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Astronomy

Big History 024 – Timekeeping

Today we talked about why keeping track of the seasons would be important to hunter-gatherers (need to know when plants will be ready to harvest and when animals will be migrating) and the different ways people have kept track of time.  We talked about the ‘calendar’ recently discovered in Scotland that is thought to be 10,000 year old!  Holes were found in the ground that marked the position of the moon throughout the year. sunrise_200-097e2f7d56aa26ecca371ecd2a9b2ec5ec5540d2-s400-c85 In Peru, they’v recently discovered the oldest known solar observatory, Chankillo,  in the Americas (photo on the left taken on June Solstice).  13 towers mark the position of the setting sun from the June to December Solstice.  We also talked about sun dials, water clocks, candle clocks  and the ‘cosmic engine’ created by Su Song, a Chinese polymath around 1000AD.  One interesting thing I learned while putting the slideshow together, is the word ‘calendar’, comes from the Romans.  A Roman prient would call out each  month when it was a new month, and the 1st day of the new month was called the Kalends, from calare – meaning to call out.

We watched the following SciShow on the History of Time in class.

And then the kids made a pocket sundial and a star clock, which allows them to tell the time by observing the orientation of the big dipper in the sky. Both of these activities are from the Universe at Your Fingertips DVD.  Here’s a slightly different version of the pocket sun clock activity.  The sun clock is shown below, you hold it so the string is pointing North and the shadow falls on the time.  For daylight savings time you have to add an hour. And the big dipper star clock can be found here.IMG_2435

The activities only took a few minutes to put together so then I had the kids work in groups of 2 or 3 and fill out one of the Big History Project activities where they compare ancient civilizations.  Each group picked one civilization, Rome, Greece, Egypt, etc. and filled out a table, time span, where its located, what crops they grew, animals they domesticated, difficulties they had (dry land, volcanoes, etc) and what they’re famous for. Once they had their table filled out, they swapped information with another group so they would have information on two different civilizations.  This is a nice lead in to the Museum exhibit project they’re going to complete this month.  Again working in groups or by themselves, they’l research one civilization and give a presentation to the class at the end of the month. They can either do as the Big History Project activity recommends and build a mini museum exhibit or do a slideshow stressing why the civilization was ‘Great’.

Big History 010 – How far is it?

Since we had run out of time in the last class, we started today’s class with The Planet: Check Your Understanding worksheet from the Earth’s Place in the Universe Interactive Organizers that I got from TeachersPayTeachers.com.  The students had to use the planet flip organizer that they made last week, and perhaps an app or google, to answer various questions about the planets.  Once they had that finished that we talked about how we made planets to scale a few years ago and that today we were going to make a model for the distances between planets that would be to scale.  pocket solar systemEach student was given a piece of adding machine tape approximately 1 meter long and put a sticker for the sun on one end of the tape (or drew it), and pluto on the other end.  We then folded the tap in half and creased it at the half way point, where they put a sticker for Uranus.  Fold it again and again, putting stickers for planets at the appropriate locations.  I got this activity from The Universe at Your Fingertips DVD and it can also be found here, its called Solar System in Your Pocket.  I was a bit skeptical that the distances worked out so well to be at exactly 1/2, 1/4, 1/8, so I checked the actual distances, and it really does work, the distance from the Sun to Uranus really is close to half the distance from the Sun to Pluto.  Its a nice activity because you really see how far apart the gas giants are and how close, relatively, the small rocky planets are to the Sun (all in the last 1/32nd of tape).  The kids were able to fold up the adding tape and tape one end into their notebooks as seen in the photo.

The last thing we did was another scale model activity from The Universe at Your Fingertips DVD, called Sizing up the Moon, where each group was given a ball of home-made playdoh about 4 inches in diameter and told to break it apart and make an Earth and a moon.  earth playdohIMG_0880Then they placed their Earth and moon on the floor at what they thought was the proper distance.  I have to say, I was very impressed, they all had pretty small moons, not too far off from the correct size and they were just a little short on the distance. earth moon models I think all the scale models we’ve done and talking about how far apart everything is in space has been successfully drilled into their brains.

While the middle school kids were doing these activities I had give the two teens the handout for A Grapefruit Saturn, again from The Universe at Your Fingertips DVD, and told them to make a model of Saturn, using a big apple (I didn’t have a grapefruit).  They had to draw the rings to scale on a transparency, cut it out and then place it on the apple.  As you can see in the photo, they used toothpicks to hold it up.  When they were done they showed it to the younger kids and explained how they made it.  This really shows how big the rings are and how far out they go from Saturn.  Next week we’l be learning about plate tectonics.Apple Saturn

Physics 010 – Gravity & Kepler’s Laws of Planetary Motion

IMG_0835We started class by playing with the SpillNot – a device used to hold a cup of water, which you then swing in a circle over your head.  If you swing it fast enough the water stays in the cup, even when its upside down.  It works really well and no body got wet.  This little demonstration goes along pretty well with circular motion and gravity, which we were covering today.  One of the topics in my slideshow today was about how the astronauts in the space shuttle and space station appear to be weightless, but gravity is still pulling on them… just like the water in the cup.  But the astronauts and the space shuttle are falling towards earth constantly, and constantly missing.  Eek, I just realized I forgot to talk about the vomit comet today, will have to do that next week.

