When Do Lunar Eclipses Happen?

Ask students what they know about the phases of the Moon, specifically about when the full moon and new moon phases occur. Clear up any misconceptions, and ask what students know about lunar eclipses. If necessary, explain that lunar eclipses occur during full moons when the Moon passes through Earth’s shadow. Tell students they are going to create a model that explains why lunar eclipses don’t occur during every full moon.

Have students draw or trace a circle 3-5 inches (8-13 centimeters) in diameter in the approximate middle of the plate.

In the center of that circle, students should draw a small circle and label it "Earth."

At the edge of the paper plate, students need to make a small arrow pointing outward that they will use as a reference in the next step and later in the lesson.

On the inside edge of the larger circle, students will mark four evenly spaced points at the 0, 90, 180, and 270 degree marks, or for students who haven't learned degrees, at the 3, 12, 9, and 6 o’clock spots, respectively. Students should align the 90 degree (12 o’clock) point with the reference mark they made in Step 4.

Students should label each point as follows

• Point 1 (at 90 degrees, or 12 o’clock)
• Point 2 (at 180 degrees, or 9 o’clock)
• Point 3 (at 270 degrees, or 6 o’clock)
• Point 4 (at 0 degrees, or 3 o’clock)

Guide students so that they all label their points in the same way.

Students will need to carefully cut along the larger circle they drew, as follows. Poking a starter hole with a pencil or other pointed object on the line will make starting the cut easier.

They should stop cutting at the 180 and 0 degree marks, leaving a few millimeters of uncut space before continuing to cut around the circle. They should be left with a circle still connected to the plate at 180 and 0 degrees.

With both cuts made, students will need to gently twist the cut circle in the center of the plate. It should pivot at 0 and 180 degrees marks so that the 90 degree point rises up slightly and the 270 degree point dips down slightly. Students should be careful not to tear the paper between the two cuts.

Tell students that the edge of the tilted circle represents the tilted orbit of the Moon around Earth. Tell them the flat part of the plate represents the plane in which Earth’s shadow falls.

Have students align the reference arrow on their plate so it's pointing away from them. Then, have them place their object representing the Sun on the opposite side of the plate from the reference arrow.

Ask students the following questions. They can use the optional student worksheet to record their answers:

In this orientation…

• Where would the full moon be located?
  Answer: Point 1
• Is it in the same plane as Earth’s shadow?
  Answer: No
• Would there be a lunar eclipse during this full moon?
  Answer: No

Have students move their plate counter-clockwise 90 degrees, or to the 9 o’clock position, relative to the Sun, while keeping their reference arrow pointed in the same direction. Tell them that as Earth orbits the Sun, the orientation of the Moon’s tilt stays the same. Ask them the questions as before: In this orientation…

• Where would the full moon be located?
  Answer: Point 2
• Is it in the same plane as Earth’s shadow?
  Answer: Yes
• Would there be a lunar eclipse during this full moon?
  Answer: Yes

Have students independently repeat these steps after moving the plate to the 6 o’clock and 3 o’clock positions relative to the Sun, and have them record their answers.