Thursday, August 10, 2017

A Trap for Theorists

EBIT Cutaway
One of the highlights of the workshop program I conduct at Lawrence Livermore National Laboratory is a tour of the Electron Beam Ion Trap or EBIT. In about an hour Dr. Greg Brown explains how this incredible instrument works and what it is used for. I have toured it many times and learn new things on each tour. Before I started working at LLNL, I had no idea what an EBIT was. Now I know it is essential for many important areas of physics and astrophysics. The physics behind how the EBIT works covers many topics encountered in an introductory physics class. The areas of physics and astrophysics it is used for are highly interesting to introductory physics students. Because of this I decided to write a post about EBIT.

Astrophysics was enabled by the invention of the spectrometer. However, it didn't get started until the connection was made between the odd lines observed in solar and stellar spectra and laboratory measurements of the spectra of elements. Many scientists and engineers contributed to this effort but Gustav Kirchoff and Robert Bunsen get the lion's share of credit for this accomplishment. Together they developed techniques to purify samples and observe and record their characteristic spectra in the uncontaminated flames of, you guessed it, a Bunsen burner. This work allowed them to discover several new elements. More importantly, they realized that their work could be used to decipher the information coming from the universe through a spectroscope connected to a telescope. By comparing the absorption and emission lines of astrophysical objects to the lines measured in the lab, astrophysicists could determine the composition, temperature, magnetic field strength, motion, and much more about astrophysical objects. This led to an explosion of discovery that resulted in humans learning about nucleosynthesis and the Big Bang.
By Poul la Cour & Jacob Appel - Historisk Fysik bind I,
Bare Uranium Spectrum by EBIT
The modern day Kirchoffs and Bunsens are working at EBIT. They use a high energy beam of electrons to create ions and hold them still. They use advanced spectrometers to measure the spectral lines of these ions. You might wonder why this still needs to be done. Haven't we already measured all the spectra of the elements? The answer is no because the spectra are different for different ions and isotopes of the same element. When you consider all the naturally occurring elements and all of their isotopes and possible ionization states, you realize that this is a herculean task. Another reason this wasn't done a long time ago is that it is extremely difficult to create some of these ions. Imagine the energy required to remove the 92nd electron from a uranium atom. That is something they have done at EBIT. These high ionization states are found in high energy environments like the accretion disks of black holes, supernova, fusion experiments, and x-ray lasers. Without EBIT and other similar research facilities, astrophysicists and fusion scientists would not be able to decipher the observations they make with their spectrometers.

We arrived a little early at EBIT for this summer's tour. Luckily, we were met by a high school physics teacher from Hawaii, Tessie Lumabao. She had completed Level 1 and 2 of the Fusion/Astrophysics Teacher Research Academy in 2016. Tessie had first seen EBIT on the TRA tour I took the teachers on last summer. Now she was working there as a summer intern. One of the primary benefits of the TRA program is the opportunity for a paid research internship at LLNL. Tessie introduced herself to this year's TRA participants and excitedly described what she was working on. I asked her to elaborate when she mentioned they were observing the spectrum of "neon-like molybdenum". She explained that it was a molybdenum atom that had 32 electrons removed by EBIT's electron beam. That left it with only 10 electrons, just like a neutral neon atom. At this point Dr. Greg Brown joined our group and started our tour. Tessie's brief talk proved helpful as Dr. Brown scattered things like "helium-like uranium" and "beryllium-like iron" in his remarks throughout the tour. We still saw a lot of Tessie as she dashed back and forth between computer monitors connected to EBIT and the spectrometers as they collected data.

The experiment that was being conducted with Tessie's help has important implications for one of the foundations of modern physics, quantum electrodynamics or QED. This theory explains how light interacts with electrons. In other words, it explains everything you have ever seen or ever will see. QED has been tested extensively and has held up very well. It is considered to be the most accurately tested theory in physics. Theorists can use it to predict what the spectral lines of a given ion will be. These are extremely complex calculations and need to be tested with something like EBIT. In many tests their predictions hold up. In some there is a troubling discrepancy. The question is, who is right, the theorists or the experimentalists?  Observations of the spectra of astrophysical objects suggest it is the experimentalists using EBIT that are correct and the theorists and possibly QED that have a problem. It struck me that we were visiting a research facility that could change a fundamental theory in physics and one of our TRA high school physics teachers was participating in this endeavor.

