Solar system

Today we blasted off from our Earth-Moon base and explored the other planets.  I started with this image of the terrestrial planets, which accurately depicts their relative sizes but not their distances. I brought in a big yoga ball to represent the Sun and we went in order from the Sun (ie from the left in that image).  For each planet I elicited what they already knew or thought they knew about each planet, and then enriched it as best I could.  For example, they knew Mercury is hot because it's close to the Sun...but what about the side away from the Sun (ie the night side, which is not always the same side)? It is actually very cold; why would that be?  To put it another way, why is the day/night temperature variation on Earth not very extreme? That led to a discussion of atmospheres, which further led to a discussion of cratering, which further led to comparisons between Mercury and our Moon (similar size, both airless and cratered, extreme day/night temperature variation).  I won't try to document each planet's discussion here, but 45 minutes flew by. (Here are links to a similar image comparing some asteroids in the asteroid belt, one comparing the gas giant planets (aka Jovian planets) and an image comparing the dwarf planets outside Neptune's orbit.)  As we went, I filled in a table of planet sizes (diameters) and distances from the Sun, for later reference.  I rounded the numbers quite a bit so kids would more easily see the comparisons.  For example, rounding the Sun's diameter to 800,000 miles and Earth's to 8,000 we easily see that the Sun is about 100 times bigger across.  This is way easier to understand than listing the exact numbers and doing the exact computation to find that it is 109 times bigger across.

Just before the break, I addressed why Pluto is no longer considered a planet. Short answer: it became clear that Pluto was just one of many smallish iceballs which are very unlike terrestrial planets and also very unlike Jovian planets, so they deserve their own class.  When Pluto was the only known example, it didn't occur to anyone to put it in its own class.  A nice example of how the way we classify things can change as we get more data.

After the break, we worked on understanding the distances and sizes by building scale models. First, we did the pocket solar system to understand the relative distances. It's quite amazing to see how relatively jam-packed the inner solar system is compared to the outer solar system, yet even in the inner solar system there are many tens of millions of miles between planets.

Next, the sizes. With the 65-cm-diameter yoga ball as the Sun, I pulled balls of various sizes out of my box: softball, baseball, tennis ball, ping-pong ball, etc.  Because I had two ping-pong balls, students suggested they could be Earth and Venus, which are nearly the same size.  Does this accurately depict how much smaller than the Sun these two planets are? Well, Earth is 100 times smaller than the Sun, so on this scale it should be 0.65 cm across, or 6.5mm (1/4 inch).  That's way smaller than a ping-pong ball, so I had to rummage around in my kit, where I found some allspice.  Allspice varies in size, but we did find some which were 6mm across.  That's right, if the Sun is a yoga ball, Earth is the size of an allspice!

Whenever we do a calculation, we have to double-check it.  I held up the yoga ball and the allspice and asked the kids if they thought 100 allspice would fit across the yoga ball.  Yes, it looks about right.  Out of curiosity, how many would fit in the yoga ball? Some of them guessed 100x100, because the yoga ball is 100x bigger in each of the two dimensions which are easily seen.  But the yoga ball is also 100x bigger in the third dimension, so its volume is 100x100x100 or 1,000,000 (a million) times bigger. One million Earths could fit into the volume of the Sun.  (The Sun's density is a bit less than Earth's, so the Sun's mass is "only" 318,000 times bigger than Earth's.  For older kids, adding density and mass to this whole discussion might make sense.)

OK, so now we have Earth and Venus.  What about Jupiter? Using the same reasoning, we found a ball about Jupiter's size (a small whiffleball, not much bigger than a ping-pong ball), and Saturn is just a bit smaller. Uranus and Neptune could be represented by small marbles.  Mars could be a small allspice or an average peppercorn, and Mercury could be a mustard seed.  Amazing! (If you're a teacher who would like to do this kind of activity, check out the peppercorn Earth website for some supporting materials.)

Finally, if these are the sizes of the planets in our scale model, what are the distances between planets? The Earth-Sun distance is about 100 Sun diameters, so we need 65 meters or about 200 feet.  That's about the distance from our classroom to the far side of the playground.  Jupiter is 5 times farther, so maybe we could put it at the KFC a block or so away.  Pluto is 40 times further than Earth from the Sun, so that would be 8,000 feet or 1.6 miles, the distance from school to home for some of the kids.  Imagine...all that space in between would be empty.  Even Mercury, closest to the Sun, would be about 80 feet away and the size of a mustard seed!

At the end, I asked the students to choose a favorite planet or moon, learn more about it, and make a poster over the next two weeks.  We'll put the posters up all over school at the appropriate distances to make a scale model.  At the center of each poster will be a small object size to match the scale model.  To fit the scale model into the school, some of them will have to be very small objects, like a grain of sand.  Teacher Brittany will work with the students on the math for that, and I'll report back on the scale model in a few weeks.