How big is Earth's orbit around the Sun?

From the November 2024 issue

Earth’s orbit around the Sun is not circular, but an ellipse. The perimeter of Earth’s orbit is roughly 584 million miles (940 million km).

Earth’s orbit is not a circle, but an ellipse with the Sun at one focus. Using the semimajor and semiminor axes of our planet’s orbit, we can estimate the perimeter of the ellipse it traces out each year. To calculate this, the axes are measured from the center of the ellipse. Credit: Astronomy: Roen Kelly

How big is the ellipse that our planet travels in a year around the Sun?

J.J. Müdespacher
Mexico City, Mexico

Earth’s orbit around the Sun is not circular, but an ellipse that is slightly elongated with an eccentricity of 0.017. (An eccentricity of 0 is a circle, while the dwarf planet Pluto has a relatively high orbital eccentricity of 0.244. The maximum eccentricity possible is 1, which forms not a closed orbit but a parabola, meaning the object will never complete a full orbit but instead be flung off into space.)

The difference between perihelion, when Earth is closest to the Sun, and aphelion, when Earth is farthest from the Sun, is only about 3 percent, or roughly 3 million miles (4.8 million kilometers). At perihelion, which occurs in January, Earth is generally about 91 million miles (147 million km) from the Sun; at aphelion in July, Earth is about 94.5 million miles (152 million km) from the Sun.

To calculate the size (perimeter) of the ellipse Earth travels each year, we need its semimajor axis (a) and semiminor axis (b). NASA gives the semimajor axis of our planet’s orbit as nearly 93 million miles (149,598,000 km). From this value and the eccentricity e of Earth’s orbit (0.017), we can calculate the semiminor axis of Earth’s orbit by rearranging the equation to calculate eccentricity: