This is a summary of my Science in the River City workshop aimed
at fifth-grade solar system standards.
I started by projecting this opinion piece responding to the release
of a new survey showing that 26% of Americans answered the question
"Does the Earth go around the Sun, or does the Sun go around the
Earth?" incorrectly. The opinion piece was a bit snarky and
unforgiving; educators know that everyday experience (eg, seeing the
Sun rise and set) is extremely powerful. People who think the Sun
goes around the Earth are at least processing what they see and
building some kind of model to account for it. The vast majority of
people who "know" that the Earth goes around the Sun are probably just
memorizing something their teachers told them, and probably could not
cite evidence or build an argument to support this statement. But the
latter is a much more valuable skill in today's society, and the Next
Generation Science Standards (NGSS) call for our kids to practice this skill.
So, I asked the teachers to role-play in pairs, one building an
argument that the Sun goes around the Earth and the other vice
versa. Try it...it's hard!
The #1 argument for Sun-around-Earth is, of course, rising and
setting, but there aren't a lot of obvious strong arguments for the
other way. Kids will try to invoke authority, such as "NASA launches
rockets that can see what's going on" but I steer them away from that;
people concluded Earth-around-Sun long before rockets were built, so
they should be able to use simpler observations. One pair of teachers
played out the two models using their bodies so they could see the
consequences of each, and that's exactly what I recommend whenever
possible. They showed that the Sun's daily motion could be explained
by the Sun-around-Earth model or by a spinning-Earth model (which is
not the same as Earth-around-Sun). Lesson: don't stop creating models
once you find one that fits, as other models might fit the data too!
If we had no other data, we might not be able to choose between these
models. In such cases we should try to bring other observations to
bear, and/or deduce further consequences of each model. An example of
the latter: if the Earth is spinning, why don't we feel it? We feel
dizzy when we spin on a merry-go-round....but if the merry-go-round
took 24 hours to complete one turn the effects might be too small to
feel. So that was worth thinking about, but inconclusive. Can we
bring other data to bear? Well, just about everything in the night sky
rises and sets and rises again in about 24 hours, so the
Sun-around-Earth model has to become the
entire-universe-turns-around-Earth model. But it's a lot easier to
believe that one thing (Earth) spins than that everything in the
universe contrives to revolve around Earth in 24 hours. Preferring the
simpler model is called Occam's razor, and we do that all the
time in real life. (Think of situations where someone is caught
red-handed doing something they shouldn't, and they say "This isn't
what it looks like" and tell a complicated story...do you believe the
simple story or the complicated story?)
The real physical proof that the Earth spins is quite subtle. One is the
Coriolis effect: we can measure the effects of being on a slowly spinning
"merry-go-round". I showed the teachers a great kinesthetic activity
for this (described in a previous post). Another is the Foucault
pendulum, which is not easy to demonstrate in a school but which kids
may have seen in science museums. A third is even more modern:
astronomers can directly measure velocities of celestial objects using
the Doppler shift (the same principle is used by radar speed guns),
and we constantly have to correct for the velocity of the observatory.
Because of Earth's rotation, stars that are rising appear to be moving toward
us and stars that are setting appear to be moving away from us.
So the Earth spinning accounts for the daily (apparent) motions of Sun
and stars...so why do we think Earth goes around the Sun? (Do you see
how much reasoning we had to do to even get to this question?) Anyone
who looks at the stars periodically must have noticed that you can't
see the same stars all year round. And because the presence of the
Sun in the sky defines when we can't see stars, that means that the
Sun moves relative to the stars. Another way to say this is that,
unlike the Sun, the stars don't take exactly 24 hours to (appear to)
go around; they take 23 hours and 56 minutes. So the Sun may be
opposite a certain star (ie the star is high up at midnight) at a
certain time of year, but over months will creep around the sky to
prevent us from seeing that star, and after 365 days the Sun will be
back to its original position relative to the stars. Earth's rotation
period is either 24:00 or 23:56; it can't be both! (It could be
neither, but considering that possibility may cause cognitive
overload.)
