Astronomy in-class activity: eclipses

Astronomy 210 In-class activity 5 v.13.06.09, fall semester 2013
Cuesta College, San Luis Obispo, CA

Students find their assigned groups of three to four students, and work cooperatively on an in-class activity worksheet to discuss the categories and causes of different types of eclipses.

Presentation: eclipses

Cats casting shadows. Eh, this looks Photoshopped. How can you tell? What clues do you look for? As we'll see, it's not just the shadows cast by objects that are important, but where we are located relative to these shadows.

In the previous presentation, we discussed the motion and cycles of the moon--specifically lunar phases--and here we'll complete our discussion of the moon with eclipses.

Just a public service announcement about watching solar eclipses--make sure you are not looking directly at the sun with the unprotected eye. Either project an image of the sun using a pinhole, binoculars, or telescope...

...or use filters specifically labeled for observing the sun. My dog can practice safe solar eclipse viewing, and so should you.

First, the boring but necessary terminology.

Make sure you can distinguish between very similar terms:

• full and new moon (well, this should be self-explanatory).
• lunar eclipse and solar eclipse (light from the sun that would illuminate the full moon is blocked by Earth; direct light from the sun that would reach Earth is blocked by the moon).
• partial, total, and annular eclipse (sun or moon partially or totally covered/darkened, respectively; annular refers to an "annulus" or "little ring," where the moon is centered on the sun, but does not completely cover the sun).

Let's observe a simulation of a total lunar eclipse in April 14, 2014, as seen from the perspective of the sun. Note that Earth blocks light to the moon. Where would you have to be located (on Earth) to observe this total lunar eclipse? Where on Earth would observers not be able to see this total lunar eclipse? What phase is the moon in during this total lunar eclipse? (Why is there not a total lunar eclipse during every full moon?) (Video link: "1-5-140415-Lunar.mov.")

Here's a simulation of a total solar eclipse from July 11, 1991, again seen from the perspective of the sun, where the moon blocks light to Earth (casting a much smaller shadow). Where would you have to be located (on Earth) to observe this total solar eclipse? Where on Earth would observers not be able to see this total solar eclipse? What phase is the moon in during this total solar eclipse? (Why is there not a total solar eclipse during every new moon?) (Video link: "1-5-910711-Solar.mov.")

Time to do a picto-quiz--you'll be shown a picture or movie clip of the moon and/or sun (assume that each of these situations is the maximum extent of something being shadowed or blocked), and then be prompted with possible responses. At that point, if you know the correct answer, shout it out--because yes, the loudest answer is the most correct answer...

Is this a solar or lunar eclipse, or not an eclipse at all? Is this eclipse partial, total, or annular?

Is this a solar or lunar eclipse, or not an eclipse at all? Is this eclipse partial, total, or annular?

Is this a solar or lunar eclipse, or not an eclipse at all? Is this eclipse partial, total, or annular?

Is this a solar or lunar eclipse, or not an eclipse at all? Is this eclipse partial, total, or annular?

Is this a solar or lunar eclipse, or not an eclipse at all? Is this eclipse partial, total, or annular?

Is this a solar or lunar eclipse, or not an eclipse at all? How do you know that this is not an eclipse? What phase is this moon?

One more slide, for you Twihards--which team are you on? (Why is there no Team Bella?)

Second, let's consider why not every full moon is a lunar eclipse, and not every new moon is a solar eclipse.

This is a to-scale simulation of the moon revolving around Earth, while Earth revolves around the sun, as seen from the perspective of the sun. A lot goes on simultaneously, so just watch this and observe the different types of motions. After we discuss what details we should be looking for, we'll run this simulation again. (Video link: "1-5-Linesofnodes.mov.")

Note that the orbit of the moon is tilted, and the bright part is closest to us (the sun) and tilted downwards, while the dim part is farthest away from us, and tilted upwards. Which phase is the moon in right now? How do you know that an eclipse is not occurring right now? How do you know this?

The tilt of the moon's orbit changes slowly over time. Which phase is the moon in right now? How do you know that an eclipse is not occurring right now? How do you know this?

