1. Announcements First Homework Assignment.
Due Thursday 14th July, At start of class.
Complete pages 79 to 85 of the Lecture Tutorial book.
There are two sections to be complete.
The causes of the Moon Phases.
Predicting Moon Phases.
This is a double homework so each section will count as a separate homework.
First Mid-term test will be Friday 15th July.
It will cover everything we do up to and including Wed 13th July.
It is worth 10% of your grade.
Based on in-class questions and lecture tutorial book questions.
About 15 questions, 30 minutes long.

2. The Seasons

3. Thought Question
Earth is closer to the Sun in summer and farther from the Sun in winter.
True.
False.
A good way to begin discussion of seasons is by posing this question about the most common misconception about the seasons.

4. Thought Question
Earth is closer to the Sun in summer and farther from the Sun in winter.
True.
False.
Hint: During summer in the United States, it is winter in Australia.
A good way to begin discussion of seasons is by posing this question about the most common misconception about the seasons.

5. Thought Question TRUE OR FALSE! Earth is closer to the Sun in summer
and farther from the Sun in winter.
Variation of Earth–Sun distance is small—about 3%.
This small variation is overwhelmed by the effects of axis tilt.
Seasons are opposite in the N and S hemispheres.
Days are longer during N summer and shorter during S winter.
So changing distance from the sun cannot be the reason for the seasons.
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…

6. Axis tilt changes directness of sunlight during the year.
This tool is taken from the Seasons tutorial on MasteringAstronomy. 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 the Flux of Sunlight Vary

7. 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.
Days are longer in Summer, this also means more sunlight in Summer.
AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.

8. How do we mark the progression of the seasons?
We define four special points:
summer solstice
winter solstice
spring (vernal) equinox
fall (autumnal) equinox

9. We can recognize solstices and equinoxes by the 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…)

10. 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

11. Phases of the Moon.
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 your lamp is bright enough, the students can remain in their seats and watch the phases as they move the ball around their heads.

12. Why do we see phases of the Moon?
Lunar phases are a consequence of the Moon’s 27.3-day orbit around Earth.
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 your lamp is bright enough, the students can remain in their seats and watch the phases as they move the ball around their heads.

13. Phases of the Moon
Half of the Moon is illuminated by the Sun and half is dark.
We see a changing combination of the bright and dark faces as the Moon orbits Earth.
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.

14. Phases of the Moon 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.
Phases of the Moon

15. Moon Rise/Set by Phase
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.
Time the Moon Rises and Sets for Different Phases

16. } } 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

17. Thought Question first quarter waxing gibbous third quarter half moon
It’s 9 A.M. 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.

18. 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.
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.

19. 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.

20. The Cause Of Eclipses

21. Lunar Eclipse Lunar Eclipse
This interactive tool goes through lunar eclipses. Use it instead of or in addition to the earlier slides on eclipses.
Lunar Eclipse

22. 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 three types of lunar eclipse.

23. Solar Eclipse Evolution of a Total Solar Eclipse
This interactive tool goes through the solar eclipses. Use it instead of or in addition to the earlier slides on eclipses.
Evolution of a Total Solar Eclipse

24. 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 three types of solar eclipse.

25. Why don’t we have an eclipse at every new and full moon?
The Moon’s orbit is tilted 5° to the ecliptic plane.
So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon.
Use this pond analogy to explain what we mean by nodes and how we get two 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.

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

27. Now work on the tutorial Booklet … Page 91-96