Explore! Marvel Moon

Penny Moon

Adapted from The Penny Moon and the Quarter Earth, Lunar and Planetary Institute.


In this 10-minute investigation, children ages 10 to 13 use a penny and a quarter to model that the Moon does indeed spin on its axis as it orbits the Earth.

What's the Point?

  • Demonstrate the motion of the Moon's spin and orbit.
  • The Moon takes the same about of time to orbit as it does to spin, so the same side is always facing us.
  • If the Moon did not spin, the opposite side would be visible to us from Earth.


For each pair of children:

For the facilitator:


Facilitator's Note: Be sure to introduce this topic separately from the reason for lunar phases; children may become confused and assume that the Moon’s rotation is related to its phases.

The Moon's period of revolution, or orbit, around the Earth is 27.3 days, which is identical to the Moon's period of rotation, or spin. Because of this, we only see the front side of the Moon.

This wasn't so at first. Over time, the Moon became tidally locked with the Earth — just as the Moon exerts tidal forces on the Earth, our planet also exerts tides on the Moon, slowing down its rotation until it matched its revolution.


1. Explain to the children that they will be working in pairs to determine whether or not the Moon rotates. Hand out the pennies and quarters so that each pair has one of each. Describe how the coins are models of the Earth and Moon.

  • Which coin represents Earth? The quarter. Which one is the Moon? The penny.
  • Which parts of the Moon do we see from Earth? Is it always the same side, or do we see all sides? We see only one side of the Moon from Earth. We see the giant impact basins (or maria or "the man in the Moon", etc.) on it.
  • Does the Moon turn? Can we see its far side? Answers may vary.

2. Invite the children to move the penny around the quarter, but ask them to keep Lincoln's nose facing the quarter, so the people on planet Quarter can always see Lincoln's nose. Have them make the "Moon" (penny) make at least one full circle (orbit) around "Earth."

  • Did the Moon (penny) spin all the way around? Yes.
  • What can you conclude about the Moon's spin from this model? The Moon's takes the same amount of time to spin around once as it does to make one orbit around the Earth approximately one month. That is a very slow rotation compared to the Earth's 24-hour spin!


Summarize the children's investigation. Let the children know that the Moon takes 27.3 days to orbit around the Earth. The Moon also takes 27.3 days to spin once around on its axis.

  • Do the children think we have we ever seen the far side of the Moon?

Robotic spacecraft have transmitted images of the far side — the Soviet Luna 3 probe was the first to document its unfamiliar craters in 1959. The Lunar Reconnaissance Orbiter is currently taking pictures of the entire lunar surface, including the far side. The Apollo 8 astronauts were the first to see it with their own eyes in 1968.

Scientists want to know more about the lunar far side. It looks very different than the side we usually see, especially since it only has a few places that were filled in with lava.

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