I came across a video I thought you'd all enjoy. It's a delightful demonstration of how gravity varies from planet to planet. Gravity depends upon mass. The more mass, the more pull. This affects how much you weigh as well as how high you can jump on a given planet, moon or asteroid.
Earth is 81 times as massive as the moon. In other words, you'd need to crush 81 moons together to equal the mass of our planet. Being much less massive, its attractive power is far less. On the moon you can jump 9 feet (2.7 meters) in the air from a standing position compared to just 1.5 feet (0.5 m) on Earth. Near Jupiter, the most massive planet, you'd only achieve 6 inches, while on Martian moon Phobos, a jump would launch you straight out into space.
Our muscular system was built to carry us around on Earth. You can jump higher on the moon because your leg muscles have the same strength as on Earth, but you're working against far less weight. A 200 pound human weighs just 33 pounds there. With this calculator you can check your weight on other planets and the moon in pounds or kilograms.
In the simulations, the astronaut stands on a powered platform at cloud-top level at the outer planets because they have no solid surfaces to stand on. The only inaccuracy I can see is at Venus, which has such a dense atmosphere that it would make jumping harder than it looks in the example.
Not only does a planet's gravity affect how high you can jump but also the rate of your fall. With less gravity tugging at your body, you descend more slowly on the moon compared to the Earth or Jupiter. The strength of gravity you experience on a planet or moon also depends upon its density — how tightly packed the material is.
If you had two planets with identical masses, but one was smaller (and therefore denser), you would weigh more on the smaller, denser planet. In fact, if you compressed the Earth into a sphere just 0.7-inches (1.8 cm) in diameter, it would become so dense it would collapse into a black hole with gravity powerful enough to rein in light. Shortly before that moment, if you could somehow stand on the rapidly shrinking Earth, you would weigh billions of tons.
But because the mass of the Earth would be unchanged, the moon would still revolve the marble-sized planet just like always as if nothing had happened. Weird, right?
The sun is by far the most massive object in the solar system, weighty enough to keep all the planets in place. But its gravity, like that of all objects, decreases with distance. The closer you are to a massive object the more you feel it's pull. The sun has a much tighter grip on Mercury, the closest planet, for instance compared to a comet at the edge of the solar system.
Gravity is everywhere. It keeps us anchored to the ground. The pull of the moon and sun creates the tides. Clouds of gas and dust called nebulae collapse to form stars and planets under its grip. Gravity even pulls on light, preventing it from leaving a massive star as it contracts into a black hole and disappears from view as in the example above.
The idea of gravity as a force traces its origin back to Isaac Newton and his apple tree. But the modern view of gravity bequeathed to us by the wispy-haired Einstein, sees gravity not as a force that holds things together so much as an object's influence on surrounding space. A massive object like the Earth warps the fabric of space, similar to how a bowling ball placed on a trampoline creates a depression in the mat. A marble placed at the mat's edge will roll downslope and stop when it strikes the bowling ball.
In the same way, the moon follows the curvature of space (think of it as a depressed trampoline mat) around the Earth and freefalls toward the planet. It doesn't crash into us like the marble hitting the ball because it possesses enough forward speed or momentum to keep from falling downhill. Its average orbital velocity is 2,288 mph (3,683 kph), fast enough to remain in orbit while escaping the clutches of Earth.
For fun, let's say the moon came to a sudden stop. Were that to happen it would immediately begin falling toward the Earth just like the marble obediently following the curvature of the rubber mat on the trampoline. The impact would re-melt the planet with catastrophic consequences. Speaking of which, the only reason you don't fall straight to the center of the Earth when you jump — the same way a "stopped" moon would plummet — is because the ground gets in the way. Remind me to kiss the blessed earth the next time I'm outside.
While Newton's gravity acts like an attractive force, Einstein interpretation of it as curved spacetime is a more complete description that explains, for instance, how light bends when passing near a star, which Newton's theory can't.
I hope all this discussion about warped space inspires you to jump up and down, the better to appreciate the gravity of the situation. Wink.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.