL SOLAR NEIGHBORHOOD and ROTATION OF THE MILKY WAY GALAXY.

5. More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in astronomy:
Although we won’t discuss dark matter until much later in the course, you might wish to mention it now to whet students’ appetites…

6. How do galaxies move within the universe?
Galaxies are carried along with the expansion of the universe. But how did Hubble figure out that the universe is expanding?
Describe the raisin cake analogy, and have students work through the numbers with you to make the table. (E.g., “How far away is Raisin 1 at the beginning of the hour? [1 cm] How far is it at the end of the hour? [3 cm] So how far would you have seen it move during the hour? [2 cm] So how fast is it moving away from you? [2 cm/hr]”

7. Hubble discovered that:
All galaxies outside our Local Group are moving away from us.
The more distant the galaxy, the faster it is racing away.
Conclusion: We live in an expanding universe.
Now relate the raisin cake analogy to the real universe…

8. Are we ever sitting still?
This slide summarizes our motion with spaceship Earth…

9. What have we learned? How is Earth moving in our solar system?
It rotates on its axis once a day and orbits the Sun at a distance of 1 AU = 150 million kilometers.
How is our solar system moving in the Milky Way Galaxy?
Stars in the Local Neighborhood move randomly relative to one another and orbit the center of the Milky Way in about 230 million years.

10. What have we learned? How do galaxies move within the universe?
All galaxies beyond the Local Group are moving away from us with expansion of the universe: the more distant they are, the faster they’re moving.
Are we ever sitting still?
No! Earth is constantly in motion, even though we don’t notice it.

11. Discovering the Universe for Yourself

12. Patterns in the Night Sky
Our goals for learning:
What does the universe look like from Earth?
Why do stars rise and set?
Why do the constellations we see depend on latitude and time of year?

13. What does the universe look like from Earth?
With the naked eye, we can see more than 2000 stars as well as the Milky Way.
Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation.

14. Constellations

15. Constellations A constellation is a region of the sky.
Eighty-eight constellations fill the entire sky.
Remind students that we often use the term “constellation” to describe a pattern of stars, such as the Big Dipper or the stars that outline Orion. However, technically a constellation is a region of the sky (and the patterns are sometimes called “asterisms”). A useful analogy for students: a constellation is to the sky as a state is to the United States. That is, wherever you point on a map of the U.S. you are in some state, and wherever you point into the sky you are in some constellation.

16. Constellations

17. Thought Question The brightest stars in a constellation:
all belong to the same star cluster.
all lie at about the same distance from Earth.
may actually be quite far away from each other.
You can use this question both to check student understanding of the idea of a constellation and as a way of leading into the concept of the celestial sphere that follows.

18. Thought Question The brightest stars in a constellation:
all belong to the same star cluster.
all lie at about the same distance from Earth.
may actually be quite far away from each other.
Worth pointing out that two stars in the same constellation can actually be farther apart than stars on opposite sides of our sky, depending on their distances… E.g.,: Procyon and Alpha Centauri are much farther apart than Procyon and Betelgeuse; But in space, Alpha Cen and Procyon are both relatively nearby stars (their distances are 11.4 and 4.4 light-years, respectively) which means they are not more than about a dozen light-years apart, while Betelgeuse is several hundred light-years away from Procyon (distance to Betelgeuse is about 430 light-years).

19. The Celestial Sphere
Stars at different distances all appear to lie on the celestial sphere.
The 88 official constellations cover the celestial sphere.
The illusion of stars all lying at the same distance in the constellations allows us to define the celestial sphere. It doesn’t really exist, but it’s a useful applet for learning about the sky. When discussing this slide, be sure to explain:
North celestial pole
South celestial pole
Celestial equator
Ecliptic
It’s also very useful to bring a model of the celestial sphere to class and show these points/circles on the model.

