The Eccentricity of Earth's Orbit: An Ellipse That Changes Everything

An Orbit Not So Circular

Earth's orbit around the Sun is often depicted as a circle, but it is actually a slightly flattened ellipse, characterized by an eccentricity denoted as \(e\). This eccentricity measures the degree of flattening of the ellipse: \(e = \sqrt{1 - \frac{b^2}{a^2}}\), where \(a\) is the semi-major axis and \(b\) is the semi-minor axis.

Variable Eccentricity

An Orbit Influenced by Giants

The eccentricity of Earth's orbit is not constant. It oscillates between 0.005 and 0.058 over cycles of about 100,000 years. These variations are caused by the gravitational perturbations exerted by the giant planets, notably Jupiter and Saturn, which slowly modify the shape of Earth's orbit.

A Small but Significant Eccentricity

Currently, Earth follows an orbit with an eccentricity of about 0.0167, which may seem close to a perfect circle. However, this small value results in a distance difference of about 5 million kilometers between the perihelion (147.1 million km) and the aphelion (152.1 million km).

N.B.: The aphelion is the point in the orbit where Earth is farthest from the Sun (~152.1 million km currently). At this position, the solar energy received is minimal.

N.B.: The perihelion is the point in the orbit where Earth is closest to the Sun (~147.1 million km currently). Solar irradiance is at its maximum, which can slightly modulate the seasons, especially in the southern hemisphere currently.

An Amplified Impact on Climate

This variation in distance results in a difference of about 6.7% in the solar energy received between the closest and farthest points from the Sun. Alone, this variation might seem minor, but it acts in synergy with other orbital parameters (obliquity and precession) and with Earth's feedback mechanisms (ice, albedo, CO₂, water vapor, biosphere). These combined effects are the origin of the Milankovitch climate cycles, which explain the glacial/interglacial alternations observed over the last few million years.

Major Climatic Consequences

Variations in eccentricity influence the average distance between Earth and the Sun, thus modifying the received insolation. During periods of maximum eccentricity, the difference between perihelion and aphelion becomes significant. Combined with other orbital parameters (obliquity and precession), this variation is the origin of the Milankovitch cycles that pace the ice ages.

Comparison of Earth's Eccentricity with Other Planets

Source: NASA – Planetary Fact Sheet

An Ellipse That Changes Everything

Even if the eccentricity of Earth's orbit seems small, its consequences on the global climate are notable on geological time scales. Combined with other orbital parameters, it shapes the major climatic trends of our planet and plays a key role in the alternation of glacial and interglacial periods.

Recent Glacial Cycles

Here are the main climatic cycles identified over the last 800,000 years:

Data from: EPICA Dome C, Vostok Ice Core (Petit et al., 1999; Jouzel et al., 2007), LR04 (Lisiecki & Raymo, 2005).