A Weighty Issue Gravity changes depending on where you are. In some places, it is weak. In others, it is strong. For example, if you visit the moon, your mass will be the same as on Earth. But your weight will be about 17 percent of what it is on Earth. T at’s because the moon’s gravity is weaker than Earth’s. On the moon, gravity pulls you down less. Why? Because the moon has less mass than Earth. What about Mars? Mars is less massive


than Earth but more massive than the moon. On Mars, you weigh 38 percent of what you do on Earth. T at’s more than what you weigh on the moon but less than what you weigh on Earth. T at’s because on Mars, gravity is stronger than it is on the moon.


Gravity’s Long Reach Speaking of the moon, let me point out that gravity is the reason there’s a moon for us to look at. T ink about it. What keeps the moon going around Earth? T at’s right: the pull of Earth’s gravity. T e moon’s gravity also aff ects Earth, for example, causing the ocean tides. T e sun is much more massive than either


Earth or the moon. It’s by far the most massive object in the solar system. So the sun’s gravity forces all the objects in the solar system, including Earth, to orbit around it. Why doesn’t the sun’s gravity suck all the


planets into it? Because they zip around it. If they stopped moving, they would indeed fall into the sun. T e closer a planet is to the sun, the more the sun’s gravity pulls on the planet, so the faster the planet must move around the sun to keep from falling into it. So Mercury, the planet closest to the sun, moves faster than Venus, the next planet out. Venus moves faster than Earth, and so on. Gravity goes way beyond our solar


system. Gravity makes the sun go around the center of our galaxy, the Milky Way, every 230,000,000 years. So: T e moon goes around Earth. Earth


goes around the sun. T e sun goes around the Milky Way, all because of gravity.


14 NATIONAL GEOGRAPHIC EXPLORER


An Important Force Even though gravity is a weak force, it’s usually the most important force in space. Why? First, it operates over a distance. Second, gravity always attracts. It never repels. You might think gravity is pretty


interesting, but it goes way beyond that. Gravity is a matter of life and death. I’m not kidding. Without it, the air would vanish; the seas would freeze; and we’d all be in big trouble. Imagine turning off all gravity in the universe. Because Earth’s gravity no longer pulls you down, you start to fl oat. T at’s fun— at fi rst. But then you notice that the air you breathe


is getting thinner. T at's because the air is also fl oating away, into space. Without gravity, Earth loses its air, and we have nothing to breathe. T at's not all. Other bad things happen. T e sun’s gravity


no longer holds on to Earth. So Earth fl ies off into the deep freeze of space. As a result, Earth gets colder and colder. T is isn’t going well. We’d better turn


gravity back on again. T at way, we have air to breathe and the sun to keep us warm. Me, I’m a big fan of gravity and think we


need to keep it around. Even if it means I end up on the ground aſt er tripping!


WORDWISE


force: a push or pull that can make something move


gravity: a force that pulls one object toward another object


mass: the amount of material in an object


weight: how heavy something is


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