20. The Celestial Sphere
Ecliptic is the Sun’s apparent path through the celestial sphere.
If you do not have a model of the celestial sphere to bring to class, you might wish to use this slide; you will probably want to skip it if you have a model that you can discuss instead…

21. The Celestial Sphere
North celestial pole is directly above Earth’s North Pole.
South celestial pole is directly above Earth’s South Pole.
Celestial equator is a projection of Earth’s equator onto sky.

22. The Milky Way
A band of light making a circle around the celestial sphere.
What is it?
Our view into the plane of our galaxy.
On the previous slide or your model, you can point out that the celestial sphere is also painted with the Milky Way. Many students may never have seen the Milky Way in the sky (especially if they live in a big city), so the photo here is also worth showing. Key points to emphasize:
We use the term Milky Way in two ways: for the band of light in the sky and as the name of our galaxy.
(2) The two meanings are closely related. We like to use the following analogy: Ask your students to imagine being a tiny grain of flour inside a very thin pancake (or crepe!) that bulges in the middle and a little more than halfway toward the outer edge. Ask, “What will you see if you look toward the middle?” The answer should be “dough.” Then ask what they will see if they look toward the far edge, and they’ll give the same answer. Proceeding similarly, they should soon realize that they’ll see a band of dough encircling their location, but that if they look away from the plane, the pancake is thin enough that they can see to the distant universe.

23. The Milky Way
On the previous slide or your model, you can point out that the celestial sphere is also painted with the Milky Way. Many students may never have seen the Milky Way in the sky (especially if they live in a big city), so the photo here is also worth showing. Key points to emphasize:
We use the term Milky Way in two ways: for the band of light in the sky and as the name of our galaxy.
(2) The two meanings are closely related. We like to use the following analogy: Ask your students to imagine being a tiny grain of flour inside a very thin pancake (or crepe!) that bulges in the middle and a little more than halfway toward the outer edge. Ask, “What will you see if you look toward the middle?” The answer should be “dough.” Then ask what they will see if they look toward the far edge, and they’ll give the same answer. Proceeding similarly, they should soon realize that they’ll see a band of dough encircling their location, but that if they look away from the plane, the pancake is thin enough that they can see to the distant universe.

24. The Local Sky
An object’s altitude (above horizon) and direction (along horizon) specify its location in your local sky.
Now we move from the celestial sphere to the local sky. The local sky looks like a dome because we see only half the celestial sphere. If we want to locate an object:
It’s useful to have some reference points. Students will probably already understand the horizon and the cardinal directions, but explain the zenith and the meridian; a simple way to define the meridian is as an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north.
Now we can locate any object by specifying its altitude above the horizon and direction along the horizon. A good way to reinforce this idea is to pick an object located in your class room, tell students which way is north, and have them estimate its altitude and direction.

25. The Local Sky
Meridian: line passing through zenith and connecting N and S points on horizon
Zenith: the point directly overhead
Horizon: all points 90° away from zenith
Now we move from the celestial sphere to the local sky. The local sky looks like a dome because we see only half the celestial sphere. If we want to locate an object:
It’s useful to have some reference points. Students will probably already understand the horizon and the cardinal directions, but explain the zenith and the meridian; a simple way to define the meridian is as an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north.
Now we can locate any object by specifying its altitude above the horizon and direction along the horizon. A good way to reinforce this idea is to pick an object located in your class room, tell students which way is north, and have them estimate its altitude and direction.

26. We measure the sky using angles.
Point out that in general we have no way of judging true (physical) sizes and distances of objects in the sky -- like the illusion of stars lying on the celestial sphere, this is due to our lack of depth perception in space. Thus, we can measure only angular sizes and distances. Use these diagrams as examples. Optional: You can show how angular sizes depend on distance by having students sitting at different distances from you in the class use their fists to estimate the angular size of a ball you are holding. Students in the back will measure a smaller angular size.

27. Angular Measurements Full circle = 360º 1º = 60 (arcminutes)
1 = 60 (arcseconds)
Use this slide if you want to review the definitions of arc minutes and arc seconds.

