Reviewing Explanation for the Moon's Changing Phases

• What have we observed about how the moon appears to move during one day? (east to west like the sun)
• How can we explain the apparent daily motion of the moon?
• Two possible explanations:

1. (sun and moon are moving around the earth) (seems obvious)
   

1. (earth is ROTATING, spinning on its axis) (less obvious but might be so)
   

• What have we observed about how the moon appears to move over several days? (west to east)
• How can we explain the apparent motion of the moon over several days? (moon is REVOLVING around the earth)
• We are making a claim (moon revolved part way around the earth over several days) based on the evidence of these observations. Note we could make the same kind of claim (moon revolves around the earth during one day) based on evidence of daily observations.

• How decide which is the case? Look for additional evidence
• What might be evidence for the earth’s spinning daily? (interesting topic, not easy for us to investigate here, Foucault pendulum, Coriolis effects; however, have cultural experiences)
• What might be evidence for the moon revolving around the earth over many days?

• What else have we observed about the moon? It seems to have different shapes at different times.

• How do those shapes change? Is there a pattern? Full, less than full but more than half lit, half lit, less than half lit. We want to build an explanatory model based on these observations.
• Let’s think about the light from the moon:

• Does the moon make its own light? (not always fully lit so probably not)

• What is a possible source of light from the moon? (reflected from the sun)

• If the sun shines light on the moon, infer the half of the moon that is facing the sun is always lit. Some of the light from the sun bounces off the moon in the direction of the earth.

• If the sun shines light on the moon, infer the half of the moon facing away from the sun is always dark. Infer the dark side of the moon is dark because that part of the moon is in the moon’s own shadow.

Let’s review the details of our observations:

• What have we seen for a full moon? (the entire lit part)
• If we point one arm at a full moon and one arm at the sun when both are visible, what kind of angle would be formed by our arms? (straight)

(didn’t actually see this, we were looking at a waning gibbous moon and the angle was almost but not quite straight)

• What have we seen for a waning gibbous moon? (more than half lit on the same side as the sun)
• If we pointed one arm at a gibbous moon and one arm at the sun when both are visible, what kind of angle was formed by our arms? (obtuse)
• What have we seen for a third quarter moon? (half lit part on the same side as the sun)
• If we pointed one arm at a quarter moon and one arm at the sun when both are visible, what kind of angle was formed by our arms? (right)

(reported by one of the students)

• What have we seen for a crescent moon? (just a sliver, lit on the same side as the sun)
• If we pointed one arm at a crescent moon and one arm at the sun when both are visible, what kind of angle was formed by our arms? (acute)

• What is the relation between the lit portion of the moon that we see and the angle formed by pointing one arm at the sun and one arm at the moon? (the smaller the angle, the smaller lit portion of the moon that we see; the sun is on the same side as the lit side of the moon)

• How can we replicate what we see in the sky with a physical model here on earth? Use ball to represent the moon, sun or lamp to represent the sun, ourselves to represent ourselves on earth looking at the moon. Have stand far away from the sun/lamp to replicate that a half lit ball is seen when pointing one arm at the sun/lamp and holding out the ball at a right angle. Can move the ball around our heads to see the changing lit portions on the ball in the same pattern as the phases of the moon.
• INFERENCE: As the moon REVOLVES around the earth, we see different amounts of the part of the moon that is lit by the sun.