Here's why:
* Earth's orbit is elliptical, not circular: Earth's orbit around the sun is slightly elliptical, meaning it's not a perfect circle.
* Perihelion and Aphelion: This means there are points in Earth's orbit where it's closest to the sun (perihelion) and points where it's furthest from the sun (aphelion).
* 1 AU is an average distance: The astronomical unit (AU) is defined as the average distance between the Earth and the Sun.
So, while 1 AU is a convenient measure for describing Earth's average distance from the sun, Earth's actual distance varies throughout the year.
