1. The Moon, Our Constant Companion
Our goals for learning:
Why do we see phases of the Moon?
What causes eclipses?
© 2005 Pearson Education Inc., publishing as Addison-Wesley

2. Why do we see phases of the Moon?
Half the Moon is illuminated by the Sun and half is dark at all times
We see some fraction of the bright and dark faces at all times
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.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

3. © 2005 Pearson Education Inc., publishing as Addison-Wesley
Phases of the Moon: day cycle
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.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

4. How Can We Predict the Rising of the Moon?
For our discussion, we will assume the phases of the Moon are on a 28 day cycle instead of the true 29.5 day cycle. It makes the numbers a little easier and introduces only a small amount of error
We need to note the following facts:
As seen from above the north pole, the Earth orbits and rotates in a CCW direction
As seen from above the north pole, the Moon orbits and rotates in a CCW direction
Because the Moon orbits the Earth so much more rapidly than the Earth orbits the Sun (0.986º/day versus 12.2º/day), we need only consider the rotation of the Earth and the Orbit of the Moon
© 2005 Pearson Education Inc., publishing as Addison-Wesley

5. How Can We Predict the Rising of the Moon?
Draw the Sun, the Earth, and the Moon with the Sun to the left, the Earth to its right and the Moon in one of eight positions:
Third Quarter
Waning Crescent
Waning Gibbous
Sun
New Moon
Full Moon
Waxing Gibbous
Waxing Crescent
First Quarter
© 2005 Pearson Education Inc., publishing as Addison-Wesley

6. How Can We Predict the Rising of the Moon?
Choose the phase you wish to investigate and imagine (or draw) a person on the opposite side of the Earth from that phase.
Third Quarter
Waning Crescent
Waning Gibbous
Sun
New Moon
Full Moon
Waxing Gibbous
Waxing Crescent
First Quarter
© 2005 Pearson Education Inc., publishing as Addison-Wesley

7. How Can We Predict the Rising of the Moon?
Consider what time each position on the Earth corresponds to. For instance, when the person is between the Earth and the Full Moon, the Sun is directly below his/her feet. Hence, that spot on Earth must be experiencing 12 AM, midnight.
Third Quarter
Waning Crescent
Waning Gibbous
Sun
New Moon
Full Moon
Waxing Gibbous
Waxing Crescent
First Quarter
© 2005 Pearson Education Inc., publishing as Addison-Wesley

8. How Can We Predict the Rising of the Moon?
Similarly, when on Earth between the New Moon and the Earth, the Sun appears to be directly overhead so that time must correspond to 12 PM, noon. When between Third Quarter and the Earth, the time is sunrise (about 6 AM).
Third Quarter
Waning Crescent
Waning Gibbous
Sun
New Moon
Full Moon
Waxing Gibbous
Waxing Crescent
First Quarter
© 2005 Pearson Education Inc., publishing as Addison-Wesley

9. How Can We Predict the Rising of the Moon?
Now that you have a person on the opposite side of the Earth from the given Moon phase, it is clear that the Moon is not visible; it is set.
Third Quarter
Waning Crescent
Waning Gibbous
6 AM
Sun
New Moon
12 PM
12 AM
Full Moon
6 PM
Waxing Gibbous
Waxing Crescent
First Quarter
© 2005 Pearson Education Inc., publishing as Addison-Wesley

10. How Can We Predict the Rising of the Moon?
The Moon just appears (rises) or disappears (sets) when the line connecting the person to the center of the Earth and the line connecting the Moon and the Earth make a 90º angle. The Moon is at its highest when the person is between the Moon and the Earth.
Third Quarter
Waning Crescent
Waning Gibbous
6 AM
Sun
New Moon
12 PM
12 AM
Full Moon
6 PM
Waxing Gibbous
Waxing Crescent
First Quarter
© 2005 Pearson Education Inc., publishing as Addison-Wesley

11. How Can We Predict the Rising of the Moon?
Thoroughly confused? Good, you need an example. Let’s consider the New Moon.
The arrow below indicates a person experiencing midnight. The Earth prevents the Moon from being seen.
6 AM
Sun
New Moon
12 PM
12 AM
6 PM
© 2005 Pearson Education Inc., publishing as Addison-Wesley

12. How Can We Predict the Rising of the Moon?
The Moon rises when the line connecting the Moon to the Earth and the line connecting the center of the Earth to the person make a 90º angle.
In the drawing below, the two lines make a 90º angle
Since the Moon was previously unable to be viewed, we have found the rise time for the New Moon (Sunrise or about 6 AM).
6 AM
Sun
New Moon
12 PM
12 AM
6 PM
© 2005 Pearson Education Inc., publishing as Addison-Wesley

13. How Can We Predict the Rising of the Moon?
The Moon reaches your meridian (i.e., it is at its highest) when it appears to be directly over the person’s head.
In the drawing below, the person is directly below the New Moon
The New Moon will cross your meridian at 12 PM, noon.
6 AM
Sun
New Moon
12 AM
6 PM
© 2005 Pearson Education Inc., publishing as Addison-Wesley