After reviewing some of Chapter 9 (Light and Matter), I gave a slideshow on Copernicus, Galileo, Brahe and Kepler to name a few.  This is a presentation I’ve given many times over the years, the first time being in a Story of the World co-op.  Along with the history, we went over Kepler’s laws of planetary motion and then Newton’s formula for the gravitational force between any two objects. draw ellipse Then I had the students draw ellipses, label the two foci and pick one to be the sun.  The way to draw an ellipse is to put two pushpins through the paper and into cardboard, put a loop of string around the pins and put your pencil inside the string and pull it taunt, forming a triangle.  Now you can pull your pencil around the pins, keeping the string tight and you get an ellipse.  How elliptical or circular your ellipse looks will depend on how close you put the push pins.  Once they had their ellipse, they labeled the perihelion (point on the orbit closest to the sun) and aphelion (point on the orbit farthest from the sun).  Then they measured the semi-major axis, a, the distance between the sun and the aphelion and c, the distance from the center of the ellipse to the sun. From these measurements they calculated the eccentricity of the ellipse, e = c/a.  If c = a, then e = 1 and you have a straight line, but if c = 0, basically the two pins are on top of each other and you have a circle.  They drew one or two more ellipses, either changing the distance between the pins, or the length of string to change the eccentricity.  kepler1_smallIn the slideshow I had shown an image of some of planets orbits and how they really aren’t very elliptical, with the exception of Mercury (and dwarf planet Pluto which isn’t shown).  I found this image on the Windows to the Universe webpage which also has some nice interactives, like this one, where you can change the eccentricity and watch the orbit change.

For Kepler’s 2nd Law, which states that planets sweep out equal areas in equal time, I modified a worksheet I found on the web by Professor van der Veen to make it a bit shorter.  The original worksheet gives the data for Mercury’s position around the sun for a complete orbit and asks the students to plot it. I knew we wouldn’t have time for that, so I made the graph mercury orbitand gave copies to my students.  They had to pick two points on the orbit where Mercury was closest to the sun and two points where it was farthest and find the area of the ‘triangular’ sections swept out between those two points. The time interval was 5 days between each data point.  The worksheet gives clear instructions on how to find the area and the students found, that indeed, the areas were almost the same.

Another fun thing to go along with topic, is the Voyager-Grand-Tour-physics-puzzle-game-appFREE Voyager: Grand Tour app.  Its a game but you have to launch your rockets at the right angle and speed to put your satellite in orbit and collect data to complete the missions and move on.  What’s really nice about this app is that it shows you the path for each trial, so you can figure out what you need to change… and it really hard not to laugh when you keep crashing directly into Jupiter, or time your launch so poorly that it hits the moon on its way to Mars.

Before class, I had asked the kids to watch these videos on youtube.

Big History 009 – Threshold 4 & Planets

We started Unit 4.0 in the Big History Project this week which means we’ve reached Threshold level 4, the formation of our solar system and planets.  I had the kids tape in the Threshold 4 card into their notebooks as they arrived and then we watched Crash Course Astronomy video on Exoplanets because I wanted to make sure they realized that our solar system is not the only one with planets.  

After the video we discussed how our solar system formed and I asked them what they knew about each planet.  Then they did the Big History Project lessoplanet card sortn 4.0, Planet Card Sort, and cut out the pictures of the solar system in various stages of development and tape them into their book in the proper order.

The last activity, making a flip organizer for the planets, took the longest and some kids didn’t quite finish.   I got this idea from the Earth’s Place in the Universe Interactive Organizer, but I was disappointed to see they gave the planet data in miles, and the acceleration due to gravity in ft/s^2!  There was no way I was handing those out, so I redid the activity in metric and changed a few of the facts, gave the mass of the planet in Earth masses so they could tell how much more or less massive the planet was compared to Earth. I also gave them blank cards with just a circle on them so they could color it in to look like one of the planets, put the name of the planet on it and then they had a sheet of planet facts they had to sort through and put the right facts on each card.  I had the facts clumped together so one table had orbit radius on it, so they could actually just put those in numerical order and the one with the shortest distance from the sun must be for Mercury, etc.planet organizer  The other set of facts had orbital period, how long it takes for the planet to go around the sun, so again, they could put the table of facts in order, shortest period would be the Mercury, etc.  I did include Pluto on the organizer because everyone loves Pluto, especially with the latest data from the New Horizon.   I had left the number of moons fact empty so they had to look that up on the web or in the Solar System app by Touchpress.  There was also a spot on the card for them to write at least one unique feature about the planet, for example, Venus is considered Earth’s evil twin, Saturn’s famous for its rings, Jupiter for its spot.  Most kids didn’t get to this in class because we ran out of time.  But when taking pictures for this post I noticed my son put Matt Damon on Mars… lol, we’re big fans of The Martian.mars

Physics 009 – Uniform Circular Motion and Black Holes.

