1W-E1 75 minutes SNC1W Science
Why Does the Moon Seem to Change Shape?
Use the Moon Phase Simulator to connect the Moon's visible shape to Sun-Earth-Moon geometry, observation position, and repeating cycles.
Lesson Focus

Anchor question: if the Moon is always round, why does it look like a crescent, half circle, or full circle from Earth? Today you will use a model to explain what changes and what stays the same.

Section 1 Observe the Pattern ~20 min

Reference

  • The Moon does not make its own light. We see sunlight reflecting from the Moon.
  • Half of the Moon is always lit by the Sun, except during an eclipse.
  • Moon phases happen because we see different amounts of the lit half as the Moon orbits Earth.
  • Waxing means the lit part appears to grow. Waning means the lit part appears to shrink.
🌙
Open the Moon Phase Simulator
Use the play controls, phase display, sky view, and Sun-Earth-Moon geometry to compare what the Moon looks like from Earth with where it is in orbit.
Open Moon Phase Sim ↗
1
Press play and watch one full cycle of phases.
2
Pause at new moon, first quarter, full moon, and third quarter.
3
For each phase, compare the sky view with the top-down geometry view.
4
Use the phase display and date display to estimate how long the cycle takes.
Phase What Does the Moon Look Like from Earth? Where Is the Moon Compared with the Sun and Earth? What Pattern Do You Notice?
New moon
First quarter
Full moon
Third quarter
Section 2 Explain the Geometry ~25 min
?
Model check The Moon is not changing shape. The amount of the sunlit half that we can see from Earth is changing.
Light Source

What lights up the Moon, and which part of the Moon is lit?

Observer View

Why does the Moon look different from Earth at different points in its orbit?

Waxing

How can you tell in the sim that the Moon is waxing?

Waning

How can you tell in the sim that the Moon is waning?

Cycle Length
Use the date display to estimate how long it takes the Moon to return to the same phase.
Write your answer as a number of days and describe how you used the sim to estimate it.
Section 3 Use the Model ~20 min
Situation Prediction Reasoning
The Moon is between Earth and the Sun.
Earth is between the Sun and the Moon.
The lit part of the Moon appears to be getting larger each night.
!
Final Task Karen says, "Moon phases happen because Earth's shadow covers different parts of the Moon every night." Correct her using evidence from the sim.
Karen Prompt
Write a short correction. Include Sun-Earth-Moon geometry and one observation from the sim.
Exit Ticket
Answer the anchor question: if the Moon is always round, why does it look like it changes shape from Earth?
Finished?
Save your completed work as a PDF when you are done. Your answers are saved automatically while you type.
If you need to come back later on the same device, your answers should still be here.