How Orbits Work: The Motion of Planets Explained

An ellipse, not a circle

For most of astronomical history, heavenly motion was assumed to be circular, since circles were considered the only shape perfect enough for the heavens. Johannes Kepler spent the early 1600s trying to fit Mars's observed positions — recorded with painstaking precision by Tycho Brahe, the astronomer he worked under — to a circular orbit, and kept failing by a few percent. Rather than fudge the data, Kepler concluded the orbit itself wasn't circular. In 1609 he published what's now called Kepler's first law: every planet moves around the Sun in an ellipse, with the Sun at one focus, not the center.

An ellipse is a circle stretched along one axis, and the stretch determines how far the Sun sits from true center. Because the Sun occupies a focus rather than the middle, a planet's distance from it changes constantly: closest at perihelion, farthest at aphelion. This isn't a quirk unique to planets — it falls directly out of Newtonian gravity. Any object moving under an inverse-square gravitational pull traces one of a family of curves called conic sections, and a bound orbit, one without enough speed to escape, works out mathematically to be an ellipse, with a circle as the rare, perfectly balanced special case.

Equal areas, unequal speeds

Kepler's second law describes what that changing distance does to a planet's speed. Draw an imaginary line connecting the planet to the Sun, and as the planet moves, that line sweeps out area like a windshield wiper. Kepler found that it sweeps equal areas in equal times no matter where the planet is in its orbit — forcing the planet to move fastest when closest to the Sun and slowest when farthest away, since a fast-moving planet needs only a short, stubby wedge to sweep the same area a slower, distant planet covers with a long, thin one.

Earth demonstrates this every year, even though its orbit is nearly circular. At perihelion in early January, Earth sits about 147.1 million kilometers from the Sun, moving at roughly 30.3 kilometers per second; by aphelion in early July, it has drifted to about 152.1 million kilometers and slowed to around 29.3 kilometers per second. The effect is subtle for Earth but dramatic for Halley's Comet, which spends most of its 76-year loop out beyond Neptune's orbit, then accelerates past 50 kilometers per second near the Sun before slingshotting back into the dark. The mechanism is the same conservation law that speeds up a spinning ice skater pulling in her arms: angular momentum stays constant, so closing the distance to the Sun trades distance for speed.

The farther out, the slower and longer the year

Kepler's third law, published a decade later in 1619, ties the solar system together with one relationship: the square of a planet's orbital period is proportional to the cube of its average distance from the Sun. Orbital periods, in other words, grow faster than distance does — a planet twice as far from the Sun doesn't take twice as long to go around, it takes roughly 2.8 times as long, because it has farther to travel and moves more slowly doing it, both effects of the Sun's gravity weakening with distance.

The real solar system shows the pattern cleanly. Mercury, at an average 0.39 astronomical units (AU) from the Sun, completes an orbit in just 88 Earth days. Earth, at 1 AU by definition, takes 365.25 days. Jupiter, at 5.2 AU, takes 11.86 years. Neptune, out at roughly 30.1 AU, takes about 165 years — only slightly more than one full orbit since its 1846 discovery. Kepler had no idea why the relationship held; he treated it as a numerical pattern in his data. It took Isaac Newton, decades later, to show that all three of Kepler's laws are exact consequences of one simpler rule: gravity weakens with the square of the distance between any two masses.

Eccentricity and tilt, without the equations

A planet's eccentricity captures how stretched its ellipse is, running from 0 for a perfect circle toward 1 for an ellipse stretched almost into a line. Venus has the most circular orbit of any planet, at just 0.0068 — its distance from the Sun barely changes over a Venusian year. Earth's is a mild 0.0167. Mars, at 0.0934, sees its distance from the Sun vary by roughly 20 percent between perihelion and aphelion, part of why its seasons are more lopsided than Earth's. Mercury sits around 0.206, and Pluto's path swings from about 29.7 AU to 49.3 AU on an eccentricity of 0.25 — stretched enough that for about twenty years of its 248-year orbit, Pluto is actually closer to the Sun than Neptune is.

Inclination is the other number worth knowing: the angle by which a planet's orbital plane tilts relative to Earth's, the reference plane astronomers call the ecliptic. Most planets hew closely to it, tilted by only a degree or two, evidence the solar system formed flattened out of a single spinning disk of gas and dust. Mercury breaks that pattern slightly at about 7 degrees, and Pluto breaks it dramatically at roughly 17 degrees — one of several clues, alongside its eccentric orbit and icy composition, that helped convince astronomers Pluto belonged with the Kuiper Belt's small icy worlds rather than the planets.

Why nothing just falls into the Sun

Gravity is pulling every planet toward the Sun at every instant. What keeps a planet from spiraling in is that it's also moving sideways, fast enough that the pull keeps missing. Newton illustrated the idea with a thought experiment: imagine firing a cannonball horizontally from a very tall mountain. A weak shot arcs down and lands nearby; a stronger shot travels farther. Fire it fast enough, and the curve of its fall matches the curve of the Earth's surface falling away beneath it — the cannonball never lands, because the ground drops away exactly as fast as gravity pulls it down. That's an orbit: continuous, perpetual falling that never gets any closer to the ground.

Planets orbit the Sun for the identical reason, just at a scale where cannonballs become worlds. Earth is falling toward the Sun every moment, and its roughly 30-kilometers-per-second sideways velocity carries it just far enough to miss, curving into the next segment of its ellipse instead. Nothing needs a constant push to keep an orbit going the way a car needs a running engine — there's no friction in the vacuum of space, so once that balance of speed and gravity is set, it repeats indefinitely with no extra energy required. That reframing — orbit as endless freefall, not a static track — let Newton derive Kepler's three laws from one law of universal gravitation, unifying the sky with the same force that drops an apple to the ground.

When orbits fall into step: resonance

Occasionally, two orbiting bodies settle into periods that form a simple ratio of whole numbers, a state called orbital resonance, and the repeated, synchronized tugs of gravity that follow can either stabilize a system or tear it apart. The most famous example keeps Pluto safe: Neptune and Pluto orbit in a 3:2 resonance, so for every three trips Neptune makes around the Sun, Pluto makes exactly two. Pluto's path does cross Neptune's, but the resonance means the two are never actually near each other when their orbits intersect — Neptune's gravity has locked Pluto into a rhythm that's kept it safe for billions of years.

Resonance can also carve empty space out of a crowded belt: an asteroid orbiting at a period that's a simple fraction of Jupiter's — one-third, say, or one-half — gets the same gravitational tug at the same point in its orbit over and over until the nudges destabilize it, leaving the Kirkwood gaps, empty lanes in the asteroid belt that Daniel Kirkwood first identified in 1866. Saturn's rings show a related effect at the Huygens Gap, near the inner edge of the Cassini Division, where particles sit in a 2:1 resonance with the moon Mimas that has swept the lane nearly clean. Jupiter's moons use the same mechanism for stability instead: Io, Europa, and Ganymede lock into a 4:2:1 pattern, Io completing four orbits for every two of Europa's and one of Ganymede's, a rhythm precise enough to keep Io's interior flexing — making it the most volcanically active body in the solar system.