During the lesson, monitor students’ understanding of the following points, and adjust their understanding as necessary, to help to ensure that they can master the targeted learning goals within the time frame:

• The solar system is almost entirely empty space.
• Astronomical distances are vast. The planets are so far away that they can only be studied with difficulty.
• The terrestrial planets and asteroids have solid surfaces; the gas giants do not.
• The balls and spheres used to represent the planets are approximately, but not exactly, to scale.
• The distances are to scale. If the Earth were the size of a sphere, Saturn would be a baseball nearly a mile away.
• Astronomers have encountered mysteries but they have been subject to scientific explanations.
• Provide feedback during class discussions to ensure student understanding of the composition of the solar system, as well as its vastness.
• Students’ use of scale to explore the vastness of the solar system will be assessed through a class activity involving the creation of a scaled model of the solar system.
• Collect models for individual assessment.
• The integral relationship between technology and modern science (astronomy) will be explored through a class discussion of past astronomical contributions and recent technological advancements.

Begin with the following narration:

“It’s called Iapetus (I-ap-eh-Tuss.) It’s a moon of Saturn that’s about half the size of our moon. When astronomers first noticed it 300 years ago they immediately ran across something spooky. They could see Iapetus on one side of Saturn. They could watch it revolve around Saturn night after night, taking more than a month to get halfway around Saturn.

“And then, after it got to the other side of Saturn, it would disappear. Eventually, it would show up on the side of Saturn where they originally saw it. Then they could start following it again—until it disappeared again when it got to the other side of Saturn.

“Moons should not just disappear and then reappear. It was something that could, and did, inspire science fiction authors. The astronomers in the late Renaissance who first noticed this oddity would not have been blamed if they had come up with unscientific explanations for the object’s bizarre behavior.”

Lead the class in a discussion of why Iapetus (which has a diameter of 1,470 kilometers) might disappear and reappear.

For instance, Iapetus was depicted as the location of an alien artifact that monitors humanity’s development in the novel version of 2001: A Space Odyssey on the apparent assumption that the periodic reappearance of Iapetus was a signal beacon. The location was changed to a moon of Jupiter in the movie of the same name, since Jupiter’s appearance was easier to simulate.

Outline what happened in reality by explaining how astronomers hypothesized four things:

1. Iapetus was locked in orbit with one side always facing Saturn, much as the Earth’s moon always keeps one side facing the Earth. This means that one side, the forward side, is always facing the direction of orbit.
2. Most of Iapetus was covered with bright material, such as ice. But this covering had been altered on the forward side by debris in Saturn’s orbit, making the forward side much darker than the other side.
3. When the darker side is facing the Earth, it is too dim to be seen with available telescopes. But it is still there.
4. When better telescopes became available they would be able to see the dim side.

Better telescopes became available within a generation and they were indeed able to follow Iapetus through its entire orbit. The dark side was found to be about one-sixth as bright as the other side. The scientific method prevailed.

Modern space probes have since mapped Iapetus, and established that the dark material really is a coating of space debris. No alien artifacts showed up.

Continue the lesson by explaining that the early astronomers had trouble establishing basic facts about Iapetus because it was so far away. The class will explore the scale of the composition of the solar system, based on a half-inch (12.7 mm) sphere equaling the size of the Earth.

Set up the lesson as follows:

• On the table, line up four half-inch spheres, explaining these represent the first four planets (Mercury, Venus, Earth, and Mars.)
• Place the BB beside the third sphere, explaining that the third sphere represents the Earth and the BB represents the scaled size of the moon. A scaled distance between the two would be about 40 centimeters.
• The first sphere represents Mercury. It should be about four-tenths the size of the Earth (about 5 mm instead of 12.7 mm) but no such sphere was available.
• The second represents Venus. It is 95 percent the size of the Earth, and at this scale you can’t tell the difference.
• The fourth represents Mars, which is slightly more than half the size of the Earth and should be 7 mm in diameter, but no such sphere was available.
• These four are the terrestrial planets, meaning they are like Earth, in the sense that they have solid surfaces that you could walk on. Mercury has no real atmosphere. Venus has a dense, hot, toxic atmosphere. Mars has a thin atmosphere composed mostly of carbon dioxide. It has barely one percent of the pressure of Earth’s atmosphere, but still produces twisters, ground frost, and some clouds. The tilt of the Martian poles and the length of the day are similar to that of Earth. Mercury and Venus have no moons, and Mars has two tiny moons each only a few kilometers in diameter.
• Depict the sun at one end of the table. At this scale it would be nearly 1.4 meters (about 4.6 feet) in diameter. It would easily be bright enough to blind you. Think of an arc welder’s spark about the size of a recliner chair.
• At the opposite end of the four spheres, note that a gap is being left to represent the asteroid belt. Nothing will be put there since at this scale the asteroids amount to dust. They have solid surfaces, are typically stony or metallic, and all but the biggest have irregular shapes. The largest, Ceres, is less than 1,000 kilometers in diameter.
• Next, put down the softball beyond the gap, representing Jupiter. At this scale Jupiter would be 143 mm in diameter while this softball is 40 percent smaller, or about 100 mm in diameter. No 143 mm ball was available.
• Beyond it, place the baseball representing Saturn, again noting that at this scale Saturn would be about something larger than the baseball. Saturn would be 120 mm in diameter while the baseball is about 71 mm in diameter. Its famous rings are not represented.
• Then place the two racquetballs representing Uranus and Neptune. At this scale the handballs at about 47 mm in diameter are almost exactly the right size. Uranus should be 51 mm and Neptune should be 49 mm in diameter.

Continue the discussion and explain to students:

• The last four planets are gas giants, which lack a solid surface. Instead, they are balls of gas. The gas gets denser and denser as you get nearer their cores, where the gas is so dense and cold as to be basically solid.
• The overall density of the gas giants is approximately that of water, with Saturn being less dense (at 69 percent that of water) and the others being a little more dense. Neptune, the densest, is 64 percent denser than water. That is a little denser than molasses at room temperature. The overall densities of the terrestrial planets differ, but they are more than concrete and less than cast iron.
• Each of the gas giants has a large collection of moons, and we probably have not discovered them all yet. Jupiter has at least 63, Saturn has at least 62, Uranus has at least 27, and Neptune has at least 13. Some are comparable in size to our moon, but most are smaller than 50 kilometers and may be captured asteroids. All have solid surfaces, although some may be mostly ice.
• Beyond Neptune is another asteroid belt called the Kuiper (pronounced Kipe-her) Belt, which extends from Neptune’s orbit to three billion kilometers beyond it.. It includes some dwarf planets like Pluto, formerly called a planet. The asteroids there differ from the ones between Mars and Jupiter in that they have heavy coatings of frozen gas and dust.
• Beyond the Kuiper Belt is the Oort (pronounced ort) Cloud, with asteroids of very irregular orbits stretching halfway to the nearest star.
• Sometimes asteroids from the Kuiper Belt or beyond are perturbed into orbits that bring them close to the sun. As they approach the sun, their coating of frozen gas and dust begins to vaporize and flow out behind them, away from the sun, creating a long, shiny tail. These are called comets. There is about one visible comet per year, but only a few, like Halley’s Comet, are spectacular.
• All the planets orbit the sun close to the plane of the ecliptic, an imaginary disk defined by the Earth’s orbit. Some asteroids and comets orbit at a steep angle to the plane, especially in the Oort Cloud.

Now that the scale is established (where a sphere equals the Earth) the class will explore the scale of the solar system.

Distribute copies taken from the Internet of the 50-meter-scale image of the school and its vicinity. Each student should have a ruler.

Direct students to pick out a location near the center of the image that they want to be the center of their solar system, and place an X there.

They should then locate and sketch in the orbits of the four terrestrial planets, based on the scale of the Earth being a half-inch sphere. The scaled orbital distances are as follows:

Mercury	58 meters
Venus	108 meters
Earth	150 meters
Mars	228 meters

Students can estimate the proper distance on the map using the scale, or you can assist students in calculating the on-map distance in millimeters.

Students should then determine what landmarks lie along the orbital paths of the four planets, such as trees or classrooms, and, below the map image, list at least one landmark for each planet.

Lead students in a discussion of their findings.

Distribute copies taken from the Internet of the 2,500-meter-scale image of the school’s vicinity.

Have students locate the center of the solar system as shown in their first maps and draw a circle where the orbit of Mars would be. The other orbits will be too small to designate.

They should then locate and sketch in the orbits of the four gas giants, based on the scale of the Earth being a half-inch sphere. The scaled orbital distances are as follows:

Jupiter	777 meters
Saturn	1,426 meters
Uranus	2,871 meters
Neptune	4,485 meters

Students should then determine what landmarks lie along the orbital paths of the four outer planets, and list at least one for each planet. In this case it could be roads, road junctions, neighborhoods, shopping strips, etc.

Lead students in a discussion of their findings. Conclude by making this point:

“Keep in mind that, before we started sending out two space probes in the 1960s, astronomy relied on telescopes peering from the bottom of Earth’s atmosphere, whose instability makes telescopic images jumpy and blurry even on the clearest nights. The telescopes were good enough to establish that Iapetus was real, although they could reveal little else about it.

“But on the other hand, there were astronomers using the best telescopes who became convinced that Mars was covered with networks of long, narrow irrigation canals, which they thought they glimpsed during rare instants when the air was calm. This was a dramatic announcement, since the presence of canals implied the presence of intelligent canal-builders.

“Other astronomers demanded better proof, knowing that no ground-based optical instrument can give you as good a view of Mars as the naked eye can give you of the moon. If there were such canals on the moon, you could not stand in your backyard and see them at a glance. (The old story about the Great Wall of China being visible from the moon is, incidentally, a myth.)

“Thanks to our space probes we now know that there are no such canals on Mars, and we have even mapped Iapetus in detail. In both cases the scientific method prevailed. Dramatic explanations were rejected for ordinary ones, and the ordinary ones were later confirmed.

“In our scaled representation, Iapetus would be a quarter the size of a BB more than a mile away. Learning anything about it would be a challenge. On the frontier of science, any science, there are always comparable situations, where the few available facts suggest a dramatic explanation. Or they can be analyzed using the scientific method, which has less entertainment value but generally leads, eventually, to reliable results.”

Extension:

• Launched in 1977, the Voyager I space probe is on a trajectory that will eventually leave the solar system for interstellar space. In early 2010, it was about 112.5 Astronomical Units from the sun. Using this lesson’s scale where the Earth equals a half-inch sphere, which puts Voyager I nearly 16.8 kilometers from the sun, find or generate a map with an appropriate scale and sketch out the orbit of Neptune as previously given. Then place the Voyager I at some landmark or road junction at the appropriate distance. Students should not sketch a circular orbit since the probe is moving away from the sun, rather than orbiting it.

• The closest star to the solar system that we have detected in interstellar space is Proxima Centuri. It is a star in the southern hemisphere that is too dim to be visible to the naked eye, and is about 4.2 light years away. Assuming a light year is 63,241.1 Astronomical Units, have students calculate how far Proxima Centuri is from the sun using our scale where the Earth is the size of a sphere, and then discuss how to map that. (In our scale an AU is 149.51 meters. This puts the star 39,710.6 kilometers from the sun. This is almost exactly the circumference of the Earth, or about a tenth of the way to the moon.)