A stone released from your hand reaches the ground. The Moon is also attracted to Earth, yet month after month it stays in the sky. Why does gravity make one object collide with the surface while another moves around the planet?
The Moon is in free fall while also moving across the direction towards Earth. Gravity curves its path, and its existing motion allows it to miss Earth’s surface. Gravity makes the orbit possible; falling does not necessarily end in a collision. NASA: how orbits work.
You can fall while moving sideways
Imagine a spherical Earth without an atmosphere, and a small body high above it. Initially, the body is at rest relative to this hypothetical Earth. Gravity accelerates it towards the centre, and it moves down along a radius until it reaches the surface.
Now give it an initial velocity perpendicular to the radius, starting from the same place. While gravity changes its motion towards Earth, the body also moves sideways. Its path becomes curved. With a small sideways speed, it still intersects the surface, just at a different place.
Increasing the initial speed changes the path’s shape. Under suitable conditions, the body moves sideways in such a way that Earth’s surface stays out of its trajectory. It continues falling around Earth. This is the idea behind Newton’s famous horizontal-launch thought experiment; we do not need an actual giant mountain and cannon to explain it. NASA: the limitations of this thought experiment.
Gravity changes velocity rather than being cancelled by it
To describe motion, we need both speed and direction: together, these make up velocity. Even when an object travels equal distances in equal times, its direction can change. That change in velocity also means it has acceleration.
On a circular orbit, velocity is tangent to the circle at every point, while gravitational acceleration points towards Earth’s centre. The direction of motion therefore turns continuously. Speed remains constant, but gravity is what keeps the path curved. OpenStax: force and acceleration in a circular orbit.
In this description there is no second force generated by speed itself that balances gravity. If the net force really disappeared, the body would continue in a straight line. Going around Earth requires a continuous change of direction, which gravity provides.
One Earth, four initial speeds
Before changing the diagram, predict what happens if sideways velocity is removed. Does any amount of sideways motion guarantee an orbit that avoids the surface?
All four launches below use the same Earth, starting position and initial direction of gravity. We change only the magnitude of the initial sideways velocity. Circular speed is the speed that makes the path a circle at that distance. The curve shows the entire selected section of the path at once; it is not an animation or a sequence of equal time intervals.
Change sideways speed, keep the same gravity
Predict what happens without sideways motion. Then compare four launches from one position. The keyboard arrows change the selection; there is no animation or running clock.
vc = 5.59 km/s. v₀ shows initial velocity direction; a shows gravitational acceleration at launch. Arrow lengths are not speed or acceleration scales. The large dot is the start; the small dot ends the displayed line.
Hypothetical launches, not the Moon’s orbit or a mission plan. Fixed spherical Earth with R = 6371 km; launch at 2R from the centre, an altitude of R. The field is GM/r², GM = 398600.435507 km³/s²; the body has negligible mass and initial velocity perpendicular to the radius. No air, other bodies, engine, rotation or terrain. All modes share a geometric scale; the test body is enlarged. Curves are analytic; nonradial paths are sampled uniformly in angle, not time. Sources: NASA, OpenStax, JPL GM, JPL R.
Zero speed produces radial fall. Half the circular speed already curves the path, but the body still hits Earth. Circular speed produces a closed path above the surface. In the fourth case, the speed is sufficient for an open trajectory: in this isolated model the body can recede without limit, although gravity continues to act.
The diagram deliberately starts much closer to Earth than the Moon is. The starting distance is two Earth radii from the centre, so the altitude above the surface is one radius. The Moon is not shown in this diagram. These hypothetical launches help separate the effect of gravity from the role of initial motion.
The Moon has this kind of motion, but its orbit is not a perfect circle
The Moon’s average distance is about 384,400 km. That is roughly 60 Earth radii from our planet’s centre. Earth still attracts the Moon at that distance, and the Moon’s motion takes it far from the surface. NASA: the distance to the Moon.
The central idea combines two facts: the Moon already has orbital motion, and Earth’s gravity continually changes its velocity. It needs no engine continually pushing it along the orbit. Its state of motion comes from the system’s previous history; gravity influences how that motion continues.
Branch · With formulasOptional · you can skip this
What speed does a circular orbit need?
Let r be the distance from Earth’s centre, M its mass, G the gravitational constant and v the speed. Gravitational acceleration is GM/(r)^(2). Motion around a circle of radius r requires an acceleration (v)^(2)/r towards the centre. Equate these two expressions for the same acceleration:
(GM)/((r)^(2)) = ((v)^(2))/(r), v = √((GM)/(r)).
Substituting GM ≈ 398 600.4 km³/s² and r = 384 400 km gives about 1.02 km/s. Earth’s gravitational acceleration at that distance is about 0.0027 m/s². The units deliberately change here: speed is in kilometres per second, while acceleration is in metres per second squared.
This is a calculation for a circular orbit around a fixed Earth, not a measurement of the Moon’s instantaneous speed. The formula explains why the required circular speed depends on distance. OpenStax: deriving circular speed, JPL: Earth’s gravitational parameter.
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What changes in the real system?
The Moon’s orbit is not an exact circle: its distance from Earth changes. The Sun also influences its motion, and the orbit’s shape and orientation change over time. Calculating the Moon’s precise position requires a more complex model than a single unchanging ellipse. NASA GSFC: features of the Moon’s orbit.
Earth also moves under the Moon’s gravitational pull. Both bodies move around a shared centre of mass. The small test body and fixed Earth in our diagram reveal the main mechanism, but they do not reproduce every motion of the real pair. JPL: planetary satellites and the centre of mass.
Free fall describes motion, not an inevitable collision
Check your understanding: if gravity instantly disappeared in our ideal model, would the body stay on its circular orbit? No. Without a force changing the direction of its velocity, it would travel in a straight line along the tangent. Gravity is precisely what the circle requires.
What if we keep gravity but set sideways velocity to zero? That is a different initial state: the body falls towards Earth along a radius. This comparison does not predict the Moon’s future; it shows what the different parts of the explanation contribute.
The Moon’s motion allows it to miss Earth, while gravity continually curves that motion. “Falling” and “orbiting” can therefore describe the same process. While observing the Moon’s changing phases, you can also notice its movement among the stars — a visible consequence of orbital motion. Next in the course, we will examine what free fall means for astronauts inside an orbiting spacecraft.