What keeps the Moon in orbit — why doesn't it fall to Earth?

The Moon is falling toward Earth — it just keeps missing. An orbit is a continuous freefall where the object moves sideways fast enough that as it falls, Earth's surface curves away beneath it. Isaac Newton famously illustrated this with a cannon: fire a cannonball fast enough and it goes so far that the Earth curves away before the ball lands — the ball is in orbit. The Moon orbits at about 1 km/s; at that speed, the Earth curves away at the same rate the Moon falls. NASA Space Place's orbits page has Newton's cannon interactive.

What are Kepler's three laws?

Johannes Kepler published his three laws of planetary motion between 1609 and 1619, derived from precise observations by Tycho Brahe: 1. Law of Ellipses — planets orbit the Sun in ellipses with the Sun at one focus. 2. Law of Equal Areas — a line connecting a planet to the Sun sweeps equal areas in equal times (planets move faster near perihelion, slower near aphelion). 3. Law of Periods — the square of a planet's orbital period is proportional to the cube of its semi-major axis: T² ∝ a³. Newton later derived all three from his law of universal gravitation. The ESA summary of Kepler's laws is a concise reference.

Why are orbits elliptical and not circular?

Circular orbits are a special case — they require an exact balance between gravity and orbital speed that rarely occurs naturally. Any slight perturbation, initial velocity at an angle, or influence of other bodies produces an ellipse. Newton showed that the only closed orbits under gravity are ellipses (circles are just ellipses with eccentricity zero). Highly elliptical orbits like those of comets take them far beyond Pluto at aphelion and very close to the Sun at perihelion. See our asteroids, comets and meteors guide for how cometary orbits work.

How do rockets change their orbit?

Rockets change orbits by firing engines to change speed. The key insight is that orbits are determined by energy: more speed = higher orbit, less speed = lower orbit. To move from a low orbit to a higher one, a spacecraft fires its engine in its direction of travel (prograde burn) — this raises the opposite side of the orbit. A second burn at the new high point circularises the orbit. This two-burn sequence is the Hohmann transfer, the most fuel-efficient way to move between circular orbits. The Mars missions use a Hohmann-like transfer taking ~7 months. Our Artemis III guide explains the translunar injection burn that takes astronauts to the Moon.

How Orbits Work — Gravity, Speed & Kepler's Laws

Gravity as a curved path

Gravity is not a force that "reaches out" and grabs objects — Einstein showed it is a curvature of spacetime caused by mass. Objects follow the straightest possible path through curved spacetime — and around a massive body like the Sun, those straight paths in curved spacetime are ellipses. Newton's earlier description — that gravity is a force pulling objects toward each other — gives the same orbits and is accurate enough for most engineering purposes.

The critical quantity is orbital speed. At a given altitude above a planet, there is exactly one circular orbital speed. Below that speed, the object spirals inward; above it, the object rises on an ellipse. At √2 × circular speed (escape velocity), the object leaves on a parabola and never returns. This is why rocket staging matters — each stage adds the speed needed to reach the next orbit.

Kepler's three laws, briefly

Ellipses. Every planet orbits the Sun in an ellipse, with the Sun at one focus. A circle is a special ellipse (eccentricity = 0). Earth's orbit has eccentricity 0.017 — nearly circular. Pluto's comet-like orbit has eccentricity 0.25.

Equal areas. The line connecting a planet to the Sun sweeps equal areas in equal times. Consequence: a planet moves fastest at perihelion (closest) and slowest at aphelion (farthest). Earth moves at 30.3 km/s at perihelion in January versus 29.3 km/s at aphelion in July.

T² ∝ a³. The square of the orbital period equals the cube of the semi-major axis (in consistent units). Double a planet's distance and its year becomes 2^(3/2) ≈ 2.83 times longer. This lets astronomers calculate orbital periods from distances and vice versa.

These laws apply to any two-body gravitational system — moons orbiting planets, binary stars, exoplanets, and artificial satellites. See how they're used in exoplanet detection, where the radial velocity method detects the orbital wobble they cause in a star.

How orbits work

Gravity as a curved path

Kepler's three laws, briefly

Frequently asked questions