Generally an eclipse can happen only if (1) the moon is either new or full, and (2) the moon's orbit is aligned edge-on. Our previous two examples had the right phase, but wrong orbit; then the right orbit, but the wrong phase. Now we have both the right phase (although it is not clear whether it is new or full) and the right orbit (edge-on), and some type of eclipse (whether solar or lunar) is occurring right now. The timing between the moon phase and slowly changing moon orbit is crucial, and if the timing between these two cycles is not perfect (as it usually isn't), this is why there is not a solar or lunar eclipse every new or full moon.

Back to our simulation. When an eclipse happens (whether solar or lunar), you all say "now." Synchronize! Approximately how many months lapsed during this time? How many eclipses (whether solar or lunar) occurred during this time?

Let's look at two full moons, and flip between them. One is a (nearly) full moon that is just about to undergo a total lunar eclipse, and the other is a full moon that will not undergo any type of lunar eclipse. Can you determine the difference(s) between these two full moons?

Remember that the reason a lunar eclipse does not occur every full moon is that the orbit of the moon is usually tilted not edge-on, but too high or too low. Notice that the "Not to be eclipsed" moon has a slight shadow on the bottom, meaning that its orbit is too high, while the "To be eclipsed" moon has sharp edges at both top and bottom, meaning that its orbit is edge-on and will pass directly behind Earth for a total lunar eclipse. Next time just before the moon is completely full, take a careful look at the top and bottom edges of the moon (you will probably need binoculars or a telescope). If you see a slight ragged edge at the top or bottom, don't get too excited, as a total lunar eclipse is not impending.

In this promotional photograph for a certain obscure TV show from a few years ago, is it plausible that all these actors and actresses were present at in the studio at the same time? How do you know? What clues do you look for?

In the subsequent in-class activity, we'll be looking carefully at the different "shadow zones" cast by either the moon or Earth, and what would be observed if were located in each of these zones: umbra, penumbra, and antumbra (not a common term, we'll refer to the antumbra as the "negative shadow").

Astronomy midterm question: simultaneous total lunar and total solar eclipses?

Astronomy 210 Midterm 1, Spring Semester 2010
Cuesta College, San Luis Obispo, CA

[20 points.] Consider the following excerpt regarding viewing total lunar and solar eclipses:

On July 6, 1982, a woman telephoned a radio show that had invited people to give their impressions of the total lunar eclipse then in progress. She said: 'We're seeing this total eclipse of the moon right now on our side of the world. Does that mean that at this same moment the people on the other side of the world are seeing a total eclipse of the sun?'"
—Guy Ottewell, Astronomical Calendar 2010, Universal Workshop, Raynham, MA, 2009, p. 43.

Decide whether seeing a total eclipse of the sun "at this same moment" is plausible or implausible. Support your answer using a diagram showing the positions of the sun, moon, and Earth.

Solution and grading rubric:

• p = 20/20:
  Correct. Draws a correct diagram of a total lunar eclipse, where the moon is on the other side of Earth than the sun; and since the moon must be on the same side of Earth as the sun for there to be a total solar eclipse, it is implausible that observers anywhere on Earth can observe a total solar eclipse simultaneously with a total lunar eclipse. May also illustrate and argue the converse argument, when a total solar eclipse is occuring a total lunar eclipse cannot be simultaneously observed.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Diagram is problematic (misalignment of sun-Earth-moon line), but essential argument that the moon cannot be at two different locations on either side of Earth correct.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Diagram missing, but essential argument correct, or vice versa.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Argument based on timing of time zones, size of moon's umbral spot, or improbable sun-Earth-moon diagrams.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 30674
p: 36 students
r: 1 student
t: 1 student
v: 6 students
x: 0 students
y: 0 students
z: 0 students

Section 30676
p: 62 students
r: 6 students
t: 23 students
v: 13 students
x: 13 students
y: 1 student
z: 3 students

A sample "p" response (from student 1802):

Another sample "p" response (from student 7129), lamenting the fact that Earth does not have two moons:

Another sample "p" response (from student 2425):

A sample "v" response (from student 9696), with two Earths?

