Launch the disc just right in the Field and it loops the sun forever, never falling in, never flying away. That is an orbit: a body falling endlessly toward a world while moving sideways fast enough to keep missing. The same balance holds the Moon, the planets, and every satellite overhead.
An orbit is a balance: the disc's sideways motion would carry it off in a straight line, while the sun's gravity pulls it inward — together they bend the path into a closed loop. Watch the speed readout: on a stretched orbit the disc races at its closest approach and slows far out, exactly as Kepler found for the planets. Launch too slowly and it spirals in; fast enough and it escapes for good.
An orbit is a closed, repeating path that one body follows around a much more massive one, held there by gravity. It is best understood as perpetual falling: an orbiting body is constantly pulled toward the centre, but it moves sideways so fast that it keeps missing, curving around instead of crashing down. The Moon orbits the Earth, the planets orbit the Sun, and thousands of satellites orbit Earth — all by the same balance of sideways motion and inward pull.
Gravity supplies the inward, centripetal force that bends the path into a loop. Kepler's laws describe the result: orbits are ellipses with the central body at one focus; a body sweeps fastest at its closest point and slowest when far; and bigger orbits take longer, so the period grows with the orbit's size. Get the speed just right for a stable orbit — too little and the body spirals in, too much and it reaches escape velocity and leaves for good.
In the Field a sun pulls on the disc the whole time it moves. Launch it at the balanced speed and it settles into a steady loop; launch it faster and the loop stretches into an ellipse, racing in close and dawdling far out, just as Kepler's second law predicts. Too slow and gravity wins, spiralling it inward; past escape velocity and it sweeps away forever. The disc never touches the sun — the entire orbit is gravity bending a fast sideways throw into a closed path.
An orbit is easiest to feel when you can set the launch speed and watch the path respond. Find the band that gives a stable loop, settle which satellite is slower, and turn an orbit's size into its period.
There is a narrow band of speed that gives a stable orbit. Below it gravity wins and the world spirals in; above the escape speed it breaks free and never returns.
The ISS orbits about 400 km up, circling the whole Earth in roughly 90 minutes — so its crew sees sixteen sunrises a day. It is in constant free fall, which is why everything aboard floats, even though gravity there is nearly as strong as on the ground.
Far higher, at about 36,000 km, an orbit takes exactly 24 hours, matching Earth's spin. The satellite hovers over one spot, so a TV or weather dish can point at one fixed place in the sky and never move.
The Moon takes about 27 days to orbit the Earth, and its gravity raises the ocean tides as it goes. Every orbiting partner tugs on the body it circles — the planets even make the Sun itself wobble slightly.
An orbit is one of nature's most elegant balances. This FAQ travels from the disc looping the sun to satellites, geostationary TV, Kepler's laws, the Moon's tides, decaying orbits, and Newton's cannonball.
An orbit is a closed, repeating path that one body follows around a much more massive one, held there by gravity. The clearest way to picture it is as perpetual falling: an orbiting body is constantly pulled toward the centre, but it moves sideways so fast that it keeps missing, curving around instead of crashing down. The Moon orbits the Earth, the planets orbit the Sun, and thousands of satellites orbit Earth, all by the same balance of sideways motion and inward pull. An orbit needs no engine — it simply coasts.
It is falling — that is the secret. Gravity pulls a satellite toward Earth just as hard as it pulls anything else, and on its own that would bring it straight down. But the satellite is also moving sideways at tremendous speed, around 8 km per second in low orbit. As it falls, the curved Earth drops away beneath it just as fast, so it never gets closer to the ground. The fall becomes a circle, and it keeps missing forever. Too little sideways speed and it would spiral in; too much and it would escape.
Newton imagined a cannon on a mountain so high it cleared the air, firing a ball horizontally. Fired gently, the ball arcs down and lands nearby; fired faster, it lands farther away; fired faster still, it falls so far that the Earth curves away beneath it. At one special speed the ground curves away exactly as fast as the ball drops, so the ball falls all the way around the planet and returns to the cannon. That is an orbit. The experiment showed, centuries before spaceflight, that an orbit is simply very fast falling.
Orbits are ellipses — stretched ovals — with the central body at one focus, not perfect circles. This is Kepler's first law. His second law says a body sweeps fastest when closest to the central body and slowest when far, covering equal areas in equal times. His third law says bigger orbits take longer, with the period squared proportional to the orbit's size cubed. Johannes Kepler deduced these three laws from careful observations of the planets in the early 1600s, decades before Newton explained why they hold, through gravity.