Yesterday was a very busy day. It started with Physics class, a quick lunch, then 2 hours in the lapidary shop, polishing rocks, swim class and then a special lecture by UC Berkeley Professor Alex Filipennko on black holes.  The black hole lecture actually complimented our physics class and big history class quite nicely.

bat toy
Unfortunately the physics lab didn’t go as planned today, at least it took more time than I thought it would so we didn’t quite finish.  The students are going to be graphing their data at home this week and we’l analyze the results next week.  I started the class with a flying bat toy hanging in my tree in the front yard.  It has batteries and if you give it a push it will fly, wings flapping and everything, in a nice little circle.  I asked the kids to use the video physics app to record the motion, either from the side, or from directly below the bat and see what they get.  The black bat with a complicated tree pattern as the background made automatic tracking freak out so the kids had to  mark each point by hand in the video but they got some really nice results.  The graphs below were done by recording the motion of the bat from below and marking the origin as the center of the circle.  As expected they got beautiful sine and cosine curves for the motion and you can easily read the period of the motion, when it starts to repeat,  off the graph as 1.5 seconds.

Uniform circular motion
Uniform circular motion

Once they finished looking at their graphs we went inside and talked about circular motion a bit before starting the next lab.  I asked the class if the bat circling in the tree was accelerating and they said no because the speed is constant, so I reminded them that acceleration is a vector and has direction and so does velocity.  So the speed may be constant, but the direction of the bat is constantly changing so its constantly accelerating towards the center of the circle and we call this centripetal (center seeking) acceleration, which is caused by a centripetal force – the string in this case.   To see how the speed of the object depends on the radius of the circle, we did an experiment where we could exert a constant centripetal force by hanging a know mass (100g) on the bottom a string which passes through a tube (empty bic pen) and is then tied to a rubber stopper (the object that is going to be experiencing uniform circular motion).

circular motion labYou can see a student swinging the rubber stop in the photo, and the brass hanger with 100g at the end of the string. Once the student has the stopper moving uniformly and quickly enough they can let go of the string and just hold the tube.  Then their lab partner measures the time it takes for the stopper to make 10 revolutions.  The lab we did was very similar to this one you can find on the University of Wisconsin website.  The students marked the radius they wanted by placing a piece of tape on the string below the tube. This way they could spin the stopper until they got the tape in the right place and know they had a certain radius.  They were to do this for 4 or 5 different lengths and then make a graph of speed squared versus the radius and find that its a straight line.    I think one of the pen tubes we used was causing too much friction, at one point the string broke and sent the rubber stopper flying! Luckily we were doing this experiment outside and no one got hit.  I’l have to wait and see how their data looks but I’m thinking we may redo this lab with better tubes and fishing line instead of string.  It would also be nice to measure the speed of the motion as a function of the centripetal force by changing the mass hanging off the bottom of the string.

lapidary shopAfter lunch 5 of the kids went with me to the local lapidary shop to grind and polish rocks.  This is a hobby my son and I got into through 4-H.  We joined the local rock club so we could use the shop more often and this summer I became a foreman so we could open the shop during the afternoon for homeschool teens.  Its a great hobby for teens and it involves quite a bit of patience and skill, not to mention they get use to cutting and grinding wheels. tigers eye Here’s a tiger eye cabochon that my son made last year.  We’ve learned how to wire wrap and actually make most of our christmas gifts now.  So if your teen is looking for a cool hobby you might contact your local rock club and see if they have a shop or offer classes.

The last event yesterday was a lecture on black holes at the local theater.  I was very impressed with the talk by with Alex Filippenko, he’s a great speaker and when it came time for Q&A, he made a special effort to take the kids’ questions first.  In his talk, he explained how we know, or at least are confident, that black holes exist.  Since we can’t see them directly, we have to look for stars that seem to be orbiting a large mass that we can’t see – basically you look for circular motion (well elliptical motion) of a star and from measuring the doppler effect (red shift and blue shift when its moving away and toward us) of the spectrum emitted you can determine the period of the motion and  the mass that must be causing it.  So basically he does what the kids did in class with the bat and the stopper – find the period of the motion and from that determine the force causing the motion.  Filippenko has co-written an introductory Astronomy textbook and he’s done a couple of Great Courses, one of which is available through the Great Courses Plus beta which ends unfortunately, this weekend.  I’m still debating whether or not we’l subscribe to that service. Its going to work like Netflix, pay a monthly or yearly fee and have access to a huge number of Great Courses.  Our biggest problem is finding the time to watch them and of course it depend on the price of the subscription.  If you have a kid interested in Astronomy I would definitely give his Great Courses a look for he’s a lively and entertaining professor.

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