There isn’t a single set of rules called “the three laws of gravity,” but the phrase usually refers to one of two foundational frameworks: Kepler’s three laws of planetary motion, which describe how objects orbit under gravity’s influence, or Newton’s law of universal gravitation combined with his laws of motion. Both work together to explain why planets orbit the sun, why the moon raises tides, and why you stay firmly on the ground. Here’s how each set of laws works and what they tell us about gravity.
Kepler’s Three Laws of Planetary Motion
In the early 1600s, Johannes Kepler used decades of astronomical observations to figure out three rules that describe how planets move through space. He didn’t know what force was driving the motion, but his laws turned out to be a near-perfect description of gravity’s effects on orbiting bodies.
The Law of Ellipses
Kepler’s first law states that every planet orbits the sun in an ellipse, not a perfect circle. An ellipse is essentially a flattened circle with two focal points instead of one center. The sun sits at one of those focal points, meaning a planet’s distance from the sun changes throughout its orbit. Earth’s orbit is close to circular, but Mars has a noticeably more elongated path, which is actually what helped Kepler crack the pattern in the first place.
The Law of Equal Areas
The second law explains how a planet’s speed changes during its orbit. An imaginary line drawn from the sun to the planet sweeps out equal areas in equal amounts of time. In practical terms, this means a planet moves faster when it’s closer to the sun and slower when it’s farther away. Earth, for example, travels slightly faster in January (when it’s nearest the sun) than in July.
The Law of Harmonies
Kepler’s third law connects a planet’s orbital period to its distance from the sun: the square of the time it takes to complete one orbit is proportional to the cube of its average distance from the sun. A planet twice as far from the sun doesn’t simply take twice as long to orbit. It takes considerably longer, because both the greater distance and the weaker gravitational pull slow things down. This law applies to any orbiting system, from moons around Jupiter to satellites around Earth.
Newton’s Law of Universal Gravitation
About 70 years after Kepler published his laws, Isaac Newton explained the invisible force behind them. Newton’s law of universal gravitation says that every object with mass attracts every other object with mass. The formula is F = G(mM/r²), where F is the gravitational force, m and M are the masses of the two objects, r is the distance between their centers, and G is the gravitational constant (a very small number: 6.67430 × 10⁻¹¹ in standard physics units, as measured by NIST).
Two things control how strong gravity is between any two objects: their masses and the distance between them. More mass means more pull. More distance means less pull. The “squared” part of the equation is especially important, because it means gravity weakens rapidly with distance.
The Inverse Square Law
The distance relationship in Newton’s formula follows what’s called the inverse square law, and it’s one of the most useful concepts for understanding gravity intuitively. If you double the distance between two objects, the gravitational force drops to one quarter of what it was. Triple the distance, and the force falls to one ninth. So two objects attracting each other with a force of 16 units would feel only 4 units of pull if you moved them twice as far apart, and roughly 1.78 units at three times the distance.
This rapid dropoff explains why the moon’s gravity affects Earth’s oceans but you don’t feel a tug from Jupiter, even though Jupiter is far more massive. Distance matters enormously.
How Newton’s Laws of Motion Connect
Newton’s law of gravitation works hand in hand with his second law of motion (force equals mass times acceleration). When you combine the two, you can calculate the acceleration that gravity produces on any object. For Earth’s surface, this gives you the familiar 9.8 meters per second squared, the rate at which a dropped object speeds up as it falls. That number isn’t a universal constant. It depends on the mass of the planet and how far you are from its center. On the moon, gravitational acceleration is about one sixth of Earth’s, which is why astronauts could bounce around so easily.
The gravitational field strength at any point in space is directly proportional to the mass creating the field and inversely proportional to the square of the distance from that mass. This is why gravity on a mountaintop is slightly weaker than at sea level: you’re a bit farther from Earth’s center.
Gravity in Action: Ocean Tides
One of the most visible effects of gravitational laws is ocean tides. The moon’s gravitational pull creates what NOAA calls the tidal force, which causes Earth’s water to bulge outward on the side facing the moon. A second bulge forms on the opposite side of Earth because the moon’s pull is weakest there, and the water effectively gets “left behind” relative to the planet’s center. As Earth rotates, different coastlines pass through these two bulges, producing two high tides and two low tides each day.
The tidal force is a differential force. It comes not from the moon’s gravity being strong in absolute terms, but from the difference in the moon’s gravitational pull between the near side of Earth, the center, and the far side. That difference stretches and squashes the planet slightly, and because water moves far more easily than rock, the effect shows up most clearly in the oceans.
Einstein’s Update to Gravity
Newton’s framework works brilliantly for everyday situations, but it has limits. In 1915, Albert Einstein’s general theory of relativity reframed gravity not as a force pulling objects together, but as a curvature of space and time caused by energy (mass being one form of energy). Massive objects like the sun warp the fabric of space around them, and planets follow curved paths through that warped space.
One key difference: Newton assumed gravity acts instantaneously across any distance. Einstein showed that gravity travels at the speed of light. If the sun vanished right now, Earth would continue orbiting for about 8.5 minutes before the absence of gravity reached us. Einstein’s theory also explains quirks that Newton’s couldn’t, like the slight shift in Mercury’s orbit over time. Mercury’s closest approach to the sun drifts gradually, tracing a rosette pattern rather than a fixed ellipse. Newton’s laws can’t account for this, but Einstein’s equations predict it precisely.
Another important distinction: in Einstein’s framework, all forms of energy generate gravity, not just mass. Heat, light, and even gravity itself produce additional gravitational effects. For most situations on Earth or in the solar system, Newton’s laws give answers that are accurate enough to land spacecraft on other planets. Einstein’s corrections become essential near extremely massive objects, at very high speeds, or when precision down to tiny fractions matters.