For many, watching the changing illumination of the Moon each month is one of the simple pleasures that set them on the road to an enduring interest in astronomy. Indeed, apart from the Sun, the Moon is the brightest object in the sky, it is easily visible with the naked eye, and can be seen from practically every point on the Earth‘s surface. This makes observing phases of the Moon one of the most common activities in all of astronomy; however misconceptions regarding the cause of these phases still remain. Using inexpensive props, this simple activity encourages participants to model the Sun, Earth, and Moon; allowing them to visualize why we see different phases of the Moon during the month.
This activity can be performed in a darkened room with a bright light source (for example, in a classroom), or outside using the Sun to provide the lighting (e.g.: for public astronomy outreach). The procedure will reflect the indoor version, with explanations occurring throughout. Teachers may want to have their students experiment with the model first, before providing the explanation. Also note that the text reflects a northern hemisphere perspective.
- Each participant should take their foam sphere, and use a pencil to carefully poke a hole into the bottom.
- Depending on the room layout, place the light source either in the center of the room, or at one end.
- Darken the room by blocking windows if necessary.
- Explain that in this activity, the bright light source is going to represent the Sun, the white sphere represents the Moon, and the part of the Earth will be played by the participant’s head. You might also want to point out at the beginning of the activity that, throughout the process, half of the Moon is always illuminated, and half is always in shadow. No matter how you orient it, there is always a ‘dark side’ of the Moon!
- Have each participant face the ‘Sun’, and hold their Moon (atop the pencil) with their arm outstretched, slightly above the source of light. Explain that this orientation represents the New Moon phase, with the far side being illuminated by the Sun, and the half facing Earth hidden in shadow. When in this position, it is very difficult to spot the Moon due to its proximity with the extremely bright Sun! (NOTE: If you are doing this activity outside, remind participants to NEVER look directly at the Sun as permanent eye damage may occur).
- Have everyone slowly rotate to the left (anticlockwise), and watch as the bright edge of their ‘Moon’ slowly grows – this is the Waxing Crescent phase.
- Pause the rotation after a quarter turn (90°) to the left. The ‘Moon’ should have its right half illuminated from the participants’ perspective. Explain that this is First Quarter phase, because the Moon has completed one quarter of its orbit around the Earth. Additionally, although half of the disk is visible, this is only one quarter of the actual surface of the Moon (two more quarters are on the far side, and one quarter faces towards us but is not illuminated). It occurs one week after New Moon.
- Resuming the slow turn to the left, the lit portion of the ‘Moon’ will continue to grow though the Waxing Gibbous phase.
- After two simulated weeks, participants will have completed half a turn, and the entire face of the ‘Moon’ will now be in sunlight. It’s a Full Moon! Ensure participants hold their ‘Moon’ high enough to avoid the shadow of their heads – though you could take the opportunity to talk about eclipses too if you wish! Now might be a good time to remind everyone that only half of the Moon is ever illuminated, but in this orientation the lit half is all that they can see from the Earth.
- Continuing its orbit will see the illuminated face of the Moon start to shrink, passing through the Waning Gibbous phase.
- Pause again briefly to point out the Last Quarter phase when the disk of the ‘Moon’ is again half illuminated from the perspective of the Earth. After three weeks, the Moon has now completed three quarters of its orbit around the Earth, hence why this phase is often called “Third Quarter“.
- One final, slow turn through the remaining 90° will see the lit portion of the Moon dwindle through the Waning Crescent phase, eventually returning to New Moon after a total of four simulated weeks.
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