This page has been translated into French by Kate Bondareva (August 2014).
There are a whole host of astronomical ideas and concepts that can be demonstrated with these simple materials and a little ingenuity on your part. Be careful to tailor the discussion to the age group of your audience. For the youngest groups, keep it simple. Just introducing them to some of the terminology and general concepts (the earth is round, the moon travels around the earth in a big circle called an "orbit," etc.) may be enough. But for older elementary, even concepts such as eclipses and the cause of seasons can be demonstrated.
For those parts of the demonstrations that require the "sun", have a slide projector set up off to one side and an adult volunteer who can turn the projector on and turn the room lights off. (Note: slide projector bulbs are BRIGHT: do not let students look into the slide projector!) Depending on your specific set-up, it may be helpful for the adult supervisor to move the beam of the slide projector to follow you during the demonstration (if you need to move around).
By no means do all of the demonstrations have to be done to be effective, and certainly not all in one sitting. Pick and choose for your particular audience and age group! Herewith some ideas:
Not so simple, when you think about it! Of course, to the naked eye, the earth looks pretty flat. Oh, sure there are hills and mountains, but we don't get the "overall" picture with our eyes.
For the youngest groups, you can mention Magellan's voyage, and demonstrate how, in continuing to go in one direction (west!) on the globe, you eventually come back to where you started. In actuality, one doesn't have to sail all the way around: from a seaport with a good view of the horizon, sharp eyed land-lubbers could see that the tall mast of a sailing ship could still be seen even after the"ship" itself was below the horizon! (You could draw this on a blackboard.)
For middle and upper elementary groups (who have better powers of visualization), you can use a broom stick or similar straight stick to demonstrate the way it was done by Erastothenes, a Greek mathematician, in about 200 B.C. The idea is simply that, on a given day and time, a vertical stick of the same length will cast a different length shadow at different latitudes (distances from the equator) because the surface of the earth is curved. How you wish to present this is up to you. [I've done this, for instance, having Erastothenes being a Greek janitor in Athens who notices the shadow of his broom handle, and gets to wondering. He writes to his janitor/friend who lives in Alexandria, Egypt (almost due south) and has him measure the length of his broomstick's shadow on a predetermined day and time (no cellular phones in those days!), and then they compare notes.) In actuality, Erastothenes not only convinced himself that the world was round, but measured its diameter, to an accuracy of about 1 percent!]
If you are in a hurry, you can always appeal to the space program! Since we've put rockets into orbit and gone to the moon and back, we've had plenty of opportunities to "see" directly that the earth is round. You could have a poster or other picture of the earth from space posted in the classroom for this part. Even one that only shows part of the earth (but shows the curvature) is sufficient to make the point.
What causes day and night? This is a simple concept, but one which the youngest age groups need reinforced. With the slide projector shining on the "earth", show how the sun shines on one side of the earth, but it's dark on the other side. Then spin the beachball (with your fingers on the north and south poles, or hang the globe from a string or piece of fishing line). The direction should be counterclockwise when viewed from above. An "ant" on the surface of the beachball will go from day into night and back into day as the ball spins. For the older children, have them think about how the sun comes up on one side of the sky (east), travels up and across the sky, and sets on the other side (west). Then have them visualize themselves as the ant on the globe: what do they see? The fact that we have day and night on the earth shows that the earth rotates on its axis. (This is the main point to get across.)
If you want, you can get into how big the earth is (about 8000 miles in diameter, or about 24,000 miles in diameter) and how long it takes to spin once (24 hours). Hence, someone standing on the earth's equator is actually traveling about 1000 miles per hour to the east!
3) Demonstrate the cause of the seasons
This one is a little trickier, and should probably be reserved for the older age groups. They need to have experienced the recurring seasons a few times, and perhaps noticed (although they may not have realized it) that the sun is much closer to being straight up overhead at noontime in the summer than it is during the winter. (If they haven't, you will tell them about it, and maybe it will sink in!)
The idea here is to demonstrate with the beachball and the light that not only does the earth rotate on its axis, but the axis is tipped with respect to the direction toward the sun. (By 23.5 degrees, but the amount isn't the important thing to communicate!) Then you have to try and get them to visualize being the "ant" again: when the north pole is tipped toward the sun, an ant in the northern hemisphere sees the sun going almost overhead every time the globe spins once. It's summertime! Then with the north pole tipped away from the light, see how the sun still rises and sets every day, but doesn't come so close to being overhead. (It's winter!)
