Orbit shape · Stretch
How orbit shape changes
Eccentricity measures how far Earth’s orbit is from a circle. It drifts between almost circular and slightly oval, which changes the gap between Earth’s closest and farthest distances from the Sun.
Main rhythms near 100,000 and 405,000 years
Orbit shape
The ellipse gets a little rounder or a little more stretched.
The near–far gap
Today Earth is 147.1 million km from the Sun at its closest and 152.1 million km at its farthest. At the most stretched, the gap is 17 million km.
Wobble amplifier
It sets how much the wobble can change seasonal sunlight.
What eccentricity is
A circle has an eccentricity of 0. Earth’s orbit today is 0.0167, close enough to a circle that a true-to-scale drawing looks round. Over the last 800,000 years it has stayed between about 0.004 and 0.05. The Sun sits at one focus of the ellipse, not at its centre.
As eccentricity grows, Earth’s closest point (perihelion) gets closer and its farthest point (aphelion) gets farther. Sunlight is stronger when Earth is closer, because the same energy spreads over a smaller area. With today’s shape the nearest sunlight is 7% stronger than the farthest. At the most stretched it is 26% stronger.
A small change with a large side effect
Over a whole year, a more stretched orbit changes the sunlight Earth receives by less than 0.2%. What it changes is the seasonal balance. When the orbit is almost a circle, the wobble hardly matters, because every point is the same distance away. When the orbit is stretched, the wobble can swing midsummer sunlight at 65°N by more than 100 W/m².
The ice record has its strongest rhythm at 100,000 years, which is odd, because the orbit’s direct push at that period is weak. Ice sheets, oceans and carbon dioxide must amplify it. Exactly how is still debated.
It is not an ice-age switch
No particular eccentricity starts an ice age. The three orbital cycles combine, and then ice, oceans and greenhouse gases amplify or damp the result.
The lab calculates sunlight for a given orbit and adds a rough estimate of ice and temperature. Predicting ice properly needs a climate model on top of that.
Try it yourself
Change the orbit
Move all three motions and watch midsummer sunlight at 65°N change.
Questions
Quick answers
Is Earth’s orbit very elliptical?
No. Even at its most stretched it is close to a circle. Drawings exaggerate it so the change can be seen.
Does it change the length of the year?
No. A year is still one trip around the Sun. Eccentricity changes the near–far gap and Earth’s speed along the orbit, which is higher when Earth is closer.