Classroom Stories: Teaching Interplanetary Distances Using a Human Solar System
10/22/2021
By Michael Dunham (State University of New York at Fredonia)
When I first started teaching my Astro 101 course, one of the concepts that I struggled to properly convey to students was the immense change of scale in our Solar System between the spacing of the terrestrial planets and the giant planets. Simply giving the numbers did little to properly convey the spacing to students not used to quantitative thinking. Diagrams also did little to help since, in order to fit a properly scaled Solar System onto it, the terrestrial planets are placed so close together that they are practically indistinguishable. Out of necessity, then, a new idea was born. Here I will describe an activity that I developed to better convey to students the true structure of our Solar System.
When we are just about to start discussing the Solar System, I take the entire class outside for one 50-minute lecture. We first meet in the classroom, where I have an inflatable Solar System consisting of beach ball-sized planets (along with the Sun, Pluto, and the Moon). I ask for 11 volunteers to each take one of the Solar System components, and then we all head outside to an area of the campus that has a straight sidewalk approximately 600-feet long. Before heading out, I do emphasize to students that they should pay careful attention to what they see, as there will be a graded assessment at the end. I have found that it really is necessary to say this, otherwise some students will treat this as a social hour and not pay attention to what they are supposed to be learning.
Once we are outside, I ask the volunteer holding the Sun to stand at the beginning of the sidewalk, and I tell the class that, using a scale where 15 feet is equal to 1 astronomical unit (AU), we are going to place each planet at its proper distance from the Sun. It is worth noting at this point that, while I use an inflatable Solar System that I purchased online, you could just as easily adapt this activity to have students wear planet name tags rather than hold inflatable planets.
Using a 25-foot tape measure that I extend along the ground, I ask the volunteers holding the terrestrial planets to stand at the 6-foot (Mercury), 11-foot (Venus), 15-foot (Earth), and 22.5-foot (Mars) markings on the tape measure. By this point, many students expect that we will place the remaining planets at similar distances and be finished with the activity within the next few minutes. I then announce that, at an average orbital distance of 5.2 AU, Jupiter is located 78 feet from our Sun (or 55.5 feet from Mars). This requires us to move our 25-foot tape measure three times (once to measure the first 25 feet past Mars, once to measure the 50 feet past Mars, and a third time to measure the last 5.5 feet to get to Jupiter).
By the time we place Jupiter, we only have three planets left. However, since Jupiter is at approximately 5 AU from the Sun, whereas Neptune is located at approximately 30 AU from the Sun, we have only traveled one sixth of our total distance. The last three planets end up being placed at 142.5 feet (Saturn), 288 feet (Uranus), and 451.5 feet (Neptune) from the Sun, requiring us to move the tape measure many times. If you decide to place Pluto as well, it ends up being placed at a distance of 592.5 feet from the Sun.
Once we have placed all the planets, I ask the remaining students to spend some time walking back and forth in order to truly take in the differences between the inner and outer planets. The terrestrial planets are so close together that the students can almost reach out and touch each other, whereas Uranus and Neptune are so isolated that the students holding them struggle to hear each other, even when shouting at full volume. I also ask the remaining students to take over for those holding the Solar System objects, especially the Sun and the terrestrial planets, so the volunteers can have a chance to walk down the sidewalk and truly appreciate how far away the giant planets are. Last, I tell all my students to be back in the classroom for the remaining 10 minutes of class.
Once students have returned to the classroom, I give them the following prompt:
Take out a blank sheet of paper and write your name on it. Then sketch a to-scale diagram of the Solar System, including (at minimum) the Sun and all 8 planets. (You don’t have to label the planets, but you do have to include all 8.) You are not graded on your artistic talent, but you must make a legitimate attempt to show, to the best of your ability, properly scaled distances between the planets. Once you finish, turn in your sketch and you are free to go.
Students absolutely love this activity. It scores very highly in evaluations where I ask students to rank their favorite and least favorite class activities. This activity gets students out of their seats and outside, and it teaches them about the Solar System in a memorable, lecture-free manner. The sketches that students turn in demonstrate that the scale of the Solar System has really sunk in, and high average scores on a nearly identical final exam question 2.5 months later demonstrate that this lesson has not been quickly forgotten. Although class time is very precious and it is always hard to “give up” 50 minutes of lecture, my informal assessments have convinced me that this activity is worth the time it takes.