If we again use majority rule or Occam's razor, it's simpler to think
that the stars are fixed and that the Sun moves relative to the Earth
(note how little this conclusion has to do with the basic observation
that the Sun rises and sets each day, which is caused by Earth
spinning). But there are at least two models which involve the Sun
moving relative to Earth, and again I had teachers play the roles of
Sun and Earth to demonstrate a model where Sun is stationary and Earth
moves, and vice versa. Both of these models account for the
observations equally well (so far), and the moving-Earth model has the
disadvantage that we don't feel the Earth move. This is one reason
many ancient Greek thinkers did not endorse the moving-Earth
hypothesis. But Galileo figured out that if you are in a laboratory
moving at constant velocity, you can't feel it move---think of a smooth plane
ride at 500 mph. Earth doesn't move at constant velocity, but it changes its
velocity so slowly that the effects are really small.
The other thing the ancient Greeks figured out is that the
moving-Earth hypothesis means that we should see parallax. Think about
sitting in a moving car. The roadside trees appear to rush by, but
the distant mountains appear to move very slowly. If the Earth moves,
we ought to be able to see an effect like this by comparing nearby and
distant stars. I taped some stars around the room and some teachers
played this out. If Earth is still, we will not see parallax. The
Greeks looked for parallax but did not see it, so they favored the
stationary-Earth hypothesis; they weren't stupid! It just turned out
that even the nearest stars are so far away that the parallax effect is
tiny and could not be measured until modern times.
Using parallax we can determine that the nearest star is about 300,000
times more distant than the Earth-Sun distance (that comparison is natural
because it is Earth's motion over that distance that gives rise to the parallax
effect). It's as if your car moved one mile but you were asked to discern the
difference in your view of mountains 300,000 miles away (eg, on the
Moon). This can be illustrated dramatically by drawing a one-inch
Earth-Sun model on the board, and then drawing a long line
representing the distance to the next star and asking the kids to stop
me when they thought I had arrived. Kids (even most adults) have no
idea how much 300,000 times is; they ask me to stop after 5 feet or
so, but I keep going. When I run out of board, I get a roll of toilet
paper and start unrolling it, as a way to illustrate a very long line.
I keep going even when they tell me to stop. Then, when I run out of
toilet paper, I go to the back room and get a cart full of hundreds of
rolls of toilet paper! It is really dramatic and fun. It's also fun
to write out the number of miles to the next nearest star on the
board: about 24,000,000,000,000 miles. (Kids and even most adults have
little idea what a "trillion" really means.)
Finally, because a light looks dimmer the greater its distance from
us, we can calculate how much light a typical star emits (as opposed
to the very small amount of its light that we receive). Correcting
for this distance factor, it turns out that stars emit about as much
light as our Sun! Some emit more, some less, but the bottom line is
that each star is a sun unto itself, or if you prefer, the Sun is just
another star. In the 1600's, long before parallax was ever measured,
the astronomer Christiaan Huygens turned this argument on its head:
assuming the star Sirius is as luminous as our Sun, how far away would
it have to be to appear so dim? His conclusion was 30,000 times as
far as the Sun; this is lower than the true value, but only because Sirius is
actually substantially more luminous than the Sun. And his number was
certainly big enough to convey some of the vastness of the universe.
The Moon does go around the Earth, so we used that to address the
gravity standard. I did the donutapult demo, which illustrates that
for anything to move in a circle there must be a force directed toward
its center; therefore the Moon is pulled on by a force directed toward
Earth's center. We can relate that to gravity (ie, the force we feel
every day here on Earth's surface) by looking at a globe and noting
that, anywhere on Earth, "down" means toward the center of the Earth.
Therefore (Occam's razor again) we don't need to hypothesize about a
mysterious force keeping the Moon in its orbit; it could well be the
same force that makes apples fall. (Proving that it's the same force
goes beyond the fifth grade standards.)
Finally, we discussed scale models, as scale is a crosscutting concept
in the NGSS. There is no better way to make people appreciate how
empty space is than to build a scale model of the solar system! Rather
than write up our discussion, I refer you to my previous blog posts on
the scale model project I led at Peregrine School: intro, poster assignment,
and completion.
A few links you may find useful:
- my previous SIRC workshop on the solar system (8th grade)
- Powers of Ten video to help students appreciate scale
- interactive tool to do the same (Warning: this tool only shows relative sizes, not the huge distances between celestial objects)
- feel free to browse the labels at right
