The path Earth takes around the Sun is known as its orbit, which is technically an ellipse. This elliptical shape means the distance between our planet and the Sun changes over the course of a year. However, the degree to which this orbit deviates from a perfect circle is remarkably small. If drawn to scale, the Earth’s orbit would look almost indistinguishable from a circle.
Defining the Measurement of Ellipticity
Scientists use a specific mathematical value known as eccentricity to quantify how much an orbit deviates from a perfect circle. Eccentricity is a dimensionless parameter that describes the shape of the elliptical path an object takes around its host star. A value of zero represents a perfectly circular orbit, where the distance never changes, while values between zero and one describe an ellipse.
The Earth’s current orbital eccentricity is approximately 0.0167, confirming the near-circular nature of the orbit. As eccentricity increases toward one, the orbit becomes increasingly elongated and narrow. The low value means the Sun remains very close to the center of the orbital path, not at a distant focal point.
Earth’s Closest and Farthest Distances
Despite its near-circular appearance, the Earth’s elliptical orbit results in two distinct points of distance from the Sun. The closest approach is called perihelion, and the greatest distance is called aphelion. These two points represent the physical consequence of the 0.0167 eccentricity value.
The Earth reaches perihelion around early January, at a distance of approximately 147.1 million kilometers. Conversely, the Earth is at its aphelion around early July, when the distance stretches to about 152.1 million kilometers. This difference of 5 million kilometers between the closest and farthest points represents only a modest 3.4% variation in distance from the Sun over the year.
Why Orbital Shape Does Not Determine Seasons
The small 3.4% distance variation is often mistakenly believed to cause seasons. The actual driver of seasonal change is the Earth’s axial tilt, also known as obliquity. Our planet’s axis of rotation is tilted by about 23.5 degrees relative to the plane of its orbit, and this tilt remains pointed in the same direction in space as Earth revolves around the Sun.
This persistent tilt means that as Earth orbits, the Northern and Southern Hemispheres alternately receive the Sun’s most direct rays. When the Northern Hemisphere is tilted toward the Sun, it experiences summer due to the more direct, concentrated sunlight and longer daylight hours. When it is tilted away, the sunlight hits at a lower, more oblique angle and is spread over a larger area, resulting in winter.
The effect of the axial tilt is far more pronounced than the small change in solar distance. The distance variation creates a notable irony for the Northern Hemisphere, which experiences winter when the Earth is closest to the Sun (perihelion in January) and summer when it is farthest (aphelion in July). While the slight difference in distance minimally affects solar radiation intensity, the angle and duration of daylight determined by the tilt are the dominant factors that drive temperature changes and define the seasons.
How Earth’s Orbit Changes Over Time
The degree of Earth’s orbital ellipticity is not a permanent fixture but changes slowly over geologic time scales. This variation in the orbital shape is one of the three components that make up the Milankovitch Cycles, which describe the collective effect of Earth’s orbital changes on its climate. The eccentricity cycle causes the Earth’s orbit to slowly stretch and compress over approximately 100,000 years.
Over this vast period, eccentricity ranges from a nearly perfect circle (low value of about 0.0034) to a slightly more elongated ellipse (high value reaching approximately 0.058). These gravitational perturbations are caused primarily by the influence of the massive outer planets, Jupiter and Saturn, tugging on Earth. While the current eccentricity of 0.0167 is low, the long-term cycle of this value has been linked to major shifts in Earth’s climate, such as the advance and retreat of ice ages.