Dr. Elmar Trabert
I wanted to learn more about this area of research and did some digging after the workshop was over. The LLNL EBIT website is very helpful but hasn't been updated recently. Searching online I found a remarkable paper by Elmar Trabert in the journal Atoms called "Critical Assessment of Theoretical Calculations of Atomic Structure and Transition Probabilities: An Experimenter's View" (note: clicking on preceding link will download the pdf file of this paper). This 70 page paper was outside the realm of my usual summer reading but the introduction drew me in and I spent a lot of time reading the parts that I found interesting (and understandable!). I will share a few items here. The page numbers I refer to are the ones in the document that start on page 15. Here is what caught my attention in the introduction on page 15:

"Burkhard Fricke... recalls advice by his elders as “If theory and experiment disagree, work on theory; if they agree, improve the experiment.” Alan Hibbert... would add a caution to this: “If theory and experiment agree, be aware of the possibility that both may be wrong.” Right. Moreover, if one considers another research practicality, one may suffer the consequences “if theory and experiment agree, you won’t get funding for further research.” "

This didn't sound like a typical research paper. It had a sense of history and humor. He continues in the introduction on page 16 to say this:

"My aim in this article is a discussion of what I as an experimenter expect from theory and atomic structure computations, how I try to assess the quality of the work, and what experiment can or can’t do towards a critical validation process of such calculations. After all, experiments and experimenters come in many flavors, too. We all make mistakes, and maybe we can learn from some of them that we have recognized by now. In this context, I dare to digress into anecdotal evidence of various problems in the interplay of atomic structure theory and experiment, into human and technical sources of error and uncertainty, before turning to a more systematic discussion of data on specific isoelectronic sequences. However, I will not present an ultimate recipe for assessing calculated data. Both, experiment and theory, are intertwined in the scientific process, and progress may involve a new perspective, if not always a paradigm shift."

I found a mention of the QED vs experiment issue on page 52:

Dr. Greg Brown
"Since neither experimenters nor theoreticians found fault with their own ways and results, the systematic discrepancy between experimental and computed line ratios was blamed on a variety of collisional-radiative processes in distant light sources that one could diagnose by the size of the discrepancy. However, Greg Brown’s extensive and tedious systematic studies at the LLNL EBIT [183,184] agreed with the stars, but could not suffer from the same collisional effects."

I chuckled at the adjective "tedious" but I am sure the importance of the measurements masked any sense of drudgery that may have been present during this work at EBIT. Following this mention of Dr. Brown's work, Trabert begins another digression that is uncharacteristic of the typical academic journal article. I found it fascinating. It starts on page 53 and I will leave it to you to download this paper and read this short passage on your own. If you are like me, you will get drawn in and spend more time scrolling through the rest of the paper for more of Trabert's wit and wisdom.

Another critical task of EBIT is to help test and design advanced spectrometers. On the tour we got to see and hear about what Dr. Brown described as the most accurate and sensitive spectrometer in the world. It is of a type I had not heard of before touring EBIT, a calorimeter spectrometer. Instead of using a grating and charge-coupled device (CCD) to measure the energy of incoming photons, it uses the temperature change photons cause when they strike the detector. EBIT helped design the x-ray calorimeter spectrometer that flew on the ill-fated Suzaku and Hitomi x-ray observatories. Suzaku lost its cryogenic coolant in just a few weeks due to a malfunction. This Japanese Hitomi spacecraft went into an uncontrolled tumble soon after it went operational and spun so fast the solar panels broke apart. Prior to that, it demonstrated the remarkable capability of the calorimeter spectrometer for x-ray astronomy. This will undoubtedly result in a follow up mission in the future.

