Why doesn’t the Moon fall to Earth?

01 · GRAVITY IS THE CAUSE

The Moon stays up because it is being pulled down

A common picture says the Moon is far enough away that Earth’s gravity nearly disappears. It does not. Gravity weakens with distance, but at the Moon it still supplies the inward acceleration that continually bends the path.

If Earth’s gravity vanished at one instant, the Moon would not remain hovering. Its existing velocity would carry it approximately along the tangent to the orbit. Gravity is what prevents that straight-line departure.

02 · SPEED HAS A DIRECTION

Gravity changes where the Moon is going

Velocity is not only speed; it includes direction. At each moment the Moon’s velocity points roughly along its path, while gravitational acceleration points toward the Earth–Moon system’s center of mass.

Adding a small inward change to the old velocity produces a new direction. Repeating that change continuously makes a curve. In an inertial frame, no outward force is needed to hold the Moon up: gravity is the real force and curvature is the result.

03 · FALLING AROUND A CURVE

The ground bends away as the Moon falls

Newton’s mountain thought experiment makes the geometry visible. A slow projectile hits nearby. A faster one travels farther before impact. With the right sideways speed and no atmosphere, the object falls by just enough while the spherical surface curves away by just enough.

At a chosen radius, circular speed is v = √(GM/r). Less or more speed generally produces an ellipse; at escape speed, √2 times the circular speed at that point in the ideal two-body model, the path is no longer bound.

“Missing Earth” is not a one-time trick. Gravity renews the inward turn at every point of the orbit.

04 · THE REAL LUNAR PATH

The Moon does not trace a perfect circle

The average center-to-center distance between Earth and the Moon is about 384,400 kilometers.

Relative to the stars, the Moon completes an orbit in about 27.3 days.

Its orbit is a slightly eccentric ellipse, so the distance varies between perigee and apogee. The Sun strongly perturbs that ellipse, while its orientation and tilt change on longer cycles. A neat fixed ring is a useful first model, not a literal track in space.

05 · EARTH MOVES TOO

Both bodies orbit their common center of mass

Earth pulls the Moon, and the Moon pulls Earth with an equal and opposite gravitational force. Because Earth is about 81 times more massive, the shared barycenter lies inside Earth, but not at Earth’s center.

The Moon’s large loop and Earth’s smaller wobble are two parts of one motion. Saying “the Moon orbits Earth” is an excellent approximation for many purposes; the barycentric description is the more complete one.

06 · AN ORBIT THAT EVOLVES

Tides slowly trade Earth’s spin for lunar orbital motion

Lunar laser ranging shows that the Moon’s mean distance is increasing by about 3.8 centimeters per year.

Earth rotates faster than the Moon goes around it. Tidal bulges are carried slightly ahead of the Earth–Moon line, and their gravity exerts a torque: Earth’s rotation slows while angular momentum is transferred to the lunar orbit. The present recession rate is measured, not a constant to extrapolate unchanged through all history.

07 · FROM CANNONBALL TO LASER PULSE

A thought experiment became a measured trajectory

Newton used a cannon fired from an impossibly high mountain to join terrestrial falling with celestial orbit. The same mechanics later made artificial satellites and lunar missions calculable.

Apollo astronauts and Soviet robotic missions placed retroreflectors on the Moon. Timing laser pulses sent from Earth and returned by those arrays now tests the lunar ephemeris, tidal evolution, the Moon’s interior and gravitational theory.