28. Thought Question The angular size of your finger at arm’s length is about 1°. How many arcseconds is this?
60 arcseconds
600 arcseconds
60  60 = 3600 arcseconds
This is a quick test of whether students understand what we mean by arcseconds.

29. Thought Question The angular size of your finger at arm’s length is about 1°. How many arcseconds is this?
60 arcseconds
600 arcseconds
60  60 = 3600 arcseconds
Now point out that 1 arc second is only about 1/3600 of the width of your finger against the sky --- yet modern telescopes routinely resolve features smaller than this….

30. Angular Size
An object’s angular size appears smaller if it is farther away.
Use this slide if you want to review the definitions of arc minutes and arc seconds.

31. Why do stars rise and set?
The answer to the question is very simple if we look at the celestial sphere from the “outside.” But of course, we are looking from our location on Earth, which makes the motions of stars look a little more complex…
Earth rotates from west to east, so stars appear to circle from east to west.

32. Our view from Earth:
Stars near the north celestial pole are circumpolar and never set.
We cannot see stars near the south celestial pole.
All other stars (and Sun, Moon, planets) rise in east and set in west.
Now explain the basic motion of the sky seen from Earth.

33. Thought Question What is the arrow pointing to in the photo below. A
Thought Question What is the arrow pointing to in the photo below? A. the zenith B. the north celestial pole C. the celestial equator
This question will check whether students understand the pattern they see in this time exposure photograph.

34. Thought Question What is the arrow pointing to in the photo below. A
Thought Question What is the arrow pointing to in the photo below? A. the zenith B. the north celestial pole C. the celestial equator
This question will check whether students understand the pattern they see in this time exposure photograph.

35. Why do the constellations we see depend on latitude and time of year?
They depend on latitude because your position on Earth determines which constellations remain below the horizon.
They depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars.
These are the two basic reasons that the visible constellations vary; next we’ll explore each one.

36. Coordinates on the Earth
Latitude: position north or south of equator
Longitude: position east or west of prime meridian (runs through Greenwich, England)
Use this for a brief review of latitude and longitude; it’s also useful to bring in a real globe to class for this purpose.
The photo at right is the entrance to the Old Royal Greenwich Observatory (near London); the line emerging from the door marks the Prime Meridian.

37. The sky varies with latitude but not with longitude.
Use this interactive figure to explain the variation in the sky with latitude. Show how the altitude of the NCP equals your latitude (for N. hemisphere)…

38. Altitude of the celestial pole = your latitude
Show students how to locate the NCP and SCP, and how the sky moves around them. (You might wish to repeat the time exposure photo of the sky at this point to re-emphasize what we see.) Can also ask students where they’d find the north celestial pole in their sky tonight…

39. Thought Question The North Star (Polaris) is 50° above your horizon, due north. Where are you?
You are on the equator.
You are at the North Pole.
You are at latitude 50°N.
You are at longitude 50°E.
You are at latitude 50°N and longitude 50°E.
This question just makes sure the students understand the altitude = latitude idea…

40. Thought Question The North Star (Polaris) is 50° above your horizon, due north. Where are you?
You are on the equator.
You are at the North Pole.
You are at latitude 50°N.
You are at longitude 50°E.
You are at latitude 50°N and longitude 50°E.

41. The sky varies as Earth orbits the Sun
As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic.
At midnight, the stars on our meridian are opposite the Sun in the sky.
Use this interactive figure to explain how the constellations change with the time of year.

42. What have we learned? What does the universe look like from Earth?
We can see over 2000 stars and the Milky Way with our naked eyes, and each position on the sky belongs to one of 88 constellations.
We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon.
Why do stars rise and set?
Because of Earth’s rotation.

43. What have we learned?
Why do the constellations we see depend on latitude and time of year?
Your location determines which constellations are hidden by Earth.
The time of year determines the location of the Sun on the celestial sphere.