Farther from Earth, gravity is weaker, so a slower sideways speed is enough to balance it and stay in orbit. At the same time, a higher orbit traces a much longer path. A lower speed over a longer distance means a far greater orbital period. This is why the low Space Station laps the Earth in about 90 minutes, a medium GPS satellite takes about 12 hours, and the distant Moon takes 27 days. Speed and altitude are linked: go higher and you must, and do, go slower.
A geostationary orbit is a high orbit, about 36,000 km above the equator, where the orbital period is exactly 24 hours. Because that matches Earth's rotation, the satellite circles in step with the ground below and appears to hover over the same spot in the sky. This is invaluable for communications, television, and weather satellites: a ground dish can be aimed once at a fixed point and receive the signal forever without tracking, and a weather satellite can watch the same region continuously, day and night.
| Low orbit | High orbit | |
|---|---|---|
| Altitude | A few hundred km | Thousands to 36,000 km |
| Speed | Fast (~8 km/s) | Slower |
| Period | ~90 minutes | Hours to a day |
| Good for | Imaging, the ISS | Navigation, geostationary TV |
At low altitude there are still faint traces of atmosphere, and this thin air exerts drag — a resistive force opposing the satellite's motion. The drag slowly saps the satellite's orbital speed, and with less speed it cannot stay as high, so it spirals to a lower altitude where the air is denser, which drags even harder. Eventually it plunges into thick atmosphere and the heat of re-entry burns it up. This gradual loss of height is called orbital decay, and it is why low satellites need occasional boosts to stay aloft.
The Moon's gravity pulls a little more strongly on the side of Earth facing it than on the far side. This difference stretches the oceans into two bulges, one toward the Moon and one away from it. As the Earth rotates beneath these bulges, each coast passes through them, seeing the sea rise and fall — high tide and low tide — about twice a day. The Sun adds a smaller pull of its own, so the highest "spring" tides come when Sun and Moon line up, and gentler "neap" tides when they are at right angles.
No — they feel almost full gravity, and that is exactly what holds them in orbit. At the Space Station's height, Earth's gravity is still about 90 percent as strong as on the ground. Astronauts float not because gravity is absent but because they are in continuous free fall along with their spacecraft: everything falls together at the same rate, so nothing presses on anything else, giving the sensation of weightlessness. Genuine zero gravity exists only far from any mass, deep in interstellar space.
They sit on a sliding scale of speed. Too slow, and a body cannot hold an orbit — it spirals in. At the right speed it traces a bound, closed orbit, circular or elliptical. Add more speed and the ellipse stretches longer and longer. At escape velocity the orbit stops closing altogether: the path becomes open, and the body coasts away forever, never to return. So an orbit and an escape are two ends of the same story, separated by the escape speed, which for Earth is about 11 km per second.
Because acceleration means any change in velocity, and velocity includes direction. A satellite in a circular orbit keeps a steady speed, but its direction turns continuously all the way around, so its velocity is always changing. That ongoing change is a centripetal acceleration, directed inward toward the Earth, and it is supplied by gravity. Remove that inward pull and the satellite would stop curving and fly off in a straight line. So a steady-speed orbit is, surprisingly, a state of constant acceleration.
In the Field a sun pulls on the disc the whole time it moves. Launch the disc at the balanced speed and it settles into a steady loop; launch it faster and the loop stretches into an ellipse, racing in close to the sun and dawdling far out, exactly as Kepler's second law describes. Launch too slowly and gravity overpowers it, spiralling it inward; launch past escape velocity and it sweeps away forever. The disc never touches the sun — the whole orbit is gravity bending a fast sideways throw into a closed path.
In principle yes — every mass has gravity, so anything can be orbited. In practice the gravity of small objects is absurdly weak, so an "orbit" around a person would require a ludicrously slow sideways speed and would be destroyed by the faintest breath of air or nudge of sunlight. Real orbits need a mass large enough for gravity to dominate, like a planet, a star, or at least a sizeable asteroid. Spacecraft have actually orbited small asteroids and comets, but only because deep space is calm enough for such gentle, slow orbits to survive.
Often by watching for the tug of an orbit. A planet and its star both orbit their shared centre of mass, so a planet makes its star trace a tiny circle, wobbling slightly toward and away from us. Astronomers detect that wobble as a rhythmic shift in the star’s light, repeating once per orbital period — a direct echo of Kepler’s laws at work light-years away. A second method watches a star dim slightly each time a planet crosses in front of it. Both turn the simple idea of an orbit into a tool for discovering thousands of distant worlds.
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