20120630

Presentation: eclipses

Cats casting shadows. Is this a 'shoop, or a plausible unretouched photograph? 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").

20120628

Presentation: motions and cycles

Last week we used a starwheel in order to look at the positions of stars and constellations on the celestial sphere on a given date and time. While it may seem that the stars are already against you tonight, there is a lot more going on in the sky besides the positions of stars and constellations.

We'll take a look at four types of motions in the sky, with an emphasis here on distinguishing between them, and their effect on the positions of the stars and the sun. (We'll cover the motions and cycles of the moon in our next presentation.)

First, rotation.

Rotation is the spinning of Earth on its axis, and this cycle takes approximately 24 hours to complete (depending on your frame of reference.)

Rotation causes the stars and constellations (and even the sun!) to appear to move counterclockwise) around the celestial north pole, as seen in this Starry NightTM simulation. (For the purposes of this presentation, we'll assume that we are always observing from San Luis Obispo, CA in the northern hemisphere.)

Second, precession.

Note that rotation is occurring every 24 hours as Earth spins on its axis. However, the direction of the axis wobbles like a top, and takes approximately 26,000 years to complete one cycle. Right now, the north end of Earth's axis points towards the star Polaris, but because of precession, the north end of Earth's axis will point towards other parts of the celestial sphere.

In this Starry NightTM simulation, in 2700 B.C. the celestial north pole pointed towards the star Thuban, such that all stars and constellations moved counterclockwise around this "north star."

Because of precession causing Earth's rotation axis to wobble, currently the star Polaris gets to be the "north star."

Eventually precession will cause the star Er Rai to be the "north star" in 4400 A.D. And yes, there will more typically be no given "north star" when the north end of Earth's axis points towards an empty portion of the night sky.

Third, revolution. Previously you were asked to ponder the origin of the word "zodiac," or at least consider what other words it might be related to.

Like "zoo," or "zoology," the "zodiac" has something to do with animals, and is literally the "arc (or line) of animals." Traditionally there are twelve of them, but not all of them are now animals today.

Revolution is the motion of Earth around the sun, and this cycle takes one year to complete. Note that the zodiac constellations are approximately equally spaced, and if you could see stars and constellations during the day, the sun would directly line up on a different zodiac constellation on certain times of the year, here Virgo.

Approximately one month later, as Earth revolves around the sun, you would then see the sun directly lining up with Libra. One month after that, then Earth revolves further around the sun such that you would see the sun directly lining up with Scorpius.

This is the basis of "sun-sign" astrology, where it matters which zodiac constellation the sun lines up with at different times of the year. Again, this assumes you can see stars and constellations during the day, but this is not a big deal because you have your starwheels to figure out where the sun is on the zodiac right now. (In fact, let's do this now--and see if we notice anything...) In this time-lapse, the sun is seen at noon every day over a month, over which time the zodiac constellations that align with the sun shift over once (from Virgo to Libra).

Fourth, tilt. Which carmakers produce(d) the Solstice and the Equinox? Did any of you list these car models in your "Astronomy in the Marketplace" activity last week?

Pontiac made the Solstice sports coupe, and Chevrolet current makes the Equinox sports utility vehicle. (Did any of you list these astronomy-related cars in the "Marketplace Astronomy" in-class activity last week?) These are special times of year in terms of the position of the sun and the number of daylight hours, as we'll see in the in-class activity today. (The two equinoxes occur twice a year, when there are 12 hours of daylight (and 12 hours of night); the two solstices also occur twice a year, on the longest day of the year, and on the shortest day of the year.)

Note the tilt of Earth's rotation axis, with its north end (always) pointed towards Polaris. When Earth is located on the right side of its orbit around the sun, the northern hemisphere is tilted towards the sun, such that San Luis Obispo, CA in the northern hemisphere will see the sun high in in the sky, and it will be our summer.

So not just tilt, but revolution is needed to cause the seasons to change over the course of a year. Six months later, Earth will be over on the left side of its orbit around the sun, but as the north end of Earth's rotation axis is still pointed towards Polaris, this means that the northern hemisphere is tilted away from the sun, such that San Luis Obispo, CA in the northern hemisphere will see the sun low in in the sky, and it will be our winter.

Here is a very long time exposure of the sun making different paths across the sky over the course of the year, which makes a high arc in the summer, when our hemisphere is angled towards the sun, and makes a low arc in the winter, when our hemisphere is angled away from the sun. We'll cover this interesting set of paths across the sky in more detail in the subsequent in-class activity today.