Date: June 7th, 2012

Title: Galileo Was Right!

Podcaster: Ken Brandt

Organization: Robeson Planetarium and Science Center

Description: Now that the transit of Venus has occurred, let’s take one more look at Venus and its orbit. 400 years ago, Galileo was looking for a way to prove the Copernican heliocentric system. One of Galileo’s students suggested he observe Venus. What Galileo saw was a complete set of phases (we saw a new Venus 2 days ago!), and more importantly, a significant change in the apparent diameter of the disc of Venus. You will get to see this change for yourself.

Bio: Ken is the director of the Robeson Planetarium and Science Center, and also teaches undergraduate astronomy. He is an ardent rockhound, and a lifelong educator who is always looking for low-cost, simple ways to demonstrate complex topics.

Today’s Sponsor: This episode of 365 days of astronomy is sponsored by – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.”

Additional sponsored by anonymously and is dedicated to the international dark-sky association


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 with time, and Venus appears to change size as well. We will use orbital models of Venus to demonstrate the following:

  1. The moon’s apparent diameter changes very little in the course of one complete orbit of the Earth.
  2. If Venus orbited the Earth, we would likewise see very little change in its apparent diameter.
  3. As Venus orbits the Sun, its apparent diameter changes greatly, and this change is consistent with a Sun-centered solar system.

What you’ll need:
A tennis ball or other sphere of similar size Stick lamp or other central single light source Open space
A partner

What you’ll do:
Arrange the stick lamp in the center of the room.

Make sure that it is the brightest single source of light so cover ‘light leaks’.

Let’s set up a geocentric model first. hold the ball at arm’s length, so that your elbow is locked at a 180 degree angle between your shoulder and the ball. Slowly spin with the ball, so it appears to go around your head one time. Does the apparent size of the ball change much? Does the pattern of light from the lamp striking the ball look familiar? It should be reminiscent of the phases of the moon. Of course the moon orbits the earth, and this nearly

circular orbit has the effect of changing it’s size only a little. Had Galileo seen this pattern in observing Venus, he would have concluded that Venus orbits the earth.

Next, take 3 paces away from the lamp. Have your partner take the ball, and step off 2 paces away from the lamp. Your partner should hold the ball so you can see it’s phase and apparent diameter. You will take rough measurements using your thumb at arm’s length. Both you and your partner should start lined up on the same side of the lamp. How big, compared to your thumb, does the ball appear? Now ask your partner to slowly walk around the lamp, keeping the 2 paces distance from the lamp constant if possible. Stop at every step, and remeasure the apparent diameter of the ball. Also note how the ball changes in phase, as the angle between you, the lamp, and the ball changes.

When your partner is as far away from you as they can get, how big is the ball compared to your thumb? You should notice a big change in apparent diameter. You also should observe that phases are similar, but the ball appears larger when it’s close and smaller when it’s far away. After you have completed the measurements, draw what the ball looked like when it was closest, and when it was furthest away. Then, look at Galileo’s drawings of Venus, linked here: http://

If the lamp represents the Sun, the ball represents Venus, and your head, the earth, you should see that Galileo was right! Please note that this model is very out of scale, and very crude.

An activity for teachers and students in a class setting can be found at the Stanford Solar Center’s web page, linked here: activities/venus.html

End of podcast:

365 Days of Astronomy
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