Why the Moon seems to change shape
1 min 46 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the Earth and Space Sciences Regents. Topic: Earth, Sun and Moon cycles, with the past questions on it.
What Dr. Mars says (the words of the video)
Why does the Moon seem to change shape?
Dr. Mars holds a ball at arm's length.
One lamp lights it, like the Sun.
The lamp always lights the half of the ball that faces it.
The Sun always lights half of the Moon.
Seen from above: his head is Earth, the ball is the Moon.
Full Moon: the whole lit half faces him.
He turns in place, and the ball goes counterclockwise around him.
The lit half still faces the lamp.
Third quarter: he sees the left half of the ball lit.
New Moon: the lit half faces away from him.
First quarter: he sees the right half of the ball lit.
Full Moon again: one cycle of phases.
In between: crescent, less than half lit, and gibbous, more than half lit.
The phase is how much of the lit half faces Earth.
One cycle, from full Moon to full Moon, takes about twenty-nine point five days.
The phases come from the Moon moving around Earth.
Check yourself! Where is the ball at full Moon: A, B, or C?
Find where the whole lit half faces him.
Answer: C! His head is between the lamp and the ball.
Dr. Mars saw the full Moon with his back to the lamp.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.