14. How Can We Predict the Rising of the Moon?
The Moon sets when the line connecting the Moon to the Earth and the line connecting the center of the Earth to the person make a 90º angle.
In the drawing below, the two lines make a 90º angle
Since the Moon was previously able to be viewed, we have found the set time for the New Moon (Sunset or about 6 PM).
6 AM
Sun
New Moon
12 PM
12 AM
6 PM
© 2005 Pearson Education Inc., publishing as Addison-Wesley

15. How Can We Predict the Rising of the Moon?
As we did for the several preceding slides; however, we need to note a few things:
Halfway between 6 AM and 12 PM is 9 AM
Halfway between 12 PM and 6 PM is 3 PM …
The New Moon, First Quarter, Full Moon, and Third Quarter last only an instant; all of the other times the phases are either crescent or gibbous
Since we assume a 28 day period, the First Quarter is 7 days after the New Moon, the Full Moon occurs 7 days later, Third Quarter occurs 7 days later, and finally—after 28 days—the New Moon occurs again
If New Moon is on day ZERO, the Waxing Crescent lasts from day 1 until day 6, Waxing Gibbous from day 8 until 13, Waning Gibbous from day 15 until 20, and waning crescent from day 22 until 27.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

16. © 2005 Pearson Education Inc., publishing as Addison-Wesley
Thought Question
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.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

17. © 2005 Pearson Education Inc., publishing as Addison-Wesley
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…)
© 2005 Pearson Education Inc., publishing as Addison-Wesley

18. Test Your Understanding
At around 1 PM you see a fingernail shaped Moon low in the western sky. What phase is it in? About how long will it be until a Full moon occurs?
© 2005 Pearson Education Inc., publishing as Addison-Wesley

19. Test Your Understanding
At around 1 PM you see a fingernail shaped Moon low in the western sky. What phase is it in? Since it is low in the western sky it must be about to set, therefore it must be in the waning crescent phase.
About how long will it be until a Full moon occurs? Since it is in the waning crescent phase, we have between 15 and 20 days before a Full Moon occurs. If you knew about how low the Moon was you could be more precise.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

20. Test Your Understranding
The Moon appears to progress eastward through the constellations by how many degrees per hour?
The Moon is about how many degrees wide?
In about fourteen days the Moon goes from rising at 6 AM to rising at 6 PM. About how many hours per day later should the Moon rise each day?
© 2005 Pearson Education Inc., publishing as Addison-Wesley

21. Test Your Understranding
The Moon appears to progress eastward through the constellations by how many degrees per hour? About 0.5 degrees eastward through the Zodiac.
The Moon is about how many degrees wide? About 0.5 degrees wide; hence, it moves eastward about one diameter every hour.
In about fourteen days the Moon goes from rising at 6 AM to rising at 6 PM. About how many hours per day later should the Moon rise each day? Dividing 12 hours by 14 days, we get that the Moon rises later by hours per day or about 51 minutes per day.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

22. © 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

23. 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.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

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 3 types of solar eclipse.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

25. © 2005 Pearson Education Inc., publishing as Addison-Wesley
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 new moon and solar eclipse at full 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.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

26. 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).
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). Alternatively, it could be said that the centers of the Earth, the Moon, and the Sun must be close to being on a single line.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

27. © 2005 Pearson Education Inc., publishing as Addison-Wesley
What causes eclipses?
This interactive tool goes through the basic cause of eclipses. Use it instead of or in addition to the earlier slides on eclipses.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

28. © 2005 Pearson Education Inc., publishing as Addison-Wesley
Solar Eclipse
This interactive tool goes through the solar eclipses. Use it instead of or in addition to the earlier slides on eclipses.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

29. © 2005 Pearson Education Inc., publishing as Addison-Wesley
Lunar Eclipse
This interactive tool goes through lunar eclipses. Use it instead of or in addition to the earlier slides on eclipses.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

30. Test Your Understanding
Why would the Umbra reach the Earth’s surface during some eclipses but not during others?
© 2005 Pearson Education Inc., publishing as Addison-Wesley

31. Test Your Understanding
Why would the Umbra reach the Earth’s surface during some eclipses but not during others? Because the Moon’s orbit is not a circle; it is an ellipse. Hence, it is sometimes closer to the Earth than at other times. When the Moon is too far from the Earth, its umbra cannot reach the Earth’s surface.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

32. © 2005 Pearson Education Inc., publishing as Addison-Wesley
Predicting Eclipses
Eclipses recur with the 18 yr, 11 1/3 day saros cycle, but type (e.g., partial, total) and location may vary.
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).
© 2005 Pearson Education Inc., publishing as Addison-Wesley

33. © 2005 Pearson Education Inc., publishing as Addison-Wesley
What have we learned?
Why do we see phases of the Moon?
Half the Moon is lit by the Sun; half is in shadow.
The appearance of the Moon to us is determined by the Sun, Earth, and Moon positions.
What causes eclipses?
Lunar eclipse: Earth’s shadow on the Moon. Can be penumbral, partial, or total.
Solar eclipse: the Moon’s shadow on Earth. Can be partial, total, or annular.
Tilt of Moon’s orbit means eclipses occur during two periods each year.
Eclipses recur with the 18 yr, 11 1/3 day saros cycle
© 2005 Pearson Education Inc., publishing as Addison-Wesley