1. Prof. D.C. Richardson Sections 0101-0106
ASTR100 (Spring 2006) Introduction to Astronomy Discovering the Universe
Order E2-41 TRANSPARENT CELESTIAL GLOBE D4-25 GYROSCOPE - TOY E2-34 BASIC PLANETARIUM E2-21 PHASES OF THE MOON E2-23 UMBRA AND PENUMBRA - EXTENDED SOURCE
Prof. D.C. Richardson
Sections

2. Celestial Sphere Zenith: Point directly overhead Horizon: Where
the sky
meets the
ground
SKIPPED THIS 1/31/08

3. Celestial Sphere North Celestial Pole: Point on celestial sphere
above North
Pole
Celestial
Equator:
Line on
celestial
sphere
above Equator
SKIPPED THIS 1/31/08

4. What is the arrow pointing to. A. the zenith B
What is the arrow pointing to? 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.

5. What is the arrow pointing to. A. the zenith B
What is the arrow pointing to? 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.

6. Review: 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.

7. 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…

8. You are at the North Pole. You are at latitude 50°N.
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.
This question just makes sure they understand the altitude = latitude idea…

9. You are at the North Pole. You are at latitude 50°N.
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.
This question just makes sure they understand the altitude = latitude idea…

10. The sky varies as Earth orbits the Sun
As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic.
Use this interactive figure to explain how the constellations change with the time of year.

11. 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.

12. The Seasons 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.

13. Hint: When it is summer in the U.S., it is winter in Australia.
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 U.S.,
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.

14. The real reason for seasons involves Earth’s axis tilt.
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…

15. 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 directness of sunlight.

16. 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.

17. Summary: The Real Reason for Seasons
Orientation of Earth’s axis relative to the Sun changes as Earth orbits Sun.
Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct. Spring and fall are in between.
AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth!

18. Why doesn’t distance matter?
Earth’s orbit nearly circular anyway.
Note: more ocean, less land means less extreme seasons in the southern hemisphere.
Note: Some planets have greater distance variation that DOES affect their seasons, e.g., Mars, Pluto.
The two notes should be considered optional. If you cover the first note, you might point out that since Earth is closer to the Sun in S. hemisphere summer and farther in S. hemisphere winter, we might expect that the S. hemisphere would have the more extreme seasons, but it does not because the distance effect is overwhelmed by the geographical effect due to the distribution of oceans.

19. How do we mark the progression of the seasons?
We define four special points:
summer solstice winter solstice
spring (vernal) equinox fall (autumnal) equinox
Here we focus in on just part of Figure 2.13 to see the four special points in Earth’s orbit, which also correspond to moments in time when Earth is at these points.

20. We can recognize solstices and equinoxes by Sun’s path across 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…)

21. Seasonal changes are more extreme at high latitudes
Other points worth mentioning:
Length of daylight/darkness becomes more extreme at higher latitudes.
The four seasons are characteristic of temperate latitudes; tropics typically have rainy and dry seasons (rainy seasons when Sun is higher in sky).
Equator has highest Sun on the equinoxes.
Optional: explain Tropics and Arctic/Antarctic Circles.
Path of the Sun on the summer solstice at the Arctic Circle

22. 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/solstices move.
Earth’s axis precesses like the axis of a spinning top.
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” on p 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.

23. Phases of the Moon

26. Why do we see phases of the Moon?
Half the Moon illuminated by Sun and half dark.
We see some combination of the bright and dark faces.
You may want to do an in-class demonstration of phases by darkening the room, using a lamp to represent the Sun, and giving each student a Styrofoam ball to represent the Moon. If you lamp is bright enough, the students can remain in their seats and watch the phases as they move the ball around their heads.

27. Phases of the Moon
You can use this tool from the Phases of the Moon tutorial to present the idea behind phases in another way. As usual, please encourage your students to try the tutorial for themselves.

28. Moon Rise/Set by Phase
SKIPPED 2/5/08. Use this tool from the Phases of the Moon tutorial to explain rise and set times for the Moon at various phases. As usual, please encourage your students to try the tutorial for themselves.

29. } } Phases of the Moon: 29.5-day cycle waxing waning new crescent
first quarter
gibbous
full
last quarter }
waxing
Moon visible in afternoon/evening.
Gets “fuller” and rises later each day. }
waning
Moon visible in late night/morning.
Gets “less” and sets later each day.

30. It’s 9 am. You look up in the sky and see a moon with half its face bright and half dark. What phase is it?
First quarter
Waxing gibbous
Third quarter
Half moon
This will check whether students have grasped the key ideas about rise and set times.

31. It’s 9 am. You look up in the sky and see a moon with half its face bright and half dark. What phase is it?
First quarter
Waxing gibbous
Third quarter
Half moon
If anyone chose “half moon,” remind them that there is no phase with that name… (and that first and third quarter refer to how far through the cycle of phases we are…)

32. We see only one side of the Moon
Synchronous rotation: The Moon rotates exactly once with each orbit.
This is why only one side is visible from Earth. ™

33. What causes eclipses? The Earth and Moon cast shadows.
When either passes through the other’s shadow, we have an eclipse.
This slide starts our discussion of eclipses. Use the figure to explain the umbra/penumbra shadows.

34. When can eclipses occur?
Lunar eclipses can occur only at full moon.
Lunar eclipses can be penumbral, partial, or total.
Use the interactive figure to show the conditions for the 3 types of lunar eclipse.

35. When can eclipses occur?
Solar eclipses can occur only at new moon.
Solar eclipses can be partial, total, or annular.
Use the interactive figure to show the conditions for the 3 types of solar eclipse.

36. Why don’t we have an eclipse at every new and full moon?
The Moon’s orbit is tilted 5° to ecliptic plane…
So we have about two eclipse seasons each year, with a lunar eclipse at full moon and solar eclipse at new moon.
Use this pond analogy to explain what we mean by nodes and how we get 2 eclipse seasons each year (roughly).
Note: You may wish to demonstrate the Moon’s orbit and eclipse conditions as follows. Keep a model “Sun” on a table in the center of the lecture area; have your left fist represent the Earth, and hold a ball in the other hand to represent the Moon. Then you can show how the Moon orbits your “fist” at an inclination to the ecliptic plane, explaining the meaning of the nodes. You can also show eclipse seasons by “doing” the Moon’s orbit (with fixed nodes) as you walk around your model Sun: the students will see that eclipses are possible only during two periods each year. If you then add in precession of the nodes, students can see why eclipse seasons occur slightly more often than every 6 months.

37. Another look…
This interactive tool goes through the basic cause of eclipses. Use it instead of or in addition to the earlier slides on eclipses.

38. Summary: Two conditions must be met to have an eclipse…
It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse).
The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes).
AND

39. Predicting Eclipses
Eclipses recur with the 18 yr, 11 1/3 day saros cycle, but type (e.g., partial, total) and location may vary.
Complications caused by precession of nodes.
Point out that even though some ancient civilizations recognized the saros cycle, precise prediction still eluded them. Use the colored bands in the figure to illustrate the saros cycle (e.g., red bands for 2009 and 2027 eclipses are 18 yr, 11 1/3 days apart).

40. Ended After a Few More Slides (history of astro) Feb 5/08
(see class04)