Orbits are ellipses, not perfect circles. An ellipse is essentially a stretched or flattened circle, like an oval, and every orbiting object in the solar system follows this shape. Johannes Kepler established this in the early 1600s as his First Law of planetary motion: each planet’s orbit around the Sun is an ellipse, with the Sun sitting at one of the ellipse’s two focal points, not at the center.
How stretched or circular a given orbit is varies enormously. Most planets trace paths so close to circular you’d struggle to see the difference in a diagram. Comets, on the other hand, can follow orbits so elongated they spend centuries in the outer solar system before whipping briefly past the Sun.
Why Orbits Are Ellipses
The shape comes directly from how gravity works. Gravity between two objects weakens with the square of the distance between them. Double the distance, and the gravitational pull drops to one quarter. This specific relationship, called the inverse-square law, produces a limited set of possible paths for any object caught in another’s gravity: circles, ellipses, parabolas, and hyperbolas. A perfect circle is really just a special case of an ellipse where the stretching is exactly zero, and it requires a precise balance of speed and distance that rarely occurs in nature.
In practice, an orbiting body speeds up as it falls closer to the object it orbits and slows down as it moves farther away. This constant exchange between speed and distance traces out the elliptical path. If you could somehow give a planet exactly the right velocity at exactly the right angle, it would maintain a perfect circle. But any slight deviation, and the orbit becomes an ellipse. Since the real solar system formed from a chaotic cloud of gas and dust with countless gravitational interactions, perfectly circular orbits essentially don’t exist.
Measuring How Stretched an Orbit Is
Scientists describe an orbit’s shape with a number called eccentricity. An eccentricity of 0 is a perfect circle. As eccentricity approaches 1, the ellipse becomes more and more elongated. At exactly 1, the path becomes a parabola, and above 1, it’s a hyperbola: both are open curves, meaning the object flies past once and never returns.
Here are the eccentricities of all eight planets, from NASA data:
- Venus: 0.0068 (the most circular orbit in the solar system)
- Neptune: 0.0086
- Earth: 0.0167
- Uranus: 0.0473
- Jupiter: 0.0484
- Saturn: 0.0539
- Mars: 0.0934
- Mercury: 0.2056 (the most elliptical planetary orbit)
For most planets, these numbers are so small that their orbits are nearly indistinguishable from circles. Earth’s eccentricity of 0.0167 means the difference between its closest and farthest points from the Sun is only about 5.1 million kilometers, a variation of 3.4 percent. Earth is actually closest to the Sun around January 3 and farthest around July 4, which surprises people who assume distance from the Sun drives the seasons (it doesn’t; axial tilt does).
Comets and Extreme Ellipses
Comets tell a completely different story. Halley’s Comet has an eccentricity of 0.967, making its orbit so stretched that at its farthest point it is roughly 60 times farther from the Sun than at its closest. It swings inside the orbit of Venus at perihelion, then retreats beyond Neptune at aphelion. Comet Hale-Bopp, the spectacular comet visible to the naked eye in 1997, had an eccentricity of 0.995. Its farthest point was about 400 times more distant than its closest approach.
Push eccentricity past 1.0 and you no longer have an ellipse at all. Comet C/1980 E1, with an eccentricity of 1.057, followed a hyperbolic trajectory. It passed through the inner solar system once, gained enough energy, and left permanently. Interstellar visitors like ‘Oumuamua follow the same kind of open path: they’re not bound by the Sun’s gravity and simply pass through.
The Moon’s Orbit
The Moon’s orbit around Earth is also an ellipse, and its variation is noticeable. The Moon’s distance from Earth ranges from about 356,400 kilometers at its closest point (perigee) to 406,700 kilometers at its farthest (apogee), a swing of roughly 12 percent from nearest to farthest. This is why the Moon appears slightly larger at some times than others, the phenomenon behind so-called “supermoons,” when a full moon coincides with perigee.
Orbits That Shift Over Time
Elliptical orbits aren’t frozen in place. They slowly rotate and change shape over time due to gravitational tugs from other bodies. Earth’s eccentricity, for instance, cycles between more elliptical and more circular over a span of about 100,000 years. At its most elliptical, about 23 percent more solar energy reaches Earth at closest approach than at farthest departure. Currently, Earth’s orbit is very slowly becoming more circular. These shifts, known as Milankovitch cycles, are one of the factors that have driven ice ages and warm periods throughout Earth’s history.
Mercury’s orbit provided one of the most famous tests in physics. Astronomers in the 1800s noticed that Mercury’s closest point to the Sun gradually shifts around in a way that Newton’s gravity couldn’t fully explain. After accounting for the gravitational pull of every other planet and even the Sun’s slight bulge from its rotation, there was still a discrepancy of 43 arc-seconds per century (about 0.012 degrees). It sounds tiny, but it was measurable and unexplained. Einstein’s general theory of relativity, published in 1915, predicted this extra shift precisely. In Einstein’s framework, the curvature of spacetime near the Sun causes orbits to slowly rotate, so a planet doesn’t retrace the same ellipse each time but instead traces a pattern like a slowly spinning spirograph. This was one of the first confirmations that general relativity was correct.
Artificial Satellites and Designed Orbits
When engineers put satellites into space, they choose orbit shapes deliberately. Geostationary satellites, the ones that provide TV signals and weather imagery, orbit at about 35,786 kilometers above the equator in paths that are kept as circular as possible. This lets them hover over one spot on Earth’s surface, completing one orbit in exactly the time it takes Earth to rotate once.
Satellites in low Earth orbit, between roughly 200 and 1,000 kilometers up, also tend toward circular paths but can be placed in polar orbits that pass over both poles. As Earth rotates underneath, these satellites eventually see every part of the planet’s surface, making them ideal for mapping and environmental monitoring.
Some satellites use intentionally elliptical orbits. A highly elliptical orbit lets a satellite spend most of its time high above one hemisphere (moving slowly near apogee) and only briefly sweep close to Earth (moving fast through perigee). Russia has historically used this type of orbit for communications satellites serving high-latitude regions that geostationary satellites can’t cover well.
Regardless of whether an orbit is natural or engineered, the underlying shape is always one of the same family of curves that gravity and motion produce: an ellipse for anything that stays in orbit, or a hyperbola for anything passing through.