Ken is on his way to Italy to teach about space. How do you teach complicated subjects to young students? Ken has some great ideas! Are you looking for new ideas for your classroom or homeschool? Maybe you just enjoy having these conversations with your kids – we did in our house! Keep reading to find out how Ken shares complicated topics with younger learners. This is great stuff to share.

Written by Ken Brandt, Robeson Planetarium and Science Center

Every year, the International Planetarium Society chooses a planetarian to go to Italy as a teaching ambassador. So what, exactly, am I supposed to teach?

I am a big fan of Sky Time, by Cherilynn Morrow and Michael Zawaski (Space Science Institute).

This lesson uses kinesthetic learning to teach rotation, revolution, seasons, and other concepts related to objects in motion, such as the earth and Moon.  I have been struggling for years to find a way to describe the data Galileo gathered as he watched an apparition of Venus in 1611’s pre-dawn skies.

Here are Galileo’s sketches, showing the change in diameter over the set of Venus’ phases.

As you can see, the crescent phase is roughly 4x larger than the full Venus. But how could I explain this to high school students?

I came up with another kinesthetic activity, which was featured in the education resources for PBS’ 400 Years of the Telescope, and The Stanford Solar Center, reprinted here:

Galileo Was Right: Proving the Sun-centered System
Developed by Ken Brandt and Jerry Vinski (Note: Jerry’s contribution was the scale notation in italics, at the end of the activity).

Summary of Activity:
This activity is useful for a quick, low-cost method of demonstrating one of Galileo’s observations using his telescope: the phases of Venus change, and Venus also appears to change size as well.

Duration of Activity:
1-2 class sessions

Student Prerequisites:
Some basic knowledge of the Sun and planets.

Teacher Preparation & Supplies Needed:
1. Ball about the size of a human head
2. Tennis balls or other spheres of similar size for students
3. Stick lamp, or other central single light source
4. Small rulers for student use
5. 10-foot (4-meter) tape measure
6. Open space (at least 30 feet, or 8 meters, in diameter)

Students will use orbital models of the Moon and Venus to demonstrate the following:

  • The Moon’s apparent diameter changes very little in the course of one complete orbit of the Earth.
  • If Venus orbited the Earth, we would likewise see very little change in its apparent diameter
  • As Venus orbits the Sun, its apparent diameter changes greatly, and this change is consistent with a heliocentric Solar System.

Apparent diameter
Orbit (rotation)
Geocentric (Earth-centered) orbit
Heliocentric (Sun-centered) orbit

Arrange the stick lamp in the center of the room. Make sure that it is the brightest single source of light (draw window shades, or cover ‘light leaks’).

Arrange your students at the edge of the room.

Give each student a small ball. This ball represents the Moon. Students should hold the ball at arms length, and slowly turn counter clockwise (to their left). They should notice that the amount of light shining on the Moon appears to change from their viewpoint. These are the phases of the Moon, and you can use this as an opportunity to defeat the misconception that the Moon’s phases are caused by shadows, or by the Moon shining using its own light.

Have students measure the apparent diameter of the Moon at any 4 points in its orbit around their heads using the ruler held 10 cm (3 inches) from the tip of their nose. The students should observe a nearly constant diameter. Point out that any object in a regular orbit around Earth would show a similar pattern.

You will then have students move away from the edges of the room, and have them cluster together closer to the lamp (leave yourself at least 3 meters between them and the lamp). Take the larger ball, and slowly walk around the students at a distance of 3 meters away. Ask the students to make observations of the ball. Try not to let your shadow fall on the ball (holding it above your head will work nicely). The students should notice that the ball is changing phase, and remaining roughly the same apparent diameter.

Have students measure the apparent diameter of the larger ball at 4 points as you orbit around them, using the same ruler held at the same distance from their nose. Tell them that you have just demonstrated what Venus would look like to Galileo if it orbited Earth.

Return the students to the edges of the room. Hold the large ball, and orbit the lamp at a distance of 3 meters (9 feet) from it. Again, hold the ball aloft to keep your shadow from interfering, and ask the students to observe what is similar and different about the ball (point out that this is the same ball you were just holding a few minutes ago). They should observe that phases are similar, but the ball appears larger when it’s close and smaller when it’s far away. Then have them measure at 4 points in your orbit of the Sun with the ruler, using the same procedure as before. They should include measurements when you are closest to them, and again when you are furthest away from them.

After they have completed the measurements, ask them to draw what Venus looked like when it was closest, and when it was furthest away. Then, show them Galileo’s drawings of Venus. Ask them what motion Venus was doing when they drew it (orbit of the lamp/Sun), and what motion Venus actually makes in the sky (orbit the Sun). Ask them if Galileo was right!

NOTE: You should point out that this model is way out of scale! If the students’ heads are the Earth, the Moon would be located about 25 feet (7 meters) away, instead of at arm’s length, Venus would be located at least 180 feet away, and the Sun would be the size of the school building (about 55 meters wide) and located more than a mile away!


You should also note that over the last 14 years, this activity has changed somewhat (as they often do over time). Now, it’s a lot simpler, and it has been incorporated into the full lesson that I’ll stress while I’m in Italy: Kinesthetic Activities You Can Do. This is a collection of six different kinesthetic demonstrations that are all pretty simple and easy to do, with only a few materials (stick lamp, thumbs, string, globe) and most can be done while sitting in a planetarium (how convenient!). 

Basically, students are sitting on one side of the room, and I’m on the other. An object in the middle of the room represents the Sun. I have them hold a thumb up at arm’s length, In our model, their thumb represents a Venus-sized object orbiting Earth. Have them sweep their thumb across their field of view, and ask them to evaluate the change in apparent diameter of their thumbnail, keeping the elbow locked.  If they’re not cheating, then their thumbnail shouldn’t change apparent diameter.  That would represent what Galileo would have seen IF Venus orbited Earth. 

I then hold my thumb up from across the room, and I have the students “eclipse” my thumb with theirs. I ask them; “whose thumb appears larger, yours or mine?” Theirs appears much larger, and this shocks some of them.  Now you can explain that this shows that Venus must orbit the Sun in order for Galileo to have seen what he drew.

Another fun activity which “crosses the streams” of my two blog series currently running for you: the answer to the question-”Can the moon hit the Sun during an eclipse?” You need both hands for this one. Let your right pinkie rest about two inches from your nose, and hold your left thumb at arm’s length. No matter what else you do, keep the distances from your eye equal. Close one eye. This represents your view from Earth.  Your pinkie represents the Moon, your thumb, the Sun. Move your pinkie so it’s in front of your thumb. Gently move the two hands so the pinkie repeatedly covers up the thumb. Did they hit? If you don’t cheat and move your pinkie or thumb, can they? Well, neither can the Moon and Sun during an eclipse.

Bear in mind that all of this is being taught in Galileo’s neighborhood of Northern Italy.  I hope to lead this activity in Galileo’s classroom at the University of Padova (Padua), where he taught for 18 years, among other places.

There will probably be several traditional “star talks” within the planetarium, which I’ll have no problem doing. I also have several Mars rover/Europa Clipper presentations that I can do if I need more than a couple of activities to lead. As you might guess, it’s always better to have plays in your playbook that you don’t have to use and can pull out of your back pocket when needed…

Fortunately for me, all programs are delivered in English!


Ken Brandt directs the Robeson Planetarium and Science Center in Lumberton, NC.  He is a volunteer in NASA’s Solar System Ambassador Program. He is also a member of the 3rd cohort of NC Space Grant Ambassadors, and an Ambassador for the Mars Society.