Every planet orbits its star in an ellipse, a slightly stretched circle with the star sitting off-center at one of two special points called foci. This was first demonstrated by Johannes Kepler in 1609 and remains one of the foundational facts of astronomy. While some planetary orbits look almost perfectly circular to the naked eye, none of them actually are.
Why Orbits Are Ellipses
An ellipse is a geometric shape defined by two interior points called foci. For any spot along the edge of an ellipse, the combined distance to both foci is always the same. A circle is just a special case where the two foci overlap into a single center point.
Kepler’s First Law states that each planet’s orbit is an ellipse with the Sun at one focus. The other focus is an empty point in space. Because the Sun isn’t at the center of the ellipse, a planet’s distance from the Sun changes continuously as it travels. At its closest approach (called perihelion), the planet moves faster; at its farthest point (aphelion), it moves slower.
The physics behind this shape comes down to two forces working together. A planet’s forward momentum would carry it in a straight line forever if nothing intervened. Gravity pulls it toward the Sun instead. The tug-of-war between these two effects bends the planet’s path into a closed, elliptical loop. If a planet had exactly the right speed and angle to balance gravity perfectly at every point, the orbit would be a perfect circle. In practice, that precise balance never quite happens.
How Eccentricity Measures the Stretch
Astronomers use a single number called eccentricity to describe how elongated an orbit is. It ranges from 0 (a perfect circle) to just below 1 (an extremely stretched ellipse). Technically, an eccentricity of exactly 1 would be a parabola, meaning the object would never return.
The formula is simple: eccentricity equals half the distance between the two foci divided by the semi-major axis (the longest radius of the ellipse). A small eccentricity means the two foci are close together and the orbit looks nearly round. A large eccentricity means the foci are far apart and the orbit is visibly oval.
How the Planets Compare
Most planets in our solar system have surprisingly low eccentricities, meaning their orbits are close to circular. Here’s how they stack up:
- Venus: 0.007 (the most circular orbit)
- Neptune: 0.011
- Earth: 0.017
- Uranus: 0.046
- Jupiter: 0.049
- Saturn: 0.057
- Mars: 0.094
- Mercury: 0.205 (the most elongated orbit)
Venus traces a path so close to a perfect circle that you wouldn’t be able to see the difference in a diagram. Mercury, on the other hand, has an orbit noticeably more egg-shaped than any other planet’s. Its distance from the Sun varies dramatically over the course of a single orbit, which contributes to extreme temperature swings on its surface.
What Earth’s Ellipse Means for You
Earth’s eccentricity of 0.017 is small, but it’s not zero. The practical result is that Earth is about 3.3% farther from the Sun in early July than it is in early January. That translates to roughly 6.5% more sunlight reaching Earth in January compared to July.
If that seems backward for anyone living in the Northern Hemisphere, where January is the dead of winter, it highlights an important point: seasons are caused by the tilt of Earth’s axis, not by the shape of its orbit. The slight difference in solar distance does have minor climatic effects, but it’s nowhere near enough to drive the seasons.
Orbits Change Over Time
A planet’s eccentricity isn’t locked in permanently. The gravitational pull of other planets tugs on each orbit, slowly reshaping it over thousands of years. Earth’s eccentricity oscillates between nearly circular and mildly elliptical on two cycles: one averaging about 100,000 years and a longer one of roughly 413,000 years. These shifts, called Milankovitch cycles, are one of several orbital variations that influence long-term climate patterns, including the timing of ice ages.
The changes are gradual. Over a human lifetime, Earth’s orbital shape is essentially constant. But over hundreds of thousands of years, even small changes in eccentricity alter how much solar energy different parts of the planet receive across seasons, enough to tip the balance between glacial and warm periods.
How Scientists Figured This Out
For nearly 2,000 years, astronomers assumed planetary orbits were perfect circles. Even Copernicus, who correctly placed the Sun at the center of the solar system in 1543, still relied on circular orbits. To make his predictions match actual observations, he had to add smaller circles on top of circles, called epicycles, just as the ancient Greek model had done.
The breakthrough came from Kepler, who spent years analyzing precise observations of Mars collected by the astronomer Tycho Brahe. When Kepler tried to fit Mars’s motion to a circle, the data wouldn’t cooperate. In his 1609 work, he showed that an ellipse fit the observations perfectly, eliminating the need for epicycles entirely. One simple shape replaced a complicated system of stacked circles, and the modern understanding of orbits was born.