What an incredible opportunity Tessie and other teachers have to work in a facility that is helping to design and test instruments that will fly in space and discover new wonders about the universe. If you share this opinion, I urge you to sign up for the Fusion/Astrophysics Teacher Research Academy next summer at LLNL in Livermore, CA. It is tentatively set for the last two weeks in June. We typically have teachers from the Bay Area and Central Valley attend, but some teachers have traveled from as far as Thailand and Hawaii for this opportunity. I will post on The Blog of Phyz and on my Twitter account, @kilroi22, when registration opens. If you are wondering what it would look like to see an object that is so energetic it has atoms like iron missing most of their electrons you are in luck. Make your way to the eclipse path on August 21st. The solar corona is like a giant EBIT!

Tuesday, August 08, 2017

How a VCR works—I had no idea

I know. I didn't think I needed to know, either. But I was wrong.

We've all used them, and we likely have a rough idea of what's going on in there. But I didn't know most of this, and confess embarrassment at the depth of my VHS ignorance.

The challenge, history, and inner workings are surprisingly enthralling. It's 18 minutes that you won't regret. Trust me on this.

The Impossible Feat inside Your VCR

These days a VCR seems more quaint than anything else. But in fact, there's a device inside every VCR that solved what seemed to be an unsolvable problem. The video head drum is a fascinating solution to the complex problem of tape speed, and therefore I think it deserves to be remembered. Plus, the video head drum shares some motor technologies with modern hard disk drives. So there's that.

Monday, August 07, 2017

Where we are with California NGSS state tests

This post is really just a link share. I don't know about you, but I often get so wrapped up in teaching physics that I lose track of where the state is in terms of mandated statewide assessments.

Gone is STAR and its CSTs. In development is the California Science Test (CAST), part of the California Assessment of Student Performance and Progress (CAASPP) [No program is legit unless it has a handy acronym or initialism.] So CAST is the product of CAASPP Science.

The skinny: The CAST is for eligible students in grades five, eight, and once in high school. It's currently in development with pilot tests being run. The test will become operational in the 2018-19 academic year.

Here's the California Department of Education's CAST web page.

Here's a link to released practice test items. When I looked, I was disappointed. Perhaps material for a post to follow. Take a look for yourself at Online Practice and Training Tests Portal.

And here's the promotional video offered on that page.

Which cup fills up first?

Depending on your social media foot print you may have seen a lot of this image over the last few days:

If you didn't achieve "genius" status the first (or second) time you looked at it, that's ok. It is a bit tricky and I don't think solving this has a direct correlation to your IQ. To be honest, I missed the closed pipes the first time I looked at it too. Some people got really upset with others over this thing. Massive debates were sparked and it was determined "Well, we just can't be sure, without doing it." Hands on experimentation to the rescue!

A facebook page called Prozix made a 3D print of this now infamous brain teaser and did the actual experiment. Now you can confidently tell your students the answer when they try to stump you. Video of the experiment is available here (or here) but make sure you try to make your own guess before viewing.

Sunday, August 06, 2017

Vector Birthdays

I will be teaching AP Physics C, Mechanics and Electricity & Magnetism, for the first time this year. Although I have been graciously gifted curriculum from several veteran teachers, you have to make your curriculum your own. So I'm tweaking what I have, splicing and dicing things together into what I think will make a good learning experience for my students. (Of course you know I'll have to rewrite it next year once I actually know what I'm doing).

One of the things I wanted my students to practice was drawing vectors. I found a lot of worksheets online but wanted something a little more open. Adding to my complicated needs is the upcoming eclipse and my plans to travel to Oregon like Dan. I need four days of sub plans right at the beginning of the year. At last I had an epiphany:

The instructions should be simple enough for students to do without help from the sub. Each student (unless they are twins) will have an entirely different vector drawing. Its a quick and easy way to practice. Below is the example I'll have the sub project:
I can ask students who has the smallest resultant, the largest or perhaps the one closest to 90 degrees. This use of child-specific numbers make me think how else I can use them. Perhaps problems with blanks students fill in with numbers only the know: their birthdays, their address, their phone number. Everyone gets to practice the same process but each problem would be unique.

Thursday, July 20, 2017

Left Behind: Sunball Selfies

Many science instructors will be making the pilgrimage to totality for the August 21 solar eclipse. This will leave science students without their instructors during the event.

In a previous post, we offered a lesson plan to fill one day of a science teacher's absence. That was a question set to accompany showing The Universe: Total Eclipse.

What can students do on Monday, August 21 at the time of the eclipse (peaking at 10:17am in Sacramento)? It's a school day and students will likely be in class during most of the eclipse.

Perhaps your school has stocked up on eclipse viewers/glasses and will distribute them for students Monday morning. Then again, maybe not. It's important to convey the danger of looking at the sun during an eclipse.

Conceptual Physics author, Paul Hewitt, reminds us that one of the best ways to enjoy an eclipse is to look down! The dappled light seen beneath trees is composed of pinhole images of the sun: "sunballs". When the eclipse is on, these sunballs turn into "suncrescents," pinhole images of the eclipsed sun. It is quite a spectacle. Some might even call it "amazeballs".

Here's an extra credit project that I will be assigning to my students for the eclipse:

Sunball Selfies (pdf) - Sunball Selfies (docx)

You will need to modify the document to use it at your school. And you don't need to be absent to use it. I used a similar assignment for the May, 2012 annular eclipse and students had great fun with it.

Sunball Selfies student album from May, 2012
2012 05 20 Eclipse Students

My own Sunball Selfies album from May, 2012
2012 05 20 Eclipse DB

Tuesday, July 18, 2017

I have stopped recommending the AP1 and AP2 exams—when will you?

I expressed my misgivings about the AP Physics 1 and AP Physics 2 exams in a previous post. Once again, this is a rant. If you're not into rants, kindly scroll to the next post.

The purpose of this post is simply to show the global exam score distributions, three years into the redesign. I used the numbers posted by Total Registration.

First up: the final three years of the AP Physics B exam scores. Fives were earned by 15% of students, fours were earned by 19%, threes by 27%, twos by 17%, and ones by 22%. (Approximately one third succeeded with 4s and 5s.)

Next up: the first years of the AP Physics 1 exam. Fives were earned by 4% of students, fours were earned by 14%, threes by 21%, twos by 30%, and ones by 31%. (Approximately one fifth of candidates succeeded.)

Try not to judge too harshly those who have lost enthusiasm for algebra-based AP Physics.

To complete the Algebra-based exams, consider the AP Physics 2 exam. Fives were earned by 9% of students, fours were earned by 15%, threes by 34%, twos by 32%, and ones by 9%.

The College Board hastens to offer explanations for the precipitous plummet, especially in regard to AP1. The AP2 scores actually match the legacy APB results reasonably well. There's reason to speculate that many (if not most) AP2 examinees are successful veterans of the AP1 exam.

1. Teachers aren't teaching the new course correctly. They haven't undergone enough in-service retraining. (This is part of the admission that the AP1 and AP2 exams aren't really about physics anymore, but about Big Ideas. Physics topics are merely the canvas upon which the Big Ideas are painted.)

2. Too many students are taking the AP1 exam. This is where I'm trying to help. And you can, too. I assure my students that I will do my best to prepare them for the exams as they are. But I encourage them to not submit to the exam. This reverses a position I held for 25 years. If we work together, we can help to reduce the glut of AP1 examinees.

The Physics team at the College Board has replaced the muscular Camaro that was AP Physics B with shiny Lamborghinis called AP Physics 1 and AP Physics 2. Emulating the college course that examinees could test out of is now a long-abandoned goal. The new courses hope to be the richest, most immersive physics courses that could be imagined.

I don't find the course syllabi to be particularly practical for high school students. The courses go well beyond virtually all introductory college and university courses. The AP Physics B exam was more demanding than those administered at the college level. And the AP1 and AP2 exams amplify that disparity to 11.

Surely this escalation of difficulty of the AP1 and AP2 exams must have resulted in a global advancement of the way colleges and universities accept and interpret passing scores. If it has, I am unaware of it. In the old days of AP Physics B, college acceptance of passing scores was a crazy patchwork. To the best of my knowledge, the crazy patchwork remains unchanged even with the more stringent AP1 and AP2 exams.