Astronomy midterm question: total lunar eclipse versus total solar eclipse duration

Astronomy 210 Midterm 1, Spring Semester 2009
Cuesta College, San Luis Obispo, CA

[20 points.] Why does an observer see a total lunar eclipse lasting much longer than a total solar eclipse? Defend your answer using a diagram showing the positions of an observer, Earth, the moon, and shadow zones.

Solution and grading rubric:

• p = 20/20:
  Correct. The size of the umbra created by the Moon makes total solar eclipses last only a few minutes, as the umbral spot passes quickly over an observer's position. The size of the umbra created by the Earth makes total lunar eclipses last a few hours, as it will take that long for the Moon to pass through this shadow zone of the Earth. Has a correct explanation, and clear, correct diagrams of a total solar eclipse and a total lunar eclipse demonstrating the effect of umbra sizes.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Typically incomplete or inconsistent diagrams.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Diagrams problematic, but at least recognizes importance of sizes of umbrae.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Plausible arguments involving varying orbital speeds, precession of nodes, varying orbital distances, with some attempt at diagrams. Or substantively correct diagrams only.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit. Involving unrelated factors.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 30674
p: 8 students
r: 6 students
t: 7 students
v: 17 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 1959):

A sample "t" response (from student 1889):

A sample "v" response, with diagram only (from student 1213):

Astronomy quiz question: in the penumbra of the Moon

Astronomy 210 Quiz 2, Fall Semester 2008
Cuesta College, San Luis Obispo, CA

[4.0 points.] An observer is located on the surface of the Earth, and this observer is also located in the penumbra of the Moon. What type of eclipse is seen by the observer?
(A) A total lunar eclipse.
(B) A partial lunar eclipse.
(C) An annular solar eclipse.
(D) A partial solar eclipse.
(E) A total solar eclipse.

Correct answer: (D)

Assuming that the umbra of the Moon never does pass over the observer (for which this would be response (E), a total solar eclipse), this will be only a partial solar eclipse. If the observer were in the "negative shadow" of the Moon (out past the umbra), then an annular solar eclipse would be observed (response (C)). Responses (A) and (B) correspond to the Moon itself being in the umbra or penumbra of the Earth, respectively.

Student responses
Section 70160
(A) : 5 students
(B) : 10 students
(C) : 0 students
(D) : 15 students
(E) : 3 students

"Difficulty level": 51%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.78

Astronomy midterm question: total lunar eclipse

Astronomy 10 Midterm 1, spring semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q2.4

At 7:30 PM at night, you are currently observing a total lunar eclipse in San Luis Obispo, CA. Suppose you decided to call a friend who lives in Seattle, WA, which is north of San Luis Obispo, CA. Explain what kind of eclipse (if any) your friend would be able to see in Seattle, WA at the same time, or why not, using a diagram of Earth, the moon, and shadow zones. Assume that the skies are clear in both San Luis Obispo, CA and Seattle, WA.

Solution and grading rubric:

• p:
  Correct. Clear and correct diagram of the observer, Earth, the moon and the sun shown. For a total lunar eclipse, the moon is completely in the umbra of Earth. All observers on the night side of Earth will be able to see this total lunar eclipse. Note that a total penumbral lunar eclipse may be discussed instead, as long as all observers on the night side of Earth see the same thing.
  
• r:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Earth-moon-sun diagram is essentially correct (showing a total lunar eclipse), but argument is based on observers being in different shadow zones, which is not possible, as all observers on the night side of Earth are in the umbra of Earth.
  
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Serious but flawed attempt at an Earth-moon-sun diagram, with major inconsistencies or errors, typically showing a total/partial/annular solar eclipse.
  
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y:
  Irrelevant discussion/effectively blank.
  
• z:
  Blank.
  

Grading distributions:
Section 5166
p: 27 students
r: 4 students
t: 16 students
v: 18 students
x: 2 students
y: 0 students
z: 0 students

Notably this midterm was given during the umbral phase of the February 20, 2008 total lunar eclipse! Some students who had finished early were able to go outside and make Danjon L-scale ratings of the darkness of the umbral shadow on the Moon.