To reinforce this, if you have a flashlight, shine it straight down, and you see a nice circle of light. Then shine it down at an angle: the circle turns into an oval or egg-shape. The same amount of light is spread over a much bigger area! (It may be easier for everyone to see if you shine the light at a blackboard instead of the floor. You could even have a volunteer outline the light-circle with chalk and compare the sizes in the two cases.) Then compare the flashlight to sunlight. When the sun is shining almost straight down it is more effective at heating that portion of the earth, which makes in warm in the summer. Likewise, less effective heating (lower sun angle) causes winter. With the older groups, it might be fun to get them to try and visualize what "ants" on other parts of the globe are seeing. When the ant in North America is having summer (tipped toward the sun), what is the ant in South America experiencing? (Winter) Now reverse it. Now, how about an ant at the north pole? (Six months constantly in the dark, followed by six months constantly in the light!) How about an ant on the equator? (Why is it always so warm near the equator?)
The particulars of this demonstration are discussed in a separate write-up on "Size Scales in Astronomy." The idea is to scale the earth down to the size of your beachball globe, and look at other distances and sizes on this scale. For a 16-inch earth, other relevant numbers are:
Moon diameter: 4.4 inches (a softball, or balloon inflated to this size) Moon distance: 40 feet! Space shuttle orbit: only 0.4 inches above the beachball!
See the details in the separate write-up "Size Scales in Astronomy" about ways of presenting this in a meaningful way.
(Since the separation of the "earth" and "moon" are so large, even on the scale of the above demonstration, you may wish to use two smaller balls for the earth and moon. Otherwise, just explain that you have to "cheat" on the actual separation for this part, and go ahead and use the same props.)
This demonstration can be done in a couple of different ways. One is with separate earth and moon balls, with the audience "imagining" they are on the earth ball's surface. The other way, which works with small enough groups, is to have the students sit in a cluster on the floor and have them collectively be the "earth." Then you can walk around them in a big circle as you orbit.
First establish the idea that the earth and the moon are not moving through space independently, but rather that they are affected by gravity. From our perspective on the earth, the moon appears to move around us in a big circle called an orbit. (To an independent outside observer, both the earth and the moon would appear to be orbiting around an invisible point called the "center of mass," but you don't have to get into this!) The moon orbits the earth once every 27.3 days (not quite a month).
Now you can get into the phases of the moon. (This is where it may be helpful to have an adult turn the slide projector to follow you as you move!) Start out in the front of the room with the slide projector to your left (audience's right). To the audience, the "moon" will be lit on the right half and dark on the left. This is like a first quarter moon. Note for the youngest groups that the "dark" part of the moon is still there--it's just in the shadow. Now move to your right (audience's left) until the audience is between you and the slide projector. The face of the moon toward the audience will be fully illuminated, looking like full moon. (Again, note that there is still a "dark" side, but it is pointing away from the earth.) You get the idea...continue around the "orbit" to third quarter, and then to "new moon" (where you are between the audience and the slide projector--you may want to kneel down for best effect).
Depending on your goals, you can also demonstrate the ideas of "cresent" and "gibbous" moon phases (i.e. the regions between new and quarter moons, or between quarter and full, respectively). You can also discuss the terms "waxing" and "waning" (the moon appearing more or less illuminated as viewed from earth, respectively). You also might want to highlight the fact that the "quarter" moon phase does NOT refer to the fact that the moon is one-quarter illuminated, but rather that it is one-quarter of the way around in its orbit! The moon is always half illuminated and half dark--it's just the portion we see that is changing.
One last thing that may be of interest is to ask the students whether the moon rotates on its axis (since we see the same "face" of the moon all the time). Someone will probably say "no." Take your moon ball (let's say it's a balloon at this point) and mark a large "X" on one side, pointing this side toward the students. Now walk in your "orbit" again (slide projector not needed), keeping the X pointed toward the students. Pause occasionally and ask them where the line from the X to the "earth" is pointing in the room. Hopefully as you walk around they will see that the moon must rotate once every time it orbits the earth once, in order to keep the same side facing the earth!
This one takes a little preparation to get the distances and sizes right (i.e. to get the shadows in the right places), and again may work better with the older elementary age groups. You may also want to have a few pictures of solar and lunar eclipses to show the audience. One other prop that may be helpful would be two pieces of flat, stiff cardboard (no smaller than about 8x10 inches, but bigger is better) with a slit cut halfway through each (so they can be slid together into an "X" pattern).
You may want to take about what an eclipse is a little bit first. In general terms, an eclipse is caused when the shadow from one object blocks out the light from another object. With the side projector shining from one side, set up the earth ball (or have a volunteer hold it up) so that it casts a long shadow cone. Now take your moon ball in orbit near the full moon phase and show how, if the alignment is just right, the moon can enter into (and pass through) the earth's shadow. Demonstrate this by passing the moon a little above or below the shadow as well, showing that it has to be lined up. Since the earth is larger than the moon, it casts a fairly big shadow cone, and the entire moon can pass through the shadow. (This is where your practice ahead of time will help you to get the distances right!)
Now demonstrate a solar eclipse, which occurs near new moon (when the moon is between the earth and the sun). Depending on the size of your room, you may want to move the earth ball to the far side of the room from the slide projector and/or use a moon ball that is a little smaller so that it's shadow won't be quite so big. In any event, you should be able to find a spot in the moon's orbit where (again, if the alignment is just right) the moon's shadow will far on the earth. But the moon is small, so only a portion of the earth will be in the shadow, and only those people in the shadow will see an eclipse of the sun. As the moon moves in its orbit, the shadow tracks across the earth from west to east. Hence, for an observer on earth, the moon's shadow passes by in a matter of a few minutes of real time.
You can follow this up with some discussion questions. When can you see a lunar eclipse? (Only at night, or at least after sunset.) How about a solar eclipse? (Only on the day side of earth.) Is it easier to see a lunar eclipse or a solar eclipse? (Lunar; basically anyone on the dark side of the earth can see a given lunar eclipse, so long as the sky is clear. For a solar eclipse you have to be in the right place, at the right time!) Finally, why don't we have eclipses at every full moon and new moon?
To answer this last question, you need to communicate the idea that an "orbit" defines a geometric plane. Hold up a piece of flat cardboard and have the students visualize that the earth is in the center. If you draw a circle on the cardboard, this can represent the moon's orbit around the earth. The moon stays in this plane as it goes around, never deviating up or down.
Now state that the same thing is true for the earth in its orbit around the sun: the earth's orbit also defines a plane. However, the moon's orbital plane and the earth's are tipped relative to one another (by about 5 degrees--not very much!). Insert the other piece of cardboard so that the two pieces make a wide, flat "X". Now one cardboard represents the earth-sun plane (i.e. where the light is coming from) and the other cardboard represents the earth-moon plane. Most of the time, full moon and new moon occur above of below the earth-sun plane (also called the "ecliptic" plane), and so the shadow cones do not intersect anything--no eclipse. Only when the new or full moon happens near where the moon's orbit intersects the earth-sun plane (the "intersection" of the "X"; also called the "line of nodes") can eclipses occur. This line points toward the sun roughly every six months, so if the new moon or full moon happen at just the right time, we can see an eclipse.
One further note of interest on solar eclipses. There is an amazing coincidence at work to make solar eclipses so impressive, and that is that the angular sizes of the moon and the sun are so nearly the same! The sun is much larger than the moon, but the moon is much closer to the earth, so they appear very nearly the same size in the sky. (This is not usually obvious because the sun is too bright to observe directly most of the time.) If the moon was smaller or farther away, it would pass in front of the sun but would not block out the entire bright disk. If the moon was bigger or closer to the earth, solar eclipses would happen much more often and cover a larger portion of the daylit side of the earth. As it is, the moon's shadow just makes it to the earth (if the alignment is right) to give us a glimpse of the magnificent sight!
I know much of the above seems very obvious, but for youngsters I have found it is hard to get too basic. If they already know something, you are reinforcing it. But more often than not, I have found these basic demonstrations to help elementary age students to use their visualization skills and "think big" in a way that helps them appreciate the things they see happening around them. I've even had many adults and parents comment to me how they had "forgotten" about some of these concepts! In any event, good luck in applying these demonstrations, and I hope they are helpful to you.
If you find this document useful, or if you have suggestions for improving it, please let me know through one of the following channels:
Telephone: 410-516-8447 Fax: 410-516-8260 E-mail: email@example.com Snail mail: Dept. of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686
William P. Blair