1. Mastering Astronomy

2. Chapter 2 Discovering the Universe for Yourself

3. Earthly Cycles Day - Earth rotates once
Month - Moon orbits the Earth once and rotates once
Year - Earth orbits the Sun once

4. 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.
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.

5. The Milky Way
A band of light that makes 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.

7. 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.

9. Constellations A constellation is a region of the sky.
88 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.

14. 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.
Sun's Apparent Path through the Zodiac

15. 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.

16. 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.

17. The Celestial Sphere
Stars at different distances all appear to lie on the celestial sphere.
The ecliptic is the Sun’s apparent path through 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 tool 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.

18. 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 the horizon
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.

20. 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.
A circumpolar star never sets
Now explain the basic motion of the sky seen from Earth.
Celestial equator
This star never rises
Your horizon

21. The path the stars take depends on your latitude.
At the North or South Poles, all stars are circumpolar
At the Equator, stars rise and set at 90o from the horizon
At other latitudes, stars rise and set and an angle

22. 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.

23. The sky varies with latitude but not 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)…

24. 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…

25. 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…

26. 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…

27. Special Topic: How Long Is a Day?
Solar day = 24 hours
Sidereal day (Earth’s rotation period) = 23 hours, 56 minutes
This slide goes with the optional Special Topic box in the text. Note: rather than simply discussing parts (a) and (b), you can do the actual demonstration in class…

28. Thought Question TRUE OR FALSE? Earth is closer to the Sun in summer
and farther from the Sun in winter.
A good way to begin discussion of seasons is by posing this question about the most common season misconception.

29. Thought Question (Hint: When it is summer in the United States,
TRUE OR FALSE? Earth is closer to the Sun in summer
and farther from the Sun in winter.
(Hint: When it is summer in the United States,
it is winter in Australia.)
This hint should make students realize that distance from the Sun CANNOT be the explanation for seasons: if it were, the entire Earth should experience the same seasons at the same time.
Note: Some students think the axis tilt makes one hemisphere closer to the Sun than the other; you can show them why this is not the case by revisiting the 1-to-10 billion scale model solar system from Ch. 1. When you remind them that the ball point size Earth orbits 15 meters from the grapefruit size Sun, it’s immediately obvious that the 2 hemispheres cannot have any significant difference in distance.

30. Thought Question
TRUE OR FALSE! Earth is closer to the Sun in summer
and farther from the Sun in winter.
• Seasons are opposite in the N and S hemispheres, so distance cannot be the reason.
The real reason for seasons involves Earth’s axis tilt.
Now that you’ve answered the T/F question, we can go on to explore the real reason for seasons. Note: You might optionally mention that, in fact, Earth is closest to the Sun during N. hemisphere winter…

31. What causes the seasons?
Misconceptions about the cause of the seasons are so common that you may wish to go over the idea in more than one way. We therefore include several slides on this topic. This slide uses the interactive version of the figure that appears in the book; the following slides use frames from the Seasons tutorial on the Astronomy Place web site.
Seasons depend on how Earth’s axis affects the directness of sunlight.

32. Axis tilt changes directness of sunlight during the year.
This tool is taken from the Seasons tutorial on the Astronomy Place web site. You can use it to reinforce the ideas from the previous slide. As usual, please encourage your students to try the tutorial for themselves.
Why Does Flux Sunlight Vary

33. Summary: The Real Reason for Seasons
Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun.
Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct.
AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.
Mars also has seasons because its axis is tilted with respect to the Sun as well.
Why doesn’t distance matter?
Variation of Earth–Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt.

34. Sun’s altitude also changes with seasons
Sun’s position at noon in summer: higher altitude means more direct sunlight.
Sun’s position at noon in winter: lower altitude means less direct sunlight.
This tool is taken from the Seasons tutorial on the Astronomy Place web site. You can use it to reinforce the ideas from the previous slide. As usual, please encourage your students to try the tutorial for themselves.

35. We can recognize solstices and equinoxes by Sun’s path across the sky.
Summer solstice: Highest path, rise and set at most extreme north of due east
Winter solstice: Lowest path, rise and set at most extreme south of due east
Equinoxes: Sun rises precisely due east and sets precisely due west.
Of course, the notes here are true for a N. hemisphere sky. You might ask students which part written above changes for S. hemisphere. (Answer: highest and lowest reverse above, but all the rest is still the same for the S. hemisphere; and remind students that we use names for the N. hemisphere, so that S. hemisphere summer actually begins on the winter solstice…)

36. How does the orientation of Earth’s axis change with time?
Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years.
— Polaris won’t always be the North Star.
— Positions of equinoxes shift around orbit; for example, the spring equinox, once in Aries, is now in Pisces!
Precession can be demonstrated in class in a variety of ways. E.g., bring a top or gyroscope to class, or do the standard physics demonstration with a bicycle wheel and rotating platform.
You may wish to go further with precession of the equinoxes, as in the Common Misconceptions box on “Sun Signs” --- this always surprises students, and helps them begin to see why astrology is questionable (to say the least!). Can also mention how Tropics of Cancer/Capricorn got their names from constellations of the solstices, even though the summer/winter solstices are now in Gemini/Sagittarius.
Precession