在我們還是孩子的時候就已經注意到這個現象了：無論我們跑得多快，月亮都會跟著我們，就算開車都甩不掉它。其實道理很簡單，因為它離我們太遠了。
According to astronomers, the reason why the Moon
and the Sun seem to be following us is because they are so far away. The Moon,
for example, is about 240,000 miles away; the Sun about 93 million miles. And no
matter how fast we drive, we just can't pass them.
When you drive by a stand of trees or a series of telephone poles near the
road, you pass them very quickly. So you see roadside objects first ahead of
you, then next to you, and finally behind you, receding into the rear-view mirror.
But when you drive (or stroll) by the Moon, it's a different story. Because
the Moon is so far away, the angle you view it from will change very little as
you move along. So mile after mile, the Moon will remain in roughly the same
spot of sky. And just as you can't "pass" the Moon, neither can you shake the
presence of the Sun, planets, or stars. Even very distant mountain ranges appear
nearly stationary as we drive by.
And far-away farms and city skylines seem to move by very slowly.
Since we can't pass the Moon, we can't pass its reflection, either. When you
walk along the beach at night, the river of moonlight reflected off the water
moves right along with you. Try to wade out into the moonlight, and you'll find
it remains tantalizingly out of reach, just as a shimmering patch-of-water
mirage retreats down the road as you drive toward it.
When you stand on the beach, moonlight bounces off the water and into your
eyes at a nearly fixed angle. As long as the Moon is in the same spot of sky and
the water level doesn't rise or fall much, the angle of reflection will remain
roughly the same. So if you can see the entire ribbon of moonlight, your eyes
are at just the right height to intercept the rays of light bouncing off the
water from the horizon to the beach.
Once you wade out into the water, however, you've also moved your eyes. The
moonlight bouncing from the water at your feet doesn't strike your eyes;
instead, it shoots right past you at a lower height. So the water at your feet
looks dark.
Friends on the beach behind you, however, will see you standing right in the
moonlight road, and could even snap a picture of it. So to bathe in moonlight,
simply sit down in the water, where your eyes can catch the silvery light at the
right angle.
Meanwhile, just as every car on the road thinks the Moon is following it,
every walker on the beach sees his (her) own ribbon of moonlight, stretching
towards the horizon.
rear-view mirror: 后視鏡
stationary: 固定的
（來源：how-come.net 實習生江巍）
