Why doesn’t the moon fall down to earth?

Why doesn’t the Moon fall down?

One commonplace statement that used to be indisputable is that what goes up must surely come down. The launching of spaceships and satellites has shown the argument to have flaws. The heavens are now littered with junk that went up but may never come down.

Why does the Moon, which hangs in the sky like a giant balloon and is pulled by Earth’s gravity, not come crashing down on us? The concern, a fear in times gone by, is based on an illusion. The Moon may appear to be motionless, but it is moving in a steady orbit round the Earth. We find that hard to detect, because the Moon is on average 384 400 km (238 900 miles) away. Objects close to us appear to be moving more swiftly than when they are farther away. For example, an aircraft taking off seems to be going very fast to observers on the runway; high in a cloudless sky its movement may be difficult to detect.

The Moon’s movement round the Earth is much like that of a ball swung steadily in a circle on the end of a string. The ball tries to fly off in a straight line (as it would do if the string broke) but the string keeps it swinging in a circle. Imagine that the Earth’s gravity is the piece of string. The Moon’s velocity, about 3700 km/h (2300 mph), is enough to keep it trying constantly to fly off into space. Earth’s gravity, which lessens with distance, is still strong enough to keep the Moon within its orbit.

As a young man in the 1660s, Sir Isaac Newton pondered the problems of why the Moon doesn’t come crashing down and why it doesn’t fly into space. Newton wasn’t the first to observe that objects tossed in the air fell to Earth, and the story that he discovered gravity when an apple fell on his head is possibly a piece of popular legend. Perhaps a falling apple, seen as he looked out on an orchard from his study window, inspired him. He reasoned quite rightly that, if objects on Earth were pulled by its gravity, so possibly was the Moon.

While other physicists had argued that this force didn’t apply to heavenly bodies, Newton believed that it did. Among other principles, he established that the Earth and Moon each have a gravitational pull, one on the other, which keeps them interacting in perfect balance.

The same principle applies to communications and other satellites orbiting the Earth. They are given sufficient velocity to hold a steady orbit. That doesn’t mean that they are not falling: indeed, they are. But, as they fall, the Earth’s surface is curving constantly away from them, so that they never get any closer.