You might wonder why AP Physics C instructors remain enthusiastic about their course and exam. They were unaffected by the AP Physics B redesign. Let's bring them into the mix.

While the AP1 and AP2 scores plummeted, the un-redesigned AP C Mechanics exam scores continued to flourish. Over the past three years, fives were earned by 31% of students, fours were earned by 28%, threes by 18%, twos by 13%, and ones by 10%. (Nearly 60% success rate.)

The score of 5 (highly qualified) now represents the mode.

The un-redesigned AP C Electricity and Magnetism exam scores did well, too. Over the past three years, fives were earned by 30% of students, fours were earned by 24%, threes by 14%, twos by 19%, and ones by 13%. (More than half succeed.)

The score of 5 (highly qualified) continues to represent the mode.

AP Physics C continues to be a physics content-based exam. That is, its blueprint refers to topics in physics instead of the more philosophical Big Ideas that form the core objectives of AP1 and AP2. This may well be a key to the continuing success enjoyed AP C examinees.

Of course there's more to this discussion. I'm obviously ventilating a bit here because this continues to be a source of frustration. AP Physics C is not a viable option at my school due primarily to its prerequisites. Intra- and inter- district competition for school enrollment prevents me from simply abandoning AP1 and AP2. If not for that, I would.

With haste.

Monday, July 17, 2017

Left Behind: The Universe - Total Eclipse

For instructors making the pilgrimage to totality for the August 21 eclipse, there is a chance school will be in session and lesson plans will need to be prepared for guest teachers (aka, substitutes).

One day could be covered by the showing of the "Total Eclipse" episode of the A&E documentary series, The Universe.

The program, episode 7 of season 5 (The Universe S5E7), originally aired September 16, 2010. It can be found online. I leave it readers to source the best version and to make it available for a classroom showing.

As I do for virtually all video programs I ever show in class, I have prepared a question set to accompany the episode. A sequence of questions that can be answered while the video plays tends to keep students appropriately focused. It is available, free of charge, at my nascent Teachers Pay Teachers website: The Resources of Phyz. I have found useful resources there and plan to add more to my own corner in the future. For now, the "Total Eclipse" question set is all that I have posted.

The Universe - Total Eclipse video questions

We will add to our "Left Behind" series with other activities for students not traveling to totality in future posts.

Thursday, July 13, 2017

Reining in the kilogram once and for all

It seems we're been here before. More than once. The kilogram: a pesky unit of measure that hasn't been behaving itself.

Derek Muller goes deep into the explanation of the newly redefined kilogram. The task is not trivial. I may need to rewatch this robust explainer a couple of times before it all sinks in. Have a seat and prepare to get your geek on: there will be mathematics!

How We're Redefining the kg

Sunday, July 09, 2017

Total Eclipse of the Sun - Part 3

As I mentioned in Part 2 of this four part series, I will be observing the August 21st total eclipse of the Sun from Madras, Oregon. My main reason for this choice is the prediction for low cloud cover as you can see on this chart from Eclipsophile:
The chart shows there are many other good choices, but Madras is one of the shortest drives for me, nine hours. Another reason I am going to Madras is I was asked to help with the Lowell Observatory Solar Eclipse Experience 2017. This event at Madras High School will include daytime activities and talks by astronomers on Sunday and Monday and night time star gazing on Sunday. If you are planning on being in the Madras area, I encourage you to purchase tickets for this event. In addition to the program of events, your $15 will get you a pair of eclipse glasses, access to a water station, food and beverage vendors, and the air-conditioned auditorium. The eclipse itself will be narrated by solar astronomers. In this post I will describe my contributions to the Lowell event and ideas for you to enhance your total and partial eclipse experience.

