People often imagine Earth’s annual journey around the Sun as a perfect circle, but the true shape is slightly different. Earth’s orbit is a subtly elongated oval known as an ellipse. Understanding this precise shape is central to grasping how our solar system functions.
The True Shape
The orbit of Earth, like that of all planets, traces an elliptical path around the Sun. An ellipse is a closed, curved shape that resembles a stretched-out circle. It has two central points, called foci, with the Sun situated at one of these focal points, not at the geometric center.
The degree to which the ellipse is stretched is measured by eccentricity. A perfect circle has an eccentricity of zero. Earth’s current orbital eccentricity is very low, approximately 0.0167, meaning its path is extremely close to circular.
If drawn on paper, the difference between the actual elliptical path and a perfect circle would be nearly impossible to detect. This slight deviation is important because it means the distance between the Earth and the Sun varies throughout the year.
Key Orbital Positions
Because the orbit is an ellipse, there are two distinct points of extreme distance from the Sun. The point where Earth is closest to the Sun is called perihelion, and the farthest point is called aphelion.
Earth reaches perihelion around January 3rd, when the distance is about 91.4 million miles (147.1 million kilometers). Conversely, aphelion occurs around July 4th, increasing the distance to about 94.5 million miles (152.1 million kilometers).
The difference between the closest and farthest points is about 3.1 million miles, a variation of only about three percent from the average distance. This slight change in distance has a minor influence on the amount of solar radiation received by Earth.
Why It Isn’t a Perfect Circle
The elliptical path results from the interaction between two forces: gravity and momentum. The Sun’s enormous gravitational pull constantly attempts to draw the Earth toward it. This force is counteracted by Earth’s forward velocity, which tries to keep the planet moving in a straight line. The combination of these forces causes Earth to fall around the Sun instead of directly into it.
A perfectly circular orbit would require a specific, improbable balance where the planet’s velocity exactly matches the gravitational pull. Since the initial conditions of the solar system’s formation did not produce this balance, the orbit is naturally an ellipse. When Earth is closer to the Sun at perihelion, the stronger gravitational force causes the planet to speed up. As it swings toward aphelion, the pull weakens, and the planet slows down, bending the path into the oval shape.
Addressing Common Misconceptions
A frequently held belief is that the elliptical shape of Earth’s orbit causes the seasons. This is incorrect because the distance difference between perihelion and aphelion is too small to cause significant temperature changes. Furthermore, if distance were the cause, both hemispheres would experience the same season simultaneously.
The true cause of the seasons is the tilt of Earth’s rotational axis, angled at approximately 23.5 degrees relative to its orbital plane. This axial tilt dictates how directly the Sun’s rays strike a given hemisphere throughout the year. For example, when the Northern Hemisphere is tilted toward the Sun in June, it experiences summer, even though Earth is near its farthest point from the Sun.
The slight variation in solar energy received due to the elliptical orbit has a minor effect, but it is entirely overwhelmed by the impact of the axial tilt. The tilt determines the angle and duration of sunlight, making it the primary driver of the seasonal cycle.