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.


Bill & Emily said...


"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. " E = m c^2 --- might be too complex. Another thought is a design of an eclipse. Face of Einstein? "Go Giants?" Probably all of these are too complex.

Marc said...

Thanks Dan! Lots of great suggestions.