Example #1
Why does the Moon exhibit phases?
We have observed that the Moon exhibits phases, appearing fully lit as a round circle at times and then dwindling away to a thin, thin crescent shape a mere two weeks later.
People sometimes think that the appearance of the Moon changes because clouds obscure the Moon, because the Moon moves further away from or closer to the Earth, because the Earth blocks the Sun's light from reaching the Moon, or even because a giant tiger is eating away at the delicious Moon.
When clouds obscure the Moon, they appear wispy (and they don't arbitrarily hide only one side of the Moon). Though the Moon's orbit does vary with time, the distance between the Earth and Moon stays quite constant. The Earth can occasionally get caught between the Sun and the full Moon and cast a shadow over the Moon's face, but this is a rare occurrence (an eclipse) and does not cause the daily changes in the Moon's appearance. (And as for giant hungry tigers, I say the jury is still out.)
The concept that best explains the changing appearance of the Moon is the idea that the Moon, like the Earth, is bright on the side that faces the Sun, and dark on the side that is hidden from the Sun. As the Moon orbits around the Earth once each month, we observe different fractions of that shiny surface over time. This is why the Moon can appear in the full phase (as a fully illuminated round circle), or as a thin crescent (when we see mostly the dark side, hidden from the Sun).
The Moon exhibits phases, or changes appearance, over the course of each month as its orbit brings the shiny, sunlit side into and out of our view from Earth.