Why does the Moon seem to follow us when we drive?

Why does the Moon seem to follow us when we drive? asks a 4th-grade class in Lancaster, OH.

Feel like you're being followed? While it seems like the Moon is always just over your shoulder on a moonlit night, the Sun is also shadowing you as you drive on a sunny afternoon. And then there are those distant mountains to worry about…

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 them, so every walker on the beach sees their own ribbon of moonlight, stretching towards the horizon.

By Kathy Wollard

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