You may have heard references made to the “dark side” of the moon. This popular term
refers to the fact that the same physical half of the moon, the “near side,” is always facing the
earth, which in turn means that there is a far side or so-called “dark side” that is never facing the
earth and can only be seen from space.
This phenomenon has nothing to do with the periodic light and dark we see as the phases
of the moon change. Sometimes people refer to a new moon as a “dark moon” because the
moon is fully in shadow as viewed from the earth, but that’s not the same thing as the dark side
of the moon. The side of the moon facing us during a new moon is the same as any other moon
phase, such as a full moon when we can see the entire face
So why can we only see one side of the moon from the earth? We all know that the earth
rotates on its own axis, so theoretically, the moon should also do the same, allowing us to get a
full picture of the planetoid. Why are we limited to seeing only half the picture? It turns out that
the speed at which the moon rotates has led to this particular phenomenon. Millions of years
ago, the moon spun at a much faster pace than it does now. However, the gravitational influence
of the earth has gradually acted upon the moon to slow its rotation down, in the same way that
the much smaller gravitational influence of the moon acts upon the earth to create tides. This
influence slowed the rotational period of the moon to match that of its orbit – about 27.3 days –
and it is now “locked in” to this period.
Another interesting fact is that actually a little bit more than half of the moon’s surface is
observable from the earth. Since the moon’s orbit is elliptical, and not circular, the speed of its
orbital travel increases and decreases depending on how close it is to our planet. However, the
rotational speed of the moon is constant. This difference between orbital speed and rotational
speed means that when the moon is farthest from the earth, its orbital speed slows down just
enough to allow its rotational speed to overtake it, giving observers a small glimpse of the
usually hidden area. The term for this “rocking motion" of the moon is called libration and it
allows for 59 percent of the moon to be seen in total
Finally, one reason that the far side of the moon is frequently referred to as the “dark
side” is because many people mistakenly think that it never sees any light from the sun. In that
sense the term “dark side” is wrong and misleading. In fact, since the moon is constantly
rotating on its own axis, there is no area of the planetoid which is in permanent darkness, and
the far side of the moon is only completely devoid of sunlight during a full moon – when the
sun is facing the moon with the earth in between.
【題組】75. What does rocking motion refer to?
(A) The process of rock formation on the moon.
(B) Wobbles felt when the moon rotates on its axis.
(C) Meteor rocks falling on the moon’s surface.
(D) The speed change in the moon’s orbital travel.