A sample of a "p" response (from student 0429) is shown below, clearly showing that observers in Seattle, WA and San Luis Obispo, CA would both see the Moon in the umbra of Earth:
Another "p" response (from student 1207) more elaborately illustrating the relevant geography:
Yet more elaborate geography in another "p" response (from student 1652):
An extended "p" response covering both total and partial lunar eclipse cases (from student 1886):

Astronomy quiz question: monthly solar and lunar eclipses

Astronomy 10 Quiz 2, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q2.4

[3.0 points.] Which one of the following choices best describes what would happen if the orbit of the Moon around the Earth was not tilted with respect to the orbit of the Earth around the Sun?
(A) There would never be any solar or lunar eclipses of any type.
(B) There would only be annular solar eclipses, and partial lunar eclipses.
(C) There would be a solar eclipse and a lunar eclipse every month.
(D) The Moon would then always block the Sun, making a permanent solar eclipse.
(E) The Earth would then always be between the Sun and Moon, making a permanent lunar eclipse.

Correct answer: (C)

The reason why there is no solar eclipse and lunar eclipse every month is that the orbit of the Moon around the Earth is tilted, such that the new Moon does not usually pass directly between the Sun and the Earth, and the full Moon does not usually pass directly between the Earth and the Moon.

Student responses
Section 4160
(A) : 4 students
(B) : 6 students
(C) : 25 students
(D) : 1 student
(E) : 3 students

Student responses
Section 5166
(A) : 5 students
(B) : 6 students
(C) : 50 students
(D) : 3 students
(E) : 2 students

Astronomy clicker question: eclipse observing

Astronomy 10, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q2.4

Students were asked the following clicker question (Classroom Performance System, einstruction.com) at the end of their learning cycle:

[0.3 points.] Consider a side view of the Moon and the Earth, their respective shadow zones, and the locations of an observer. The Sun is located to the left, far off of the page.

Which view of the Moon does this observer see?Correct answer: (D)

The Moon is partially in the umbra and penumbra of the Earth, and this would correspond to a dark red/brown shadow, and a barely noticeable shadow on the surface of the Moon, respectively. Thus an observer on the Earth would see response (D).

However, many students chose response (A), the view of the Earth blocking the Sun, which is what would be seen from the surface of the Moon, in its northern hemisphere (the portion of the near side of the Moon that is in the penumbra).

Student responses
Section 5166
(A) : 18 students
(B) : 3 students
(C) : 1 student
(D) : 27 students

Section 4160
(A) : 4 students
(B) : 6 students
(C) : 0 students
(D) : 17 students

Total lunar eclipse: August 28, 2007

070828lunareclipse.jpg
Originally uploaded by Waifer X

Total lunar eclipse sequence, photographed through Cuesta College's Dobsonian-mounted 8" reflector using a point-and-shoot 4-megapixel digital camera (Pentax Optio S40).

Astronomy current events question: no west coast U.S. lunar eclipse

Astronomy 10L, Spring Semester 2007
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events (skytonight.com, from Sky & Telescope magazine), and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)

[0.2 points.] On March 3, 2007 there will be a lunar eclipse for observers in Europe, and observers on the east coast of North America. Why will observers on the west coast of North America not be able to see this happening? Circle your answer below.
(A) Observers on the west coast of North America are in the northern hemisphere.
(B) There will not be enough horizon illusion.
(C) The lunar eclipse will end before the Moon rises.
(D) The Moon will be too dark.
(E) The Moon will be too far away.

Correct answer: (C).

Student responses
Section 4137
(A) : 3 students
(B) : 3 students
(C) : 17 students
(D) : 1 student
(E) : 1 student

Section 4138
(A) : 2 students
(B) : 1 students
(C) : 15 students
(D) : 3 students
(E) : 1 student

Section 4139
(A) : 0 students
(B) : 2 students
(C) : 16 students
(D) : 1 student
(E) : 0 students

However, the August 28, 2007 lunar eclipse will be visible for nearly its entire duration for observers on the west coast of North America. Students (here at Cuesta College in San Luis Obispo, CA) will be looking forward to that event...hopefully.