How does Earth's elliptical orbit change the AU over a year?

The Astronomical Unit (AU) is defined as the average distance between Earth and the Sun — about 149.6 million kilometers. However, since Earth’s orbit is elliptical (oval-shaped), the actual distance between Earth and the Sun keeps changing throughout the year.

At perihelion (early January), Earth is about 147.1 million km (0.983 AU) from the Sun. At aphelion (early July), it’s about 152.1 million km (1.017 AU) away. That means the Earth–Sun distance varies by roughly 5 million kilometers, or about 3% of one AU.

Despite this change, the AU itself is a fixed constant used as a standard unit — scientists define it precisely as 149,597,870.7 km. The orbit’s ellipticity doesn’t change the length of the AU but does affect Earth’s orbital speed and sunlight intensity.

In January, Earth moves faster and receives slightly more solar energy; in July, it moves slower and gets slightly less. This variation, though small, slightly influences seasonal temperature differences between hemispheres.

So, while the value of 1 AU stays constant, Earth’s actual distance from the Sun keeps changing slightly throughout the year due to its elliptical orbit.