Rich Krueger contacted me last year about contributing to the Lowell Observatory Eclipse Event. Rich teaches at Flagstaff Arts and Leadership Academy and is a fellow SOFIA Airborne Astronomy Ambassador. Rich flew on SOFIA with Lowell Observatory Curator, Samantha Thompson. Lowell tapped Rich to help fill out the outstanding program they are preparing for the day before and day of the eclipse. Rich originally wanted me to recreate Eddington's total eclipse observation that confirmed Einstein's Theory of General Relativity. I had read Donald Bruns' article in Sky and Telescope and knew this was beyond my skill and bank account. Even Mr. Bruns is only doing half of the experiment. He plans to use undeflected star positions from the Gaia catalog. Eddington did not have this luxury! I assured Rich I would bring something else that would interest and engage the crowd. Others may try to measure the star field around the eclipsed Sun at the Eclipse Experience, but the deflections are so tiny nothing will be evident that day.

My first thought was to bring the Spacetime Simulator that I use in the viral Gravity Visualized video. I would do demonstrations like the ones in the video plus the new ones I have added. Another idea is to make a bent spacetime game by distributing a bunch of 0.5, 1, and 2 kg masses on the fabric. Place a goal on one side and launch a marble from the other. The marble represents a photon following the geodesic of the warped spacetime. The winner reaches the goal in the longest amount of time. Doing these activities should keep me pretty busy, but I wanted to develop something new too. I also suspect that I won't be the only one bringing a spacetime simulator since so many people have built their own with the help of this DIY video.

My next idea was to make a large convex mirror using a spaceblanket and a kiddie pool. I briefly alluded to this at the end of my post about making concave and convex mirrors with trashcan lids and spaceblankets. I was inspired to create a large convex mirror by the fisheye lens pictures taken in previous total eclipses. They show a 360 degree sunset with a brilliant but small eclipsed sun floating in the twilight. I also remembered the picture of the 1991 total eclipse over Mauna Kea. You can see the eclipsed sun and sky in the reflection of the silvered observatory dome on the left. I wondered what you could see in a large convex mirror placed flat on the ground. Maybe it would give a similar view to the unaided eye in real time.
From Sky and Telescope 35mm slide set
Wolfgang Strickling, 3/9/2016 Eclipse

I picked up a 45" wading pool from Big 5 Sporting Goods.  I cut the presta valve from a bicycle tube, put it through a hole in the side of the pool, and sealed it with silicone adhesive. I used the top of the pool to mark a circle on the spaceblanket and cut it out.  I used strapping tape to attach the spaceblanket over the top and started inflating it. It worked great, here is the result:
45" Spaceblanket/Kiddie Pool convex mirror
Outside the mirror showed the horizon and the whole sky. I should have left well enough alone but I was curious about how much more I could inflate it. I attached my portable air pump and added more air. The mirror stretched a little bit more but then reached its limit. Unfortunately I did not notice this until the side of the pool buckled. I would need to take it apart and reinforce it. Instead I decided to start from scratch with a bigger pool. I found a 59" kiddie pool at Toys backward R Us. Since spaceblankets are 52" wide, I obtained an oversized one that is 71" wide. To help prevent the pool from buckling I got some lawn edging to attach around the outside of the pool. I drilled holes and used bolts to secure it, then sealed each bolt with silicone adhesive. To prevent buckling, I used a

string to measure the mirror while inflating it. When the string showed the spaceblanket was not stretching anymore, I stopped inflating it. The larger mirror gave a satisfying view inside my classroom and outside in the full sun. I can't wait to see what it looks like during a total eclipse of the Sun. If I had to do it again I would stick to the smaller version. The large mirror barely fits into my Honda Odyssey and the oversize spaceblanket is not as reflective as the regular size. I may even make another smaller version to take instead of the large one since I will need room to transport a lot of equipment and gear to Madras.

