An elliptical orbit is a curved, oval-shaped path that one object follows around another under the pull of gravity. Every planet in our solar system, along with most moons and satellites, travels in an elliptical orbit rather than a perfect circle. The concept dates back to Johannes Kepler, whose First Law of planetary motion established that planets orbit the Sun in ellipses, not circles as astronomers had assumed for centuries.
The Shape of an Ellipse
An ellipse looks like a stretched circle. It has two special interior points called foci (singular: focus). In any orbit, the body being orbited sits at one of these two foci, not at the center. So when Earth orbits the Sun, the Sun is positioned at one focus of the ellipse while the other focus is just an empty point in space. This off-center arrangement is what causes the orbiting body’s distance to change continuously throughout its trip.
The amount of stretch in an ellipse is measured by a value called eccentricity, which ranges from 0 to just under 1. An eccentricity of 0 is a perfect circle. As the number climbs toward 1, the ellipse becomes increasingly elongated and narrow. A value of 0.5 looks noticeably oval; a value of 0.95 is an extremely stretched loop where the orbiting object swings far out and then dives back in close.
How Speed Changes Along the Orbit
Objects in elliptical orbits do not travel at a constant speed. Kepler’s Second Law describes this: an imaginary line drawn between the orbiting body and the central body sweeps out equal areas of space in equal amounts of time. In practical terms, this means the orbiting object moves fastest when it’s closest to the body it orbits and slowest when it’s farthest away.
The physics behind this is the conservation of angular momentum. As an orbiting object falls closer to the central body, the decreasing distance forces an increase in speed to keep angular momentum constant. As it swings back out to its farthest point, the increasing distance causes it to slow down. This is the same principle that makes a figure skater spin faster when pulling their arms in.
The closest point in an orbit has a generic name: periapsis. The farthest point is called apoapsis. These terms get customized depending on what’s being orbited. For orbits around the Sun, the terms are perihelion (closest) and aphelion (farthest). Around Earth, they become perigee and apogee. Jupiter uses perijove and apojove, the Moon uses perilune and apolune, and so on.
Earth’s Elliptical Orbit
Earth’s orbit is very close to circular, with an eccentricity of roughly 0.017. That small value still produces a measurable difference in distance from the Sun over the course of a year. At perihelion, which occurs around January 3 each year, Earth is about 91.4 million miles from the Sun. At aphelion in early July, that distance stretches to roughly 94.5 million miles. The difference is about 3.2 million miles, a variation of 3.4 percent.
This distance change has almost no effect on seasons. Seasons are driven by the tilt of Earth’s axis, not by how close Earth is to the Sun. (Northern Hemisphere winter actually coincides with perihelion, when Earth is closest.) However, over much longer timescales, shifts in Earth’s eccentricity do influence climate. Earth’s orbit cycles between more circular and more elliptical over roughly 100,000 years, and these Milankovitch cycles have played a role in triggering ice ages. Earth’s eccentricity is currently decreasing very slowly, meaning the orbit is gradually becoming more circular.
Eccentricity Across the Solar System
The planets in our solar system span a range of orbital eccentricities. Venus has the most circular orbit of any planet, with an eccentricity of just 0.007. Mercury, by contrast, has the most eccentric planetary orbit at 0.206, meaning its distance from the Sun varies significantly over the course of its year. At its closest, Mercury is roughly 29 million miles from the Sun; at its farthest, about 43 million miles.
Comets push eccentricity to its extremes. Long-period comets, the kind that take thousands or millions of years to complete one orbit, typically have eccentricities above 0.8. Some approach values so close to 1 that their orbits are nearly parabolic: they plunge in from the far reaches of the solar system, whip around the Sun at tremendous speed, then sail back out to distances tens of thousands of times farther than Earth. This is why comets are visible for only a brief window as they pass through the inner solar system.
Why Satellites Use Elliptical Orbits
Not all satellites are placed in circular orbits. Highly elliptical orbits, or HEOs, are deliberately chosen for specific tasks. One well-known type is the Molniya orbit, which has an apogee (highest point) of about 40,000 kilometers and an orbital period of 12 hours. A satellite in this kind of orbit spends most of its time near apogee, moving slowly over a large region of Earth, then quickly swings down through perigee before climbing back up again.
This design solves a real problem for high-latitude regions like the Arctic. Geostationary satellites, which hover over the equator in circular orbits, can only observe latitudes up to about 60 degrees effectively. Beyond that, the viewing angle becomes too steep and data quality drops. Communication links to Arctic weather buoys and automatic stations also suffer. Molniya-type orbits allow satellites to linger over polar regions for hours at a time, providing quasi-continuous weather monitoring, wind speed and direction data, cloud observations, and ice cover tracking that geostationary satellites simply cannot deliver at those latitudes.
Russia’s Arctica satellite system uses this approach, collecting hydrometeorological data across the Arctic to improve weather forecasting for northern territories. These satellites also relay information from ground, sea, and air-based observing platforms. The favorable radiation environment at highly elliptical orbits gives these satellites a useful lifespan of about seven years.
Elliptical vs. Circular vs. Other Orbit Types
A circular orbit is technically a special case of an ellipse where the eccentricity equals exactly zero. In reality, perfectly circular orbits don’t exist in nature because gravitational influences from other bodies always introduce some degree of elongation. Even orbits described as “circular” in practice have tiny eccentricities.
Beyond ellipses, there are open-ended orbit types. An eccentricity of exactly 1 produces a parabolic trajectory, and anything greater than 1 is hyperbolic. Objects on parabolic or hyperbolic paths are not bound to the central body. They pass by once and never return. Interstellar objects like ‘Oumuamua followed hyperbolic paths through our solar system, meaning they had more than enough energy to escape the Sun’s gravity entirely. Elliptical orbits, by definition, are bound orbits: the object keeps looping back, tracing the same oval path indefinitely (assuming no outside forces change things).