Even though the Sun is now below the horizon, we know about where it is because we saw where it just set.
[Point out place on horizon where the Sun set.]
We can still see the Moon.
How would you describe the shape of the Moon right now?
[Crescent, banana, finger nail clipping.]
If we were to watch the sunset from this same spot three or four days from now, do you think the Moon would look the same? [No.]
How might it be different?
[Different shape; different location.]
The Moon could change its shape in one of two ways, getting fatter or getting thinner. If you think that in three days the Moon will appear narrower, please indicate so by raising your hand. Raise your hand if you think the Moon will appear fuller three or four days from now. Let’s see if our predictions are correct by moving ahead in time using our time machine.
[For young audiences, have them count the days going by with you.]
DIGITAL EFFECT: 4 Days to Quarter
Run diurnal forward 4 days in 4 seconds to get to a quarter moon phase. During this time, the Moon and stars are off, and the atmosphere and Sun are on. This gives the effect of seeing the Sun race across the sky and making the daylight flash to give the illusion of time passing by quickly. Then, turn on the Moon and turn off the atmosphere and Sun.
Those who thought the Moon would grow fatter are indeed correct.
What shape would you say this Moon is now?
Oddly enough, and for reasons that you will learn in a few minutes, astronomers refer to this shape of Moon as a “quarter Moon.”
Is the Moon in the same place as it was when it was crescent, three days ago? [No.]
Does it seem closer to or farther from the Sun? [Farther.]
Guess where you think the Moon will be and what shape it will be three days from now.
DIGITAL EFFECT: 3 Days to Gibbous
Run diurnal forward 3 days in 3 seconds to get to a gibbous moon phase. As with the jump before, the Sun races across the sky and the daylight flashes during the time travel. Advance daily motion forward such that the Sun is guaranteed to be below the horizon.
Was your guess correct?
When the Moon is this shape, bigger than quarter but not yet full, it is called a “gibbous Moon.” It seems even farther from the Sun than when it was quarter. Let’s step just three or four more days into the future. Again, try guessing where the Moon will be.
DIGITAL EFFECT: 4 Days to Full Moon
Run diurnal forward 4 days in 4 seconds to get to a full moon phase. As with the jump before, the Sun races across the sky and the daylight flashes during the time travel.
Note: You may continue to view the waning phases. Run diurnal motion until just before sunrise. Then view the Moon at its respective phases: “4 days to gibbous”, “3 days to quarter”, “4 days to crescent”, and “4 days to new moon”. Do this without seeing the Sun and daylight flash by. Instead, leave the Moon on so the audience can observe the Moon shrinking and moving in the sky (during the waxing phases, the point was for the audience to make predictions). However, if you want to save time, just skip the viewing of the waning phases.
The Moon is very nearly full. Remember where the sunset occurred. Note that the Moon is all the way on the other side of the sky from where the Sun is setting. Whenever a full Moon occurs, you can expect it to rise in the eastern part of the sky right around the time of sunset. The different shapes of the Moon that we observe are known as phases of the Moon.
Why does the Moon seem to change shape as we have just observed?
[Accept any answers.]