I have been observing the Sun, solar eclipses, and transits with students for many years. As a result I have a good collection of solar observation equipment. The pièce de résistance is a Coronado Solarmax 70mm H-Alpha telescope (discontinued). I purchased this many years ago with a grant from the Home and School Club, Los Gatos High's version of the PTA. I made sure to come to one of their meetings after getting the telescope to show them the prominences, filaments, plages, and supergranules that students would get to see. If you want to get something similar for your school, the Coronado Personal Solar Telescope is very capable and more affordable at $599. I will bring a Meade 10" SCT with solar filter, 20x80 binoculars with solar filters, a box of eclipse glasses, and an 80mm refractor with a Sun Funnel. What is a Sun Funnel? That is what I thought when I first came across this solar observing aid. It is a safer way to do eyepiece projection, pointing an unfiltered refractor at the Sun and aiming the eyepiece toward a screen. Once focused, a nice large image of the Sun shows up on the screen. I have done this many times but always worried someone would look in the eyepiece and injure themselves. The Sun Funnel is placed over the eyepiece and projects the image on the back of a screen fastened to the other end. I followed the very clear instructions and made one in about 15 minutes after assembling the materials. If you have access to a small refractor, I highly recommend you build one. Small refractors can be purchased for less than $100 and work well with the Sun Funnel as long as you use an eyepiece and star diagonal that are made of metal, no plastic.
The partial phases of the eclipse can be observed without any additional tools. You can simply overlap the fingers of each hand, making a waffle shape, and project pinhole images of the Sun on the ground or other convenient surface. The images can be improved by making pinholes in a screen and holding it above a flat, light colored surface. The further the pinhole is from the surface, the larger but dimmer the image will be. If the screen is in a shaded area, the images can be seen easier.  Some people even create a design with the pinholes, creating an image made up of crescent Suns. There are many creative plans for pinhole projectors that are described elsewhere. You can even measure the diameter of the Sun with a pinhole projector. The ratio of the diameter of the image to the distance between the pinhole and the image is the same as the ratio of the diameter of the Sun to the distance to the Sun. If there are trees, bushes, or other objects with small openings, you may notice small crescent images scattered across the ground, walls, etc. If you have access to your eclipse site ahead of time, scout around at the same time as the eclipse to locate the best examples. I won't have that opportunity so I am stacking the deck. I am creating a pinhole pattern out of a prototyping or perf board. These are used for electronics projects and have a grid of tiny, closely spaced holes. I use them for an inverse square activity in workshops (click to download). I ordered a large one (18x12.8 cm) that I will create a pattern on by masking selected holes. I am undecided what that pattern will be, if you have an idea, leave a comment. In Part 2 of this series I advised against using precious total eclipse observing time to take pictures. However, you will have plenty of time during the partial phases to photograph the crescent pinhole images, natural or contrived. That's what I did for the June 10, 2002, partial eclipse at my home in California. Sky and Telescope published my photo in the December issue of that year.

[Dean adds: for the May, 2012 eclipse, I made "eclipse portraiture" an extra credit project. My own "before and during" has been published in a few places.] 

Another way to reveal the partial phases of the eclipse is to reflect a solar image from a mirror. I use this to bring the Sun into the classroom. I use a front-surfaced mirror to get a crisper image. I mask off the surface of the mirror except for a hole made with a paper punch. This produces a brighter image than a pinhole so you can project it over a longer distance to create a larger image. You still need to project the image into a shaded location to make it easily visible. I am going to place a sheet of white paper in a box for the eclipse.
Solar Spectrum using Red Tide spectrometer with Logger Pro

Projecting a solar image is useful even if there isn't an eclipse going on. Imagine a solar image projected through your classroom door onto your whiteboard for the whole class to see. Large sunspots should be visible. After a little time, students will notice that the solar image moves. Propose the idea of determining how much time it would take the solar image to move all the way around, back to where it started. Draw a circle around the image and time how long it takes to move completely out of it.  How many images would it take to complete one circle? The Sun's apparent size is about 0.5 degrees so 360/0.5 = 720. Multiply the time measured by 720, this should give about 24 hours. How about that! You can improve your answer by calculating the exact apparent angular size of the Sun for the current distance to the Sun on the day you do this activity. I also use a spectrometer to display the solar spectrum using the projected solar image as the light source.

As you can see, I will be very busy for the total eclipse on August 21st. Even if you can't make it under the path of totality, I hope you will be busy sharing this event with your students, family, and friends. I have used this post to give you some ideas to enhance the experience. I urge you to use them in your classroom beyond the Great American Eclipse day. The concluding post of this series will describe my experience in Madras. It will be interesting to contrast it to my first total eclipse experience that I described in Part 1 of this series. I also will be posting live